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06Z FOR Nov 23 puts some extreme cold in the East and South all the way to the coast, with some snow later as the system breaks down and another moves in

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Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
201 AM EST Sun Nov 24 2019

Valid 12Z Wed Nov 27 2019 - 12Z Sun Dec 01 2019

  • Several Major Storms for the Thanksgiving Holiday Period

Guidance/Predictability Assessment:

The latest models and ensembles show better than average forecast
clustering/predictability
regarding the evolution of a shortwave
trough exiting the Central Rockies at the end of the short range
period, and steadily deepening as it lifts into the Great Lakes to
begin the medium range period on Wednesday. This should spin up a
rather impressive cyclone at the surface which tracks from the
Great Lakes to the Northeast on Wednesday-Wednesday night.
The CMC
is a hair faster with the upper shortwave/closed low as it exits
the Northeast early Thursday and so a blend of the better
clustered GFS/ECMWF with minor contributions from the ensemble
means was used, which also maintains good continuity with the
previous WPC forecast.

The biggest forecast differences in the medium range are with the
evolution of highly amplified troughing along the West Coast as it
slowly inches inland and shortwave energy eventually ejects into
the Central Plains around day 6 to day 7.
Through day 5, models
show reasonable agreement/clustering, but its after this time
where timing and intensity differences begin to arise. The GFS
(and to some degree the GEFS mean) are much quicker to eject
energy from the Southwest into the South-Central Plains, depicting
a wrapped up closed upper low over the Central Plains by as early
as day 6. Most other models and ensembles hold the energy back a
bit, not depicting a closed upper low to emerge until the system
reaches the Midwest on day 7. Given the uncertainty at the longer
range time scale, opted to go for a blend of the ECENS/GEFS means,
along with some percentage of the ECMWF which is better in line
with the ensemble means. Regardless of timing though, the models
do agree that another potent storm system could affect portions of
the north-central U.S. by the end of the period potentially with a
very similar track of the earlier storm days 3-4.


Weather Highlights/Threats:

Moderate to locally heavy precipitation is likely Tuesday into
Wednesday on the north and west side of the surface low from the
Central Plains to the Upper Midwest. Some accumulating snows may
be possible on the northern edge of the precipitation shield. As
the storm lifts into the Northeast on Wednesday, wintry
precipitation is possible across northern/interior New England. A
period of high winds appears likely to accompany this system as
well.

Meanwhile, heavy mountain snows are likely across the Great Basin
and Rockies Wednesday-Friday with snow levels expected to drop
significantly. Another round of potentially heavy rainfall with a
threat of runoff problems are also especially likely across
Southern CA and the Southwest (including burn scars) by mid to
late next week. Heavy rainfall is also possible from portions of
the Southern Plains to the lower Ohio Valley as multiple shortwave
impulses round the base of the Western U.S. trough and tap some
mid-upper level moisture from the tropical east Pacific. The
northwest edge of this precip shield across eastern New
Mexico/Texas Panhandle may also see accumulating snowfall with
surface high pressure and sufficiently cold air diving southward
across the Plains. With the next potential storm system winding up
in the Central U.S. into next weekend, accumulating snows
(potentially heavy or significant) are possible to the north and
west of the surface low from the Northern Plains to the Upper
Mississippi Valley next Friday and Saturday.

The biggest temperature anomalies will focus across the Western
U.S. through much of the period with daytime highs averaging 10 to
20 degrees below normal. In response to this trough, ridging
builds across the south-central U.S. which should bring a return
of above normal temps (anomalies +5 to +10 degrees) to this region
by the end of the period. In the East, above normal temperatures
on Wednesday should moderate back to normal or slightly below
normal at the end of the week.

Santorelli

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Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
210 AM EST Mon Nov 25 2019

Valid 12Z Thu Nov 28 2019 - 12Z Mon Dec 02 2019

  • Increasing confidence in a major winter storm for the Thanksgiving Holiday Period

Pattern overview and Guidance/Predictability Assessment:

The medium range period begins Thursday with the first major storm
system exiting the Northeast while anomalous (500mb heights 3 to 4
standard deviations below normal) and very amplified
troughing/closed upper low is situated along the West Coast.
The
Western U.S. system is the main system of interest in the medium
range period as it slowly meanders over the Western U.S.,
eventually shifting eastward as vigorous short wave energy ejects
out of the Rockies and into the Central U.S.. This will induce
cyclogenesis in the lee of the Rockies and a rather potent surface
low tracking from the Central Plains to the Northeast through the
busy upcoming Holiday weekend.
This system appears to take a very
similar, albeit slower, track to the previous system now in the
short range period.

For days 3-4, the models show good enough agreement to warrant a
majority deterministic model blend.
After this, questions begin to
arise mainly on timing of the shortwave energy ejecting into the
Central Plains day 4-5 and a tightly closed upper low forming and
tracking towards the Upper Great Lakes thereafter. The GFS has
been the most consistent both with timing and intensity of this
feature. The latest run (12z/Nov 24) of the ECMWF is quite a bit
faster than the GFS, though run-to-run continuity is poor. It at
least appears the ECMWF may be trending in the direction of the
GFS though, which lends to a bit more confidence in the GFS
solution
. Meanwhile, the CMC and UKMET (the latter of which is not
available past day 5) are much slower than the GFS, not actually
ejecting the energy into the Plains until almost a full day later.
The ensemble means are generally in good agreement and support a
solution closer to that of the faster GFS/ECMWF and is a good
middle ground between the two as well. After day 5, the WPC progs
trended towards the ECENS/GEFS means, with small contributions
from the GFS to help give some definition to the typically washed
out look of the means. This maintains good continuity with the
previous WPC forecast.


Weather Highlights/Threats:

Heavy mountain snows are likely across parts of the Central Great
Basin and Rockies Thursday and Friday, with snow levels expected
to drop significantly. Additionally, another round of heavy
rainfall with a threat for runoff problems is likely across
Southern CA and the Southwest (including burn scars) Thursday into
Friday. As the surface low moves out into the Plains, a swath of
heavy to potentially significant accumulating snowfall is possible
to the north and west of the low from the Northern Plains to the
Upper Mississippi Valley and Upper Great Lakes Friday and
Saturday. Depending on surface low intensity, a period of heavy
winds could accompany this system making for hazardous travel
across this region.
Across the South, shortwave energies tapping
into mid-upper level moisture from the tropical East Pacific will
result in heavy to possibly excessive rains shifting from the
Southern Plains to the Ohio Valley along and ahead of the main
cold front.

The biggest temperature anomalies will focus across the Western
U.S. through much of the period with daytime highs averaging as
much as 20 to 25 degrees below normal for some places. In
response, temperatures will be warm and above average from Texas
to the Gulf Coast states and into the Ohio Valley Friday and
Saturday, trending cooler by the end of the period following the
cold front passage.

Santorelli

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Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
159 AM EST Tue Nov 26 2019

Valid 12Z Fri Nov 29 2019 - 12Z Tue Dec 03 2019

  • Major winter storm likely for the Thanksgiving Holiday Period

Pattern overview and Guidance/Predictability Assessment:

The medium range period (Friday to Tuesday) begins with a broad
upper level low within very amplified troughing over the West
Coast. Shortwave energy rounding the base of the trough ejects out
into the Central Plains by late Friday-early Saturday with a
rather deep upper level low developing in the Northern Plains and
tracking quickly eastward through the weekend. Compared to recent
days, the models have come into much better agreement with this
feature, with some mostly minor timing and intensity differences

remaining. The notable outlier continues to be the UKMET which is
almost a day later to eject the energy into the Plains than the
rest of the guidance. This results in a rather potent surface low,
and potentially major winter storm, tracking from the
north-central Plains to the Upper Great Lakes/Midwest, and off the
Northeast coast by Monday.

