Monday, November 20

[NorthGeorgiaWX]

Good morning!

I hope everyone has been enjoying the nice fall weather that we've had in place for a week or two, as that will be coming to an end on Tuesday.

First, we will be under a Wind Advisory beginning at 7 pm this evening. 

 

Our chances for severe weather have increased, although it's still not going to be a major threat. Here's the thoughts from the Storm Prediction Center.

Quote

...Parts of AL/GA and the FL Panhandle...
   While the primary surface cyclone will continue moving away from the
   region on Tuesday, convection will likely persist through the day
   within a confluent regime along/ahead of the front across parts of
   AL and FL Panhandle, and eventually move into parts of GA. Generally
   weak instability (with MLCAPE generally less than 500 J/kg away from
   the immediate coast) may tend to limit storm intensity, but
   favorable moisture and deep-layer shear will continue to support the
   potential for at least transient supercell structures. 0-1 km SRH
   generally in the 150-250 m2/s2 range will support some tornado
   threat where sufficient destabilization can occur. 

   The threat may peak early in the day across south AL and the FL
   Panhandle, in association with any supercells that develop during
   the D1/Monday period and persist into Tuesday morning. A small
   Slight Risk has been included across this area, as a continuation of
   the threat from late on D1/Monday. Some threat for locally damaging
   gusts and/or a brief tornado may persist into the afternoon across
   parts of GA, in areas where modest diurnal heating can augment
   generally weak instability. 

 

 

As you can see, most of north Georgia lies outside of the areas with the greatest chance for severe weather, but that doesn't rule out severe for the northwest corner. 

Here are the thoughts from the Atlanta NWS office.

Quote

Given the scale and complexity of the breaking wave,
model and ensemble spread seems to be particularly
high with this system. There is the potential for a Low CAPE
(60Kts 10m-500mb) event, with current
GFS/ECMWF STP values of 0.5-2 fairly consistently over the last
several runs across western GA. So, here are a couple important
factors which will have a big impact on how Tuesday plays out.

1. First, the developing low looks to move to the Northeast (towards
   the Great Lakes), however significant uncertainty remains in just
   how far north the low will form, how deep it will get, how far north
   will the track be, etc. Should the low move further south, greater
   synoptic forcing may aid thunderstorm development across GA as well
   as increase moisture/heat advection.
2. Second, there remains uncertainty in the speed of the system. This
   is partly related to the first, however the current timeframe of
   cold front passage looks to be earlier on Tuesday before diurnal
   support can be established. Should the cold front propagate slower or
   back build, diurnal heating may allow for increased CAPE values ahead
   of the cold front and more vigorous thunderstorms. Even with the
   slower prorogation, CAPE values will likely remain below 500 J/Kg
   given the small warm sector. This will be the difference between
   thunderstorms or rain-showers.

Given the factors discussed above, the most likely outcome will be a
mix of scattered showers and thunderstorms, however we cannot rule
out some isolated severe.

The main hazard would be damaging winds
(given mixing occurs from the 50+kt 925mb LLJ) however 0-3km SRH
>250 m2/m2 may lead to shallow rotating thunderstorms capable of
isolated brief tornados. The greatest thunderstorm potential will be
across West GA (South of I-20) where MUCAPE is greatest aligned with
upper level support. Precipitation amounts will be generous (1-2")
with the greatest amounts across North GA. This may help to improve
but will not be enough to fix drought conditions currently across
GA.

Another potential hazard ahead of the front on Tuesday will be the
intense southerly flow. Current models seem fairly consistent in the
development of a robust LLJ near 900mb of 50+kts. Sfc winds will
likely be elevated as well with gusts well over 30kts and sustained
winds over 25kts across North and Northwest GA. This may necessitate
a Wind Advisory in future updates.
 

 

So, let's take a look at the models, and see for ourself how this might turn out.

It appears CAPE will not be a factor, as it's pretty much non-existent, at least across north Georgia, and that will be our saving grace in terms of severe storms. There isn't much time between the departing high and the advancing cold front, so we won't see very warm temperatures in the return flow air mass ahead of the front. That factor alone will reduce the chances for severe storms.

 

There are several other factors, that increase the chance for a rotating thunderstorm to develop, and that is wind shear and helicity. Here are the shear values. Wind shear is defined as a wind direction and/or speed change over a vertical or horizontal distance, and in this case, the winds are turning as you go up in altitude. Obviously, we will have very high amounts of shear.

 

Helicity is a measure of how the air twist as it increases in height. Think of a corkscrew and you have a better picture of helicity. Helicity is a function of shear, so generally the more shear you have, the more helicity you have. Certain ranges of values aid to determine storm strength and whether tornadoes might be possible.

(150-300 J/kg) - Possible supercell

(300-400 J/kg) - Supercells favorable

(400+ J/kg) - Tornadic possible
 

You can see from these images, tornadic activity may be possible. This is called a low CAPE/high shear environment, and it will be conducive for weak quick spin up tornadoes. 

 

This map shows the K Index, or the relative strength of the possible thunderstorms, and for north Georgia, we will probably be looking at storms slightly below the severe level, but with heavy rain.

 

I think our greatest threat will be damage from high winds, and here's a look at the potential wind gusts. The official wind readings are taken at 33 feet or 10 meters, and that is what this image shows. This is the reason for the wind advisory.

 

These are 925 mb winds, or roughly 2500 feet. This is in knots, and the bottom right of the image shows a max of 63 knots or 72 mph in this image. If there are any leaves left on the trees right now, they will be gone by Wednesday. You can see the wind vectors in the image that show those southeast winds in advance of the front.