Behind this main system, upper ridging builds across the Central
U.S. while reinforced troughing gets established just west of the
West Coast. There is enough run-to-run variability in the
deterministic models with the evolution of various embedded vorts
that an ensemble mean blend is the best approach at this point.


Across the board, this cycle of the WPC progs used a blend of the
GFS/ECMWF with the ECENS/GEFS means. Good agreement with the major
Central U.S. upper low warranted a majority deterministic weight
days 3-5, with increasing usage of the means thereafter. This
maintains good continuity with the previous WPC forecast.

Weather Highlights/Threats:

The Central U.S. storm will bring a variety of weather hazards to
a large part of the country for the upcoming Thanksgiving holiday

weekend. Heavy mountain snows will remain possible on Friday for
much of the Central and Southern Rockies coming to an end by
Saturday as the system shifts east. As the surface low moves into
the Plains, a swath of heavy to potentially significant
accumulating snowfall is likely north and west of the low from the
Northern Plains to the Upper Great Lakes Friday and Saturday,

shifting into New England on Sunday. High winds may also accompany
this system making for difficult and hazardous travel across much
of the region. Along and ahead of the attendant cold front, heavy
to possibly excessive rainfall is likely from the Southern
Plains/Gulf Coast to the Ohio Valley. The latest SPC severe
weather outlook also shows a 15 percent chance of severe weather
for Friday across the Southern Plains.

The greatest temperature anomalies will focus across the West
through the weekend with daytime highs 20 to 25 degrees below
normal for some but should trend back towards normal (though still
slightly below) by the end of the period. The Midwest to Deep
South are warm Friday and Saturday, with cooler than average
temperatures moving in by late weekend and early next week. Cold
front exiting the East day 6/Tuesday will bring a return to below
normal temperatures.


Santorelli

 

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Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
145 AM EST Wed Nov 27 2019

Valid 12Z Sat Nov 30 2019 - 12Z Wed Dec 04 2019

  • Major Winter Storm to impact the Northern Plains to the Northeast during the Thanksgiving weekend

Pattern overview and Guidance/Predictability Assessment:

The medium range period (Saturday-Wednesday) begins with shortwave
energy ejecting into the Central U.S. inducing cyclogenesis in the
lee of the Rockies. This results in a deep low pressure system and
major winter storm tracking from the north-central Plains to the
Great Lakes this weekend.
Additional southern stream energy
rounding the base of this cold core upper low allows for a new
surface low to form off the Mid-Atlantic coast early Monday which
then tracks up the New England Coast through Tuesday. Models and
ensembles continue to show fairly good agreement on the track and
intensity for this storm, with only minor timing and intensity
differences remaining. A general model compromise worked well and
shows good continuity with the previous shift.

Meanwhile, energies dropping southward from the Gulf of Alaska
establishes another trough into or just off the West Coast by day
4/Sunday. For days 3-4, there is good enough model agreement to
warrant a mostly deterministic blend between the latest runs of
the GFS and ECMWF. After this time, there remains question on the
evolution of an upper level low off the West Coast. The latest run
of the GFS (and CMC) show an upper low lingering through most of
the day 5-7 period well off the Southwest Coast, but run-to-run
continuity in the GFS is poor with this. The ECMWF shows better
run-to-run continuity in showing an upper low shifting southward
along the coast and eventually into the Southwest on day 7. The
ensemble means also seem to support this idea of weakening the low
and bringing it inland as well. Therefore, for days 5-7,
increasing weighting of the ensemble means was used with a good
chunk of the ECMWF which is the deterministic model closest to the
ensemble means.


Weather Highlights/Threats:

A major winter storm with widespread hazardous weather is expected
for parts of the Central and Eastern U.S. over the Thanksgiving
holiday weekend.
To the north and west of the surface low, heavy
and significant accumulating snow is likely from the Dakotas to
the Upper Great Lakes on Saturday, with accumulating snow likely
shifting into much of the Northeast Sunday into Monday. High winds
will also accompany this storm, making for difficult and hazardous
travel across much of the region. To the south, locally heavy
rainfall is possible along the attendant cold front as it shifts
eastward across the Deep South/Ohio Valley/Mid-Atlantic states
this weekend. Severe thunderstorms are also possible across the
Lower Mississippi Valley on Saturday as indicated by the latest
medium range SPC severe outlook.

In the West, expect another round of heavy mountain snows across
the higher terrain of northern and Central California, with rain
in the lower elevations as well as up the Pacific Northwest Coast.
More rain should also shift into parts of Southern California by
Tuesday as well.

Temperatures remain cold across the West Saturday into Sunday with
daytime highs 10 to 20 degrees below normal in some spots.
Temperatures should moderate back towards normal (though still
slightly below) through the rest of the period. Ahead of the cold
front in the Central U.S., warmer than average temperatures are
expected Saturday, but should quickly get replaced with below
normal temperatures by Monday which also sweep into the East
Tuesday/Wednesday following the cold frontal passage.

Santorelli

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Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
145 AM EST Thu Nov 28 2019

Valid 12Z Sun Dec 01 2019 - 12Z Thu Dec 05 2019

  • Major Winter Storm will track from the Upper Midwest to the Northeast Sunday and Monday

Pattern overview and Guidance/Predictability Assessment:

The medium range period (Sunday-Thursday) begins with a developed
low pressure system over the Upper Midwest/Great lakes region with
an attendant cold front sweeping through the Mid-Atlantic and
Southeast. Additional southern stream energy rounding the base of
the associated cold core upper low should allow for the
development of a new surface low off the Mid-Atlantic coast late
Sunday, which then tracks up the New England coast through
Tuesday. Models and ensembles show fairly good agreement on this
system, though the latest run of the CMC is slightly faster than
the remaining deterministic models with the track of the upper
low. There are some minor differences regarding how quickly the
surface low exits the New England coast as well, which of course
would have implications for precipitation along the coast. The GFS
is a tad slower with its exit than the ECMWF, but the means are
roughly in the middle.

Meanwhile, an upper level low off the Northwest coast should drop
southward off the West coast through about Tuesday before a
weakened state of the system begins to shift inland across the
Southwest Wednesday and Thursday. Days 3-5 the deterministic
models show relatively good agreement with this system, with
timing differences arising by day 6 with how quickly the energy
moves inland. The last few runs of the GFS and the latest run of
the CMC are notably faster than the ECMWF. The means are
understandably quite weak and washed out by day 7 with the
shortwave, though it seems they would favor a slower shortwave
closer to the ECMWF.

For both systems of note, the forecast blend for days 3-5 was
based mostly on the ECMWF and GFS, with small contributions from
the UKMET on day 3 and the ECENS/GEFS means on days 4 and 5.
Beyond that, leaned more on the means mainly due to continued
uncertainty with the upper low/trough in the West. This maintains
good continuity with the previous WPC forecast.


Weather Highlights/Threats:

The major winter storm to start the period over the Upper Midwest
will continue spreading heavy and potentially significant
accumulating snow across the Upper Great Lakes and the interior
Northeast on Sunday, and into New England on Monday.
High winds
may also accompany this storm, making for difficult and hazardous
travel across much of the region. To the south, locally heavy
rainfall is possible along the attendant cold front as it shifts
through the Mid-Atlantic and the Southeast on Sunday.
Behind the
cold front temperatures should trend cooler up and down the East
Coast through Tuesday, with a moderation back towards normal by
Wednesday and Thursday.

In the West, another round of heavy precipitation affects
California through Monday with heavy mountain snows expected in
northern and central California, and rain in the lower elevations.
Locally heavy rain and mountain snow may also shift into Southern
California and parts of the Southwest next week as well. A chilly
start on Sunday in the West should warm through the period, with
temperature anomalies Monday-Thursday generally less than 10
degrees below normal.


Santorelli

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Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
139 AM EST Fri Nov 29 2019

Valid 12Z Mon Dec 02 2019 - 12Z Fri Dec 06 2019

  • Significant winter storm to affect the Northeast early in the week
  • One or more precipitation events likely over the West Coast states

Pattern Overview and Guidance/Predictability Assessment:

A strong upper low forecast to be over the Mid-Atlantic as of
early Monday will progress to the east and then northeast Mon-Tue.
Expect an associated low pressure system to track just offshore
the Northeast and then into the Canadian Maritimes, with
meaningful winter weather effects likely to the north/west of the
low track.  Behind this system guidance depicts a tendency for
cyclonic mean flow over the eastern U.S. with the best defined
shortwave likely to pass over the East around midweek.  Meanwhile
most solutions suggest that an upper low and surrounding
positively tilted trough energy initially off the West Coast will
consolidate during the first half of the week.  This feature
should weaken as it moves inland over the Southwest/southern half
of the Rockies Wed-Thu with continued progression of the shortwave
thereafter.  An upstream trough will likely amplify over the
eastern Pacific during the latter half of the week.  This feature
should eventually spread moisture into a greater portion of the
West Coast region than the leading one whose precipitation will
focus more over southern areas.

Clustering among the guidance is decent for the storm affecting
the Northeast early in the week.  Latest UKMET runs have been a
bit slower than the primary consensus including the GFS/ECMWF/CMC
and the GEFS/ECMWF means.  The upstream shortwave reaching the
East by midweek will bring a modest northern Plains front
southeastward into East followed by offshore development whose
track should be far enough east to have limited effect on the East
Coast.  There is good agreement in principle on the shortwave but
some details have low predictability for being a few days out in
time.

The system heading into the Southwest Wed into Thu shows decent
agreement in the guidance considering that there are at least a
couple separate pieces of energy involved.  Typical timing
differences develop as the weakening feature crosses the southern
tier during the latter half of the week with the GFS runs showing
their faster tendency versus the ECMWF/CMC.  The GEFS/ECMWF means
and historical verification suggest the GFS may be too fast but
the mean pattern looks sufficiently progressive for an
intermediate timing to be plausible.  Through day 7 Fri the
primary uncertainty with the amplifying eastern Pacific trough is
if/where an embedded closed low may form.  Thus far the
operational models are generally suggesting higher potential for a
closed low than the ensemble means, while GFS runs have been
tracking the closed low north/northeast of the 12Z ECMWF and
latest CMC runs.  Over multiple runs the GFS has been somewhat
more consistent than the ECMWF with the general evolution.  Based
on guidance available through the 18Z cycle a compromise among the
12Z ECMWF/CMC, 18Z GFS, and 18Z GEFS/12Z ECMWF means would provide
a good starting point to represent somewhat more flow separation
than the means without committing to lower predictability closed
low details for the time being.


Weather Highlights/Threats:

The storm system tracking just offshore the Northeast early in the
week will bring a period of significant wintry precipitation to
areas from the extreme northern Mid-Atlantic into New England.
Winds may be sufficiently strong to make travel even more
difficult/hazardous.
 Parts of Great Lakes should see one or more
periods of snow but with mostly light accumulations.  Over the
West, expect weekend precipitation to linger over California into
Mon followed by a brief break.  Then the system moving into the
Southwest will bring an episode of focused precipitation with
highest totals most likely over favored terrain inland from the
southern California coast and a lesser maximum over central
Arizona.  Higher elevation snow will be possible from the Sierra
Nevada into the central Rockies.  This system may produce an area
of mostly rain over and east of the central-southern Plains late
in the week.  The upper trough amplifying over the eastern Pacific
late in the week should spread moisture into the West Coast states
at that time.  Timing and exact focus of heaviest precipitation
will be sensitive to lower predictability details within the
trough.  Currently the best potential for highest totals exists
over northern California.

The eastern states will be chilly to start the week with some
parts of the South seeing highs 10-15F below normal on Mon.
Eastern U.S. temperatures will gravitate toward normal by the
latter half of the week
.  On the other hand expect the central
U.S. to see above normal temperatures for most of the period with
some pockets of plus 10F or greater anomalies for min and/or max
readings from Tue onward.  The expected pattern will support
moderately below normal highs over the Great Basin and at times
into southern California while most of the West should see morning
lows closer to or above normal.

Rausch

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Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
159 AM EST Sat Nov 30 2019

Valid 12Z Tue Dec 03 2019 - 12Z Sat Dec 07 2019

  • Significant winter storm to affect the Northeast into Tuesday
  • Low pressure brings midweek heavy rain to southern California

Pattern Overview and Guidance/Predictability Assessment:

Latest guidance is showing better than average agreement for the
overall mean pattern during Tue-Sat as well as individual features
in principle, though with some ongoing detail/timing
uncertainties
.  From a multi-day mean perspective the flow aloft
will have a tendency toward troughing just off the West Coast
while a mean ridge prevails over the west-central U.S. and a mean
trough resides over the East.  Features within the eastern mean
trough will include the strong system affecting New England into
Tue before departing, an upstream shortwave reaching the East by
Wed and supporting Atlantic low pressure that should track far
enough offshore to have limited influence on the East Coast, and
then an initial northeastern Pacific shortwave forecast to reach
southwest Canada by Wed and the Northeast by Sat.  Farther west an
upper trough/closed low will move into the Southwest around
midweek, weakening as it passes through the longer term mean ridge
and further shearing downstream as it becomes increasingly
embedded in progressive flow.  An upstream trough will amplify
into the eastern Pacific mid-late week with its axis likely
approaching the West Coast by Sat.

At the start of the period the system just off the New England
coast exhibits good clustering in general but with some
small-scale differences that would have some influence on New
England but have fairly low predictability three days out in time.
 The upper trough reaching the East on Wed still exhibits some
detail differences with corresponding spread for developing low
pressure over the western Atlantic and into the Canadian
Maritimes.  The common theme among guidance so far is a low track
sufficiently offshore to keep the system's moisture shield away
from the East Coast states.  Consensus shows the next shortwave in
the series bringing a frontal wave into southwest Canada on Wed
with increasing divergence thereafter regarding low track and
strength/shape farther east.  Thus far the ensembles and CMC runs
have been much weaker with the surface wave than latest/recent
GFS/ECMWF runs, while the new 00Z UKMET has trended somewhat
deeper/northward versus its prior run.  The current array of
guidance argues for a defined wave but with somewhat less vigor
than the GFS/ECMWF until there is better agreement.

Over the past day there has been little change in the guidance
distribution for the system heading into the Southwest around
midweek and continuing eastward thereafter, with operational GFS
runs still faster than other solutions (including the GEFS mean)
to varying degrees.  Early in the period the 18Z GFS compared
least favorably with other solutions relative to other recent runs
as it was farthest northeast with the upper low off California.
An average among non-GFS guidance still appears to be the most
likely scenario but the progressive nature of mean flow could
still leave the door open to nudging the dominant cluster a tad
faster at some point.  There is still a fair degree of uncertainty
over exact evolution of the surface reflection which should become
increasingly weak/suppressed to the east of the Plains.  Within
the larger-scale upper trough amplifying off the West Coast
mid-late week, confidence is steadily increasing that an upper low
will close off for a while per recent operational model runs.
Among the means the 12Z ECMWF mean finally closed off a low and
now the 00Z GEFS mean has done likewise.  Potential influence from
energy to the north becomes an uncertainty by day 7 Sat with some
solutions indicating the trough could open up by then.  Clustering
and trends favor primarily an operational model emphasis into days
5-6 Thu-Fri followed by a more even weight of models/means for day
7 Sat as detail confidence decreases.

Based on guidance available through the 18Z cycle, the initial
blend started with the 12Z GFS/ECMWF/CMC early-mid period and
began incorporating a very modest 20 percent total input of the
18Z GEFS/12Z ECMWF means on day 5 Thu.  Ensemble weight increased
to 30-50 percent by days 6-7 respectively and GFS input switched
from the 12Z run to 18Z run as the 12Z run strayed to the fast
side with height falls reaching the West late in the period.


Weather Highlights/Threats:

Effects from the early week storm off the Northeast coast will
continue over New England into Tue with Maine seeing the best
potential for meaningful snow.  Brisk winds making conditions more
hazardous will likely taper off after Tue.  Shortwave energy
immediately upstream may promote a period of lake effect snow,
most likely to the lee of Lake Erie/Lake Ontario in the Wed-Wed
night time frame.  The system heading into the Southwest around
midweek should produce a brief period of enhanced precipitation
with highest totals over favored terrain inland from the southern
California coast and then a lesser maximum over central Arizona.
High elevation snow is possible from the Sierra Nevada across the
ranges of the Great Basin to the central Rockies. Expect this
system to produce an area of mostly rain over and east of the
central-southern Plains into the Southeast late in the week.  Some
pockets of locally moderate-heavy rain are possible but with low
confidence in timing/location.  Late in the period guidance is
maintaining the signal for potentially heavy precipitation (rain
and higher elevation snow) along parts of the West Coast with
strongest focus over northern California and the Sierra Nevada.
So far the detail differences exhibited by the models/ensemble
means seem to suggest more uncertainty with the precise onset of
the event (a little slower in the operational models versus
ensemble means) rather than the event itself.

Chilly temperatures will persist over parts of the East into
Tue-Wed.  Expect the greatest anomalies over the Southeast/Florida
where some locations may be 10-15F below normal for max and/or min
readings.  Morning lows could drop to near freezing as far south
as far northern Florida.  Eastern U.S. temperatures should be
closer to normal late in the week but another frontal passage may
push highs back down to 5-10F below normal over the
Mid-Atlantic/Northeast next Sat.
 Expect the central U.S. to see
above normal temperatures on most days, more so for morning lows.
Warmest such anomalies should be over the northern Plains Tue-Wed
ahead of a cold front and across the southern tier Wed-Fri with
clouds/precip accompanying the system crossing the region.  Highs
about 5-10F below normal should persist over the Great Basin into
parts of Arizona/southern California while most of the West should
see near to above normal morning lows.

Rausch

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Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
200 AM EST Sun Dec 01 2019

Valid 12Z Wed Dec 04 2019 - 12Z Sun Dec 08 2019

  • Southern tier system to affect the Southwest midweek/Mississippi Valley and Southeast late week
  • More moisture to spread into the West late week/next weekend


Pattern Overview and Guidance/Predictability Assessment:

The large scale pattern that has tended to feature mean troughs
over the eastern Pacific and eastern U.S. with a mean ridge over
the west-central U.S. will begin to progress eastward by next
weekend as the upper trough amplifying over the eastern Pacific
during the latter half of the week moves into the lower 48--albeit
with some timing uncertainty.  Latest D+8 multi-day means have
been showing a prominent negative height anomaly center over the
North Pacific and/or Aleutians, with teleconnections supporting
some eastward progression of the positively tilted trough beyond
the end of the medium range period to an axis from near the Great
Lakes to the southern Rockies (with a modest hint of stream
separation).  At the same time a ridge should begin to build into
western North America with best positive height anomalies over
western Canada.

There should be two dominant weather producers this period:  a
leading southern tier system that should be just offshore southern
California as of early Wed and then become more suppressed with
time from the southern Plains into Southeast Thu-Fri as the energy
aloft becomes incorporated into progressive flow, and the
amplifying eastern Pacific trough which by next weekend should
progress into the western and possibly parts of the central U.S.
Meanwhile a couple northern stream shortwaves may achieve a
negative tilt by the time they reach the Northeast (one Wed-Thu
and another around Sat).  Most precipitation associated with these
shortwaves will likely be on the lighter side of the spectrum
though locally enhanced activity could be possible.

For the southern tier system a recent slower trend in the GFS has
aided guidance clustering in the 12Z/18Z cycle.  The new 00Z UKMET
has strayed slower than consensus but remaining solutions maintain
good continuity.  The most significant difference for the
larger-scale eastern Pacific into CONUS trough is timing, with the
GFS and ECMWF exhibiting their fast/slow tendencies respectively.
The 12Z CMC was close to the GFS and adjusted a little slower in
the new 00Z run.  By next weekend the GEFS/ECMWF means ultimately
support an intermediate timing for the upper trough, with a
positive tilt that appears to lead well into the
teleconnection-favored pattern expected to prevail soon after next
Sun.  Therefore the updated forecast trended 50-60 percent to the
ensemble means late in the period (with modest input of offsetting
operational runs) after reflecting the 12Z/18Z operational model
consensus early-mid period.  Potential for stream interaction
within the overall trough contributes to ongoing uncertainty
mid-late period.  The preferred blend helped to downplay lower
confidence detail uncertainties with northern stream shortwaves
affecting the northern tier/Northeast.  Consensus continues to
show that low pressure with the first shortwave will track well
off the Northeast coast while there has been somewhat of a weaker
trend with the surface wave associated with the trailing shortwave.


Weather Highlights/Threats:

The system tracking across the southern tier will first bring a
brief period of locally heavy rain (perhaps some snow at highest
elevations) over favored southern California terrain around
midweek with a lesser relative maximum likely over central
Arizona.  Also expect higher elevation snow from the Sierra Nevada
into the central Rockies.  Farther east guidance is starting to
show a somewhat more coherent signal for locally moderate
(possibly briefly heavy) rainfall over and just east of the lower
half of the Mississippi Valley late in the week before a cold
front approaching from the north suppresses the system.  With
continued uncertainty in the exact onset, expect moisture ahead of
the amplified Pacific upper trough to reach the West Coast states
late in the week and push eastward with time during the weekend.
Thus far guidance has been consistent in highlighting northern
California and Sierra Nevada for highest precipitation totals.
Lower but still meaningful activity is possible over the Pacific
Northwest.  Lake effect snow aided by the first shortwave crossing
the Northeast early in the period will tend to favor locations to
the lee of Lake Erie/Lake Ontario with lighter activity to the
northwest.  The next shortwave aloft and surface wave/frontal
system may produce some relatively light amounts across the
upper/eastern Great Lakes and New England.

The greatest temperature anomalies (up to 10-20F) during the
period will be of the warm variety and for morning lows in
particular--in association with the mid-late week southern tier
system, over the northern tier Wed-Thu ahead of a cold front
drifting south from Canada, and from the West into the Plains late
week/next weekend ahead of the large scale upper trough moving
from the Pacific into the West.  Meanwhile the Great Basin into
the Southwest should continue to see highs around 5-10F below
normal while the East Coast states will see temperatures vary with
frontal progression but on average tend to be somewhat below
normal.

Rausch

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Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
938 AM EST Mon Dec 02 2019

Valid 12Z Thu Dec 05 2019 - 12Z Mon Dec 09 2019

  • Southern tier system to affect the Lower Mississippi Valley/Southeast late week
  • More moisture to spread into the West late this week/weekend and the East by next Monday

Pattern Overview and Guidance/Predictability Assessment:
A progressive but fairly amplified flow pattern is being dictated
by a mid-level vortex across central Canada and a longwave trough
moving into the eastern Pacific.  Downstream, ridging builds near
the West Coast by early next week which acts to slow down the
progression of a trough moving through the Southwest.  The 00z-06z
guidance showed very good agreement into Saturday morning before
the 00z Canadian became more progressive with the trough moving
through the Southwest, which is not supported by the broader
pattern.  The pressures, 500 hPa heights, and wind grids were
driven by a compromise of the 00z ECMWF, 06z GFS, 00z Canadian,
00z UKMET, and continuity through Saturday morning before the
UKMET and Canadian are replaced by the 00z bias corrected NAEFS
mean and 00z ECMWF ensemble mean.  This led to some slowing of the
trough moving through the Southwest but otherwise maintained
reasonable continuity.  For weather, sky cover,
temperatures/extremes, and dew points, used a little bit of the
00z Canadian early.  For QPF, tried to tame the 12z NBM and 06z
in-house ensemble bias-corrected QPF for day 7 (next Monday) as
they looked way too wet considering the model spread and the lower
deterministic QPF maxima seen on the 06z GFS, 00z ECMWF, and even
the wet 00z Canadian.


Weather Highlights/Threats:
Expect relatively light southern Rockies precipitation to taper
off Thursday with the responsible southern tier system bringing a
brief period of locally moderate/isolated heavy rainfall to the
Lower Mississippi Valley and parts of the Southeast Thursday
night-Friday before being suppressed to the south.
 Moisture ahead
of the eastern Pacific system should reach the West Coast late
this week and then spread inland over the West through the
weekend.  Consistent signals from the guidance point to the
greatest rainfall/higher elevation snowfall totals focusing over
favored terrain in the Sierra Nevada and across northern
California, possibly extending into the extreme southwest corner
of Oregon.  The Pacific Northwest as well as windward terrain
across the northern two-thirds of the Interior West/Rockies will
see less extreme but still meaningful totals.  The upper trough
and leading wavy cold front should reach far enough eastward for
precipitation (mostly rain) to expand over the eastern half of the
country by next Mon.  Some activity may be locally heavy if the
orientation and timing of the upper trough allow for sufficient
input of low level Gulf moisture.
 Earlier in the period expect
one episode of lake effect snow to taper off to the lee of the
eastern Great Lakes on Thu followed by a modest event over the
northern/eastern Great Lakes and New England approximately
Thursday night-Friday night.

Locations near the East Coast should see near to moderately below
normal temperatures from late week into the weekend.  New England
may see readings as much as 10-15F below normal Saturday into
early Sunday.
 Clouds/precipitation associated with the upper
trough progressing inland from the Pacific will spread above
normal morning lows across the West into the weekend while
maintaining moderately below normal highs over the Great
Basin/Southwest.  Cool highs will expand into the Rockies by the
start of next week.  Meanwhile expect a pronounced warming trend
over the eastern half of the country is expected Sunday into
Monday, with a broad area of 10-20F+ warm anomalies for morning
lows and pockets of 10F+ or so positive anomalies for highs.

Roth/Rausch

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Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
158 AM EST Tue Dec 03 2019

Valid 12Z Fri Dec 06 2019 - 12Z Tue Dec 10 2019

  • Moisture spreading into the West late week-weekend with heaviest rain/snow over northern California/Sierra Nevada
  • Precipitation to increase in coverage and intensity over the East early next week

Pattern Overview and Guidance/Predictability Assessment:
Guidance continues to show that a narrow but very amplified ridge
aloft moving into the eastern Pacific/northwestern North America,
and eventually reaching the West Coast next week, will encourage a
deepening mean trough to develop over the central U.S. by next
Tue.  Meanwhile a deep upper low initially off the West Coast will
likely open up by the weekend with the surrounding trough moving
into the West.  Some of the eastern Pacific trough energy will
eventually interact with the amplifying Canadian troughing (which
will push a cold front into the northern tier during the weekend)
while the rest should lag behind across the southwestern states
into northern Mexico.

From the big-picture perspective there was better than average
agreement among most 12Z/18Z models and means through the period. 
The one questionable solution was the 12Z CMC which brought the
eastern Pacific trough into the lower 48 at a much faster pace
than consensus (still a bit fast in the new 00Z run but better
than the 12Z version).  Thus the updated forecast emphasized the
18Z GFS and 12Z ECMWF/UKMET for about the first half of the period
and then trended toward about even weight among the GFS/ECMWF and
their ensemble means.

Recent trends have favored slower progression of eastern Pacific
trough energy that moves into the southwestern U.S./northern
Mexico, with latest GEFS means/UKMET runs plus a slower adjustment
in the 00Z GFS adding to that theme.  Then later in the period
there has been a general slower trend with the wavy front that
reaches the eastern U.S.--which is hard to dispute considering the
amplifying evolution aloft.  Ongoing uncertainty over the timing
of incoming Pacific energy and typically low predictability for
important smaller scale details within the overall late-period
CONUS upper trough continue to temper confidence in frontal
specifics mid-late period.  Upstream expect some energy to
approach the eastern Pacific/West Coast ridge aloft toward Tue. 
Majority cluster of the GEFS/ECMWF means and GFS runs along with
the new 00Z CMC suggest that the 12Z ECMWF may be too quick to
bring the energy into the ridge.


Weather Highlights/Threats:

The eastern Pacific system moving into the West will spread a
broad shield of moisture across the region from late this week
through the weekend.  Guidance continues to highlight areas from
the extreme southwest corner of Oregon through northern California
and the Sierra Nevada for highest rainfall/high elevation snow
totals with multi-inch liquid totals likely over favored terrain. 
The Pacific Northwest and locations farther inland through the
Rockies will also see a period of rain/snow but with less extreme
amounts.  Much of the West should trend drier by the first part of
next week.  Meanwhile expect increasing coverage and intensity of
precipitation over the eastern half of the country from about Sun
night onward as a wavy cold front approaches.  Guidance currently
suggests enough low level Gulf inflow to produce at least some
areas of locally heavy rainfall.  Best probability for highest
totals extends from near the Tennessee Valley into the
southern-central Appalachians/upper Ohio Valley but confidence in
specifics is not yet very high
.  Snow potential should be confined
to northern areas including the Great Lakes and northern/interior
New England.  Finally, late this week a weakening wave near the
Gulf Coast will produce some light-moderate rain in its vicinity
as it weakens and descends into the Gulf while a vigorous
shortwave aloft and more modest surface low crossing the Great
Lakes/Northeast will produce areas of mostly light snow.

The system crossing the Northeast late this week will bring chilly
temperatures (highs 5-15º F below normal) to the East Coast for Sat.

 Farther westward, an area of above normal temperatures will make
its way across the country from west to east corresponding to
upper trough progression/amplification into the lower 48, followed
by a cooling trend.  One exception to the warmth over the West
early in the period will be modestly below normal highs over the
Great Basin/Southwest.  Greatest anomalies over the
central/eastern states from the weekend into early next week
should be for morning lows (plus 10-20º F and locally higher) while
warmest highs should be 10-15º F above normal.
 Coldest temperatures
versus normal by next Mon-Tue should settle into/near the Upper
Midwest and vicinity with some readings 10-20º F below normal.

Rausch

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Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
156 AM EST Wed Dec 04 2019

Valid 12Z Sat Dec 07 2019 - 12Z Wed Dec 11 2019

  • Moisture spreading through the West during the weekend with heaviest rain/snow over northern California/Sierra Nevada
  • Precipitation to increase in coverage and intensity over the East early next week
  • Much below normal temperatures settle into the northern Plains/Upper Midwest next week

Pattern Overview and Guidance/Predictability Assessment:

In most respects guidance is consistent and agreeable in showing
an evolution toward fairly deep upper troughing over the
central-eastern part of North America, downstream from a narrow
but very amplified upper ridge that should drift to the West Coast
by next Tue.
 The one notable trend in the guidance since a couple
days ago--first showing up in yesterday's 00Z cycle--has been
toward a slower and sharper ridge that allows for greater westward
extent of cyclonic flow over the continent.  As initial West Coast
energy and amplifying Canadian flow head eastward, expect a cold
front to drop south from Canada during the weekend and a wavy
front to cross the eastern half of the U.S. during the first half
of next week.
 Toward the end of the period next Wed expect some
approaching eastern Pacific shortwave energy to push into the
western ridge but with models/individual ensemble members
diverging quite rapidly for specifics.

As for guidance details, the CMC continues to be the fast extreme
with the initial West Coast trough starting fairly early in the
period so it was not used in the forecast beyond day 3 Sat.  The
UKMET has been on the amplified side with the shortwave as it
reaches the southwestern U.S. and northwestern Mexico.  The 12Z
version also had a central U.S. surface evolution that differed
from consensus by day 5 Mon--eliminating that run from the
forecast after day 4 Sun.  The new run looks better in that
regard.  The 12Z ECMWF was slowest to eject the West Coast trough
energy and multi-day trends in some guidance allowed for partial
inclusion of the ECMWF in the forecast.  Faster trend in the new
00Z ECMWF seems to validate the blend approach though.  By
mid-late period a combination of the 18Z GFS/12Z ECMWF and their
ensemble means represented the large scale pattern well while
toning down smaller scale details that have lower confidence
several days out in time.  Along the West Coast late in the
period, recent trends have favored slower arrival of incoming
Pacific energy but the sharpness and northern extent of the ridge
do favor some undercutting flow to extend into the West around the
end of the period.  An average of latest solutions (minus the CMC
that is again fast) appeared reasonable given the detail spread
and lower predictability existing at that time.


Weather Highlights/Threats:

The system beginning to affect the West Coast late in the week
will spread its rain and higher elevation snow across the West
during the weekend.  Expect highest totals with this event to be
focused over northern California/extreme southwest corner of
Oregon and the Sierra Nevada.  Favored terrain from the Pacific
Northwest to the northern/central Rockies will likely see
significant but less extreme amounts.  Southern parts of the West
may see some lighter activity with greater sensitivity to exact
details of flow aloft for which models are still attempting to
resolve.  The West will be relatively dry early next week while
some moisture may begin to reach the West Coast around midweek,
with low confidence in precipitation coverage/intensity.

The eastern half of the country should see a fairly broad area of
precipitation develop early next week ahead of a wavy cold front. 
Overall not a lot has changed with respect to forecast details
compared to 24 hours ago.
 There is still potential for enough low
level Gulf inflow to enhance rainfall over some areas but
important shortwave specifics aloft are small enough in scale to
lower predictability.  For the time being the most favored axis
for heaviest rainfall extends from the Tennessee Valley/southern
Appalachians northeastward across the northern Mid-Atlantic and
southern/eastern New England.
 Locations mainly in the northern
half of the Great Lakes should be cold enough to see snow from a
combination of the synoptic system and lake effect/enhancement. 
Some snow will also be possible over northern/interior New England.

East Coast states will see a chilly start to the weekend with
highs in the Northeast up to 10-15F below normal, followed by a
warming trend.  Warm flow ahead of the ejecting West Coast system
and upper trough amplifying into the central U.S. will bring well
above normal temperatures (though mainly just for morning lows
over the West) across the lower 48 from west to east during the
period.  Some areas may see a day or so with morning lows 10-25F
above normal between Sun and Tue while highs should be a little
more moderate, 10-15F above normal.  Cold Canadian air feeding
into the northern tier behind the front will bring readings down
to 10-25F below normal over the northern Plains/Upper Midwest
Mon-Wed.  

Rausch

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Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
159 AM EST Thu Dec 05 2019

Valid 12Z Sun Dec 08 2019 - 12Z Thu Dec 12 2019

  • Rain/snow spreading through the West this weekend will taper off early next week
  • Precipitation to increase over the East early next week but with uncertain specifics
  • Much below normal temperatures settle into the northern Plains/Upper Midwest early-mid week


Pattern Overview and Guidance/Predictability Assessment:
Latest guidance has been showing fairly good agreement with the
large scale flow but with some medium to smaller scale
discrepancies that have significant sensible weather implications.

 For the most part a blend approach (more operational model input
early and more ensemble means late) continues to offer the best
way to navigate forecast uncertainty.

From day 3 Sun through the first half of the upcoming week
guidance continues to show strong upper trough amplification over
central-eastern North America in response to a sharp but very
amplified ridge extending from the eastern Pacific/West Coast
north through northwestern North America and into the Arctic
.  As
the upper trough amplifies, expect a strong cold front dropping
south from the northern Plains to interact with southern U.S.
flow/central Plains low pressure, with strengthening low pressure
likely to track through the Great Lakes into eastern Canada

Mon-Tue while a trailing cold front reaches the eastern states. 
By days 6-7 Wed-Thu the eastern trough aloft should become more
shallow as eastern Pacific trough energy filters through the
longer term mean ridge near the West Coast (rebuilding by Thu).

One ongoing uncertainty in the forecast is the ultimate evolution
of trough energy near the West Coast as of early Sun, with
individual models and ensemble members continuing to exhibit wide
spread.
 Through the 12Z/18Z cycles CMC runs had been on the fast
side while ECMWF runs have tended to be slowest--with the 12Z
ECMWF mean starting to hint at a slower/more separated feature. 
GFS runs have been moderately progressive with the GEFS mean
somewhat slower--a trend adopted by the 00Z GFS.  The major
revelation in the new 00Z cycle thus far is a complete about-face
by the CMC to the slow scenario, but to add more confusion the new
00Z ECMWF has adjusted faster.  At the same time there are
timing/amplitude differences with northern stream flow.  These two
issues put together lead to significant discrepancies in
precipitation coverage and intensity/duration over the eastern
U.S., with low confidence in a specific solution.

Later in the period the issues with the initial western energy
still impact the forecast, as exhibited by the the 12Z ECMWF's
Gulf Coast system versus nearly all other guidance that has no
such system.
 However there are enough 12Z ECMWF members that
suggest at least a more subdued feature (whether from the initial
energy or the second bundle of energy that passes through the West
Coast ridge later in the period) to suggest some southern tier
precipitation potential.  One other question mark late in the
period is the orientation of northern tier U.S./southern Canada
flow and influence on temperatures over/near the northern Plains. 
Teleconnections relative to the most prominent positive height
anomaly centers in latest D+8 multi-day means (near the West Coast
and over the North Atlantic) favor a pattern more like the
ECMWF/CMC and their means that would lead to a warming trend
versus the GFS/GEFS mean that keep cold air entrenched over the
northern Plains.  For now went with an intermediate approach given
the distant time frame and the occasional tendency for some
guidance to displace cold air too quickly.

The early to middle part of the period used various components of
the 18Z GFS/GEFS mean, 12Z and 00Z/04 ECMWF, and 12Z UKMET.  The
low-confidence Gulf Coast low in the 12Z ECMWF required phasing it
out of the blend late in the period.  By day 7 Thu the forecast
leaned about 2/3 to the GEFS/ECMWF means with a very slight tilt
in the ECMWF/ECMWF mean direction.


Weather Highlights/Threats:
Expect areas of rain and higher elevation snow over much of the
West on Sun though with a drying trend already in progress over
parts of the Pacific Northwest.  Best focus will be over favored
terrain but with amounts beginning to trend lighter than what are
expected during the late short-range period.  Much of the West
will see drier conditions early next week.  Shortwave energy
reaching the West Coast by Tue-Wed and then flow around the
periphery of a rebuilding Pacific ridge should tend to focus rain
and high elevation snow over the Pacific Northwest and possibly
the northern Rockies by mid-late week.

The forecast for precipitation specifics over the eastern U.S. has
become even more uncertain than in the past couple days.
 There
are still signals for heavy rainfall potential over some locations
within a broad area from the South northeastward into the
Mid-Atlantic and southern/eastern New England.  However latest
guidance has been too diverse and variable with separate important
features aloft to have much confidence in determining where
highest precipitation totals will be and their magnitude.
 Best
potential for snow remains over the Upper Midwest/Upper Great
Lakes which could see a combination of synoptic snow and lake
effect/enhancement, with lesser probabilities extending into New
England.  Lake effect snow should spread across the remainder of
the Great Lakes Tue onward as colder air moves into the region.

During the first half of the period the warm sector ahead of the
front advancing toward the East will lead to a day or so of well
above normal temperatures.  Some areas may see plus 15-25F
anomalies for morning lows while plus 5-15F anomalies for highs
should be common.
 Behind the front expect cold air feeding into
the northern tier to bring readings down to as much as 15-25F
below normal to parts of the northern Plains/Upper Midwest from
Mon onward.  In modified form this cold air will continue
southward/eastward with the East seeing temperatures generally
5-15F below normal by Wed-Thu.

Rausch

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Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
200 AM EST Fri Dec 06 2019

Valid 12Z (7 AM) Mon Dec 09 2019 - 12Z (7 AM) Fri Dec 13 2019

  • Heavy rain potential for Mid-South to Northeast early next week
  • Well below normal temperatures over the northern Plains/Upper Midwest early-mid week


Pattern Overview and Guidance/Predictability Assessment:

The pattern aloft by early-mid next week will highlight a closed
vortex over Hudson Bay as troughing/cyclonic flow rotates
underneath from a chilled and unsettled north-central to
northeastern U.S. This is supported by an amplified upstream ridge
aloft on the North American West Coast. A southern stream flow
meanwhile features shortwave trough ejection from the Southwest
Monday to the Southern Plains Tuesday that then phases with a more
dominant northern stream. Recent GFS/ECMWF runs are trending to a
compromise amplitude and progression solution that is becoming
increasingly consistent with their latest ensembles.

A composite 00 UTC model/ensemble solution still also suggests
that gradual rounding of the West Coast ridge later next week
would let increased eastern Pacific impulse energy/moisture work
inland from the Northwest/Rockies to the Plains. This may also set
the tone for robust east-central U.S. upper trough amplification
to keep an eye on heading into next weekend.   


Weather Highlights/Threats:

Over the West...a building West Coast upper ridge should keep most
of the West dry through early next week beyond some lingering
Rockies snows in northwest flow. The pattern should become more
wet and elevation snowy for especially the Pacific Northwest, but
also inland to the Rockies as shortwave energies round the
northern part of the ridge mid-later week.

There is a strong signal for heavy rainfall over a broad area from
the Mid-South to the northern Mid-Atlantic and southern/eastern
New England Monday/Monday night with frontal approach and support
aloft.
Locally heavy snow is possible in the comma head of this
low over the Upper Midwest Sunday night through Monday with
potential Lake Superior enhancement. Lake effect snow should
spread across the remainder of the Great Lakes Monday into Tuesday
in the wake of the low which is particularly cold. Polar plunges
are expected behind the cold front with temperature anomalies of
15 to 25 degrees below normal centered over the Upper Midwest with
reinforcing shots of cold air.

Schichtel

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Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
221 AM EST Sat Dec 07 2019

Valid 12Z (7 AM) Tue Dec 10 2019 - 12Z (7 AM) Sat Dec 14 2019

  • Much below normal temperatures for the Northern Plains/Midwest/Great Lakes


Pattern Overview and Guidance/Predictability Assessment:

A southern stream flow associated with an amplifying Southwest
upper trough early week still offers some uncertainty in terms of
magnitude and ejection timing for the U.S. southern tier Tuesday
into Thursday, but guidance clustering has improved to bolster
forecast confidence.

Overtop, a cold Hudson Bay upper vortex lingers next week. A
notable upper trough rounding underneath amplifies to the central
then eastern lower 48 states Tuesday/Wednesday, with less
amplified troughing/cyclonic flow for much of the rest of week. An
upstream ridge amplifies up the North American West Coast early
week before being weakening midweek by eastern Pacific shortwaves
that cut inland from the Northwest to the snowy Rockies. A favored
and reasonably well clustered GFS/ECMWF and GEFS/ECMWF ensemble
solution suggests  this energy may subsequently act to amplify
central to southern/eastern U.S. upper troughing. This may lead to
organized cyclogenesis to focus moisture and lift to produce a
risk of heavy rainfall across the Southeast.
Subsequent system
deepening, northward lifting, and interaction with receding lower
atmospheric cold air damming over the East heading into next
weekend may offer an emerging wintry snow/ice threat to the north
to monitor.   


Weather Highlights/Threats:

There remains a strong signal for a swath of moderate rainfall
ahead of a main cold front from the lower MS Valley to the
Appalachians/Mid-Atlantic and southern/eastern New England
Tuesday.
Activity will shift offshore midweek. Frontal waves and a
dynamic upper trough/jet also supports some enhanced snows on the
cooled northwest periphery of the precipitation shield from
Appalachians to the Northeast. Lake effect snow should also persist
across the Great Lakes in the wake of the cold front which is
particularly cold. Polar plunges are expected behind the cold
front offer temperature anomalies of 15 to 30 degrees below normal
centered over the Upper Midwest with reinforcing shots of cold
air.

Over the West...a building West Coast upper ridge should keep most
of the West dry through early next week beyond some Rockies snows.
The pattern should become more wet and elevation snowy for
especially the Pacific Northwest, but also inland to the Rockies
as shortwave energies round the northern part of the ridge later
week.

Schichtel

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Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
200 AM EST Mon Dec 09 2019

Valid 12Z Thu Dec 12 2019 - 12Z Mon Dec 16 2019

  • Heavy Southeast Rains to lift northward over a Wintry/Cold Air Dammed East
  • Wet flow from the Pacific Northwest to the Snowy North-Central Great Basin/Rockies

Pattern Overview and Guidance/Predictability Assessment:

The cold Hudson Bay mean upper vortex will linger this week. Flow
underneath over the lower 48 will become less amplified, but
remain quite active and unsettled/stormy this week as a long
series of northern and southern stream systems traverse overall
mean cyclonic flow.

The WPC medium range product suite was mainly derived from a blend
of reasonably compatible GEFS/ECMWF ensemble means. This is amid
rapidly growing forecast spread that includes significant system
and stream phasing uncertainties despite a similar larger scale
pattern evolution. This maintains good WPC continuity vs more
run-run inconsistent model solutions.


Weather Highlights/Threats:

Models still offer varied low scenarios from the Gulf of Mexico to
the Southeast then up the Eastern Seaboard into next weekend.

Forecast spread has been considerable, especially with recent
outlier GFS runs, but seems to trending a bit better with the 00
UTC guidance with respect to the timing/stream phasing,
cyclogenesis potential and eventual system tracks. This pattern
seems likely to focus moisture and lift to produce a risk of heavy
rainfall across the Southeast.
Subsequent system lift up through
the East Coast and interaction with a slow to recede lower
atmospheric cold air damming offers an interior snow/ice threat to
monitor. As is often the case, the degree of phasing/interaction
between the two initially separate streams and baroclinic
zone/coastal setup will be determining factors of eventual
evolution and any potential for any deeper coastal storm
developments and threats.

Out West, the pattern will become wet across the Pacific Northwest
into later this week as Pacific shortwave energies/height falls
punch onshore. Moderate to heavier snows will develop with system
progressions inland, especially for favored terrain of the
north-central Great Basin/Rockies later week through the weekend.

Schichtel

 

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Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
1234 AM EST Tue Dec 10 2019

Valid 12Z Fri Dec 13 2019 - 12Z Tue Dec 17 2019

  • Eastern Seaboard heavy rain threat Friday/Saturday and lead wintry precipitation risk for the interior Mid-Atlantic/Appalachians & Northeast
  • Wet Pacific Northwest to snowy N-Central Great Basin/Rockies flow into the weekend leads to Deep South low/rainfall development early next week


Pattern Overview and Guidance/Predictability Assessment:

A cold upper vortex will reform and persist over Hudson Bay this
period.
Over the lower 48, an active and complex pattern remains
as several streams laden with a series of impulses interact. Model
and ensemble solutions have become better clustered, but forecast
confidence remains somewhat tempered by recent run to run
continuity issues and flow complexity. Opted for a composite
GFS/ECMWF and GEFS/ECMWF ensemble mean solution blended with the
National Blend of Models as a basis for the WPC medium range
product suite.


Weather Highlights/Threats:

Southern stream shortwave progression and subsequent northern
stream flow interaction should support low development and track
from the Gulf of Mexico/Gulf Coast to the Southeast then up the
East Coast Friday/Saturday. This will bring heavy rain up the East
Coast mainly from the Southeast to the Appalachians, Mid-Atlantic
then Northeast. System interaction with lower atmospheric cold air
damming offers an interior snow/ice threat to monitor
. The degree
of phasing between two initially separate streams and baroclinic
zone/coastal details will be determining factors of eventual
evolution and potential coastal system threat.

The pattern will also become wet upstream over the Pacific
Northwest into late week as Pacific shortwave energies punch
onshore. Moderate to heavy snows will develop as the system
penetrates inland, especially over favored north-central Great
Basin/Rockies terrain through the weekend. By next Monday into
Tuesday, the exiting wave will eject and then interact with Gulf
moisture to spawn a rainfall focusing low pressure/frontal system
forecast to move over the Deep South, then potentially again up
the East Coast.

Schichtel

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hazards_d3_7_contours191210.thumb.png.1294900285d4fdd7bd3139ebbc29b3de.png

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Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
159 AM EST Wed Dec 11 2019

Valid 12Z Sat Dec 14 2019 - 12Z Wed Dec 18 2019

  • Eastern U.S. heavy rain and interior wintry weather threat into Saturday and again Monday/Tuesday
  • Heavy mountain snows possible this this weekend from Northwest to Rockies

Pattern Overview and Guidance/Predictability Assessment:

An active and complex pattern should persist through much of the
upcoming medium range period (Saturday-Wednesday) as
northern/southern stream shortwaves interact
. As one main lead
shortwave/trough exits the Northeast this weekend, another dives
south and eastward from the eastern Pacific into the West, moving
into the South-Central U.S. by early next week, then to the East.
Through the period, there is fairly good agreement in the large
scale pattern, though forecast confidence remains somewhat
tempered due to run to run inconsistencies with the more complex
details.

The WPC medium range product suite was primarily derived from a
composite blend of reasonably well clustered model and ensemble
forecasts and the National Blend of Models Saturday into Monday.
Increased weighting significantly to the more run to run
consistent GEFS/ECENS ensemble means into Days 6/7 amid growing
forecast spread. Even so, the overall active pattern seems to
offer above normal predictability and decent continuity.


Weather Highlights/Threats:

Southern stream shortwave progression and subsequent northern
stream flow interaction will support low development and track up
the East Coast into Saturday. This will bring heavy rain from the
Appalachians/Mid-Atlantic through the Northeast. System
interaction with lead/receding lower atmospheric cold air damming
and wrap-back/post-frontal flow offers an interior heavy snow/ice
threat and Lake effect snows.
The degree of phasing between two
initially separate streams and baroclinic zone/coastal details
will be determining factors of eventual evolution and potential
coastal system threat.

The pattern will also become wet upstream over the Pacific
Northwest/California into Saturday as Pacific shortwave energies
punch onshore. Moderate to heavy snows will develop as the system
penetrates inland, especially over favored north-central Great
Basin/Rockies terrain through the weekend. By next Monday into
Tuesday, the exiting shortwave will eject and then interact with
Gulf moisture to spawn a rainfall focusing low pressure/frontal
system forecast to move over the Southern Plains and Deep South,
then with some added uncertainty up the East Coast.
This scenario
also sets the stage for a threat of heavy snow/ice on the northern
periphery of the precipitation shield from the S-Central Plains to
the Mid-MS/OH Valleys then Northeast.

Schichtel

Links

hazards_d3_7_contours191211.thumb.png.113bbf8b3afce61dc77d29a66a2c9cd4.png

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