 

It appears that the Weather Prediction Center and the local NWS offices are in agreement regarding the rainfall totals from this system, and some areas across north Georgia may see up to 2" of rainfall. That won't be enough to end the drought, but it will go a long way toward reducing it. 

 

We will cool off behind the front to more seasonal temperatures for Thanksgiving with pretty nice weather, but that chance for some much needed rain returns on Friday. These are the high and low temperature anomalies for Thanksgiving Day.

 

[NorthGeorgiaWX]

Climatology for tomorrow.

[NorthGeorgiaWX]

Busy Weather to Start the Week:

A strong upper level trough will surge through the region during
this period. In advance of the trough a warm front will lift north
through Georgia Tuesday morning, then a cold front will track across
the state Tuesday night. The primary weather concern with this
system will be gusty winds across north Georgia Monday night and
Tuesday morning. There is also be a Marginal Risk (Level 1 of 5) for
severe thunderstorms on Tuesday. Rainfall totals for this event
should range from 0.50 to 2.00 inches, with the higher amounts over
the north Georgia mountains.

Regarding the Potential for Strong Winds:

The approaching trough looks to be the most dynamic system of the
season so far. A strong low level jet (50 to 65 kt) will develop out
ahead of the trough tonight. While the full extent of this low level
jet won`t be felt at the surface, boundary layer mixing and terrain
effects should allow wind gusts in the 35 to 45 mph range to mix
down to the surface in northern Georgia. The strongest gusts will be
felt along the mountain ridgelines tonight. A Wind Advisory has been
issued for portions of north Georgia from 7 PM this evening through
1 PM Tuesday. Winds of this magnitude could blow down tree limbs or
result in isolated power outages.

Regarding the Marginal Risk for Severe Storms:

With the latest Day 2 Convective Outlook the SPC has extended the
Marginal Risk (Level 1 of 5) northward through Georgia and into the
Carolinas. Instability during the day on Tuesday remains at the low
end of modest (MUCAPE values peaking near 500 j/kg). However
hodographs suggest shear profiles favorable for supercells and the
HREF progs mean 0-1 km SRH in the 150-250 m^2/s^2 range. These
parameters are sufficient for a conditional risk for severe storms
with either wind gusts to 60 mph or a brief lower end tornado. The
potential for storms could increase if instability over achieves.

Regarding Rainfall Amounts and the Flood Risk with this System:

As noted above, rainfall totals of 0.50 to 2.00 inches are
anticipated in the region between tonight and Wednesday morning. The
WPC`s Day 2 Excessive Rainfall Outlook has north Georgia highlighted
in a Marginal Risk (Level 1 of 4) area. Given the current drought
conditions in north Georgia that have the local one hour flash flood
guidance hovering near 2.50 inches, and 3 hour flash flood guidance
at or above 3.75 inches, the overall flood risk appears very
conditional. Essentially we would need storms to train over a region
for a prolonged period or rain rates in excess of 2 inches per hour.
Neither appear overly likely, thus flooding is on the lower end of
our concerns list for this event.

[NorthGeorgiaWX]

Afternoon update:

------------------------

Key Messages:

•  - Elevated fire concerns across northern GA this afternoon into
  the overnight hours due to high winds.
• - Wind Advisory for portions of north GA tonight through
  tomorrow afternoon.
• - Slight Risk (2 out of 5) clips Columbus and SW Georgia.
  Marginal Risk (1 out of 5) is in place across all but far northern GA.

Forecast Overview:

Quite a dynamic system will be moving through the area today into
tomorrow. Broad 500 mb trough located from 4 corners region into the
Great Plains will experience some significant stretching as Rossby
wave break occurs to the north of it. The vortmax within the trough
is responsible for current showers across the area as WAA has
already commenced ahead of developing surface low. Embedded within
that wave break will be a TPV/PV anomaly diving quickly to the
south across the Great Plains. Altogether, this will cause trough
to have 3 separate shortwaves/vortmaxes within it:

1. one that will break to southwest over Mexico and become a player in long term forecast
2. one that will move northeast with attendant surface low and begin driving cold front towards
   CWA tonight through tomorrow
3. and one that will rotate towards the CWA tomorrow, driving a secondary push of moisture
   into CWA and providing some upper level support for severe threat during afternoon.

Cold front will slow and slowly creep to southwest tomorrow night, finally
clearing CWA by tomorrow morning. Complicating all this will be
the presence of the wedge/CAD, driven into the area by strong high
pressure located over the northeast. Broad area of rain and
embedded storms will move into CWA starting tonight. Secondary
push of moisture and storms will likely occur on Tuesday afternoon
as cold front progresses through the area, with some severe
weather possible ahead of that.

Wind Threat:

CAMs and global guidance are in pretty good agreement that winds
should stay up tonight at the surface thanks in part to pretty
strong sfc pressure gradient and mixing from aloft due to very
strong low level jet (LLJ). Winds at 925 mb are 40+ kts across many
models and are 50-60 kts at 850 mb as the developing surface low
passes to our north. Very strong 1035+ mb sfc high is located over
the northeast and sfc low passing to our north will continue to
deepen through the evening. These combined should drive winds across
northern GA of 15-25 mph sustained with gusts up to 45 mph.
Especially impacted may be the ridgelines, which will take the brunt
of the LLJ and could see even higher wind gusts. Wind Advisory
remains in effect until 1 PM tomorrow.

Fire Threat:

While RHs will remain relatively high overnight, north GA has
ongoing drought conditions that have impacted longer term 100 hour
and 1000 hr fuels in the region. With very strong winds forecasted
tonight, any fire that can establish itself will be dangerous. A
Fire Danger Statement has been issued for these areas through 05Z (1
AM EST). Please head any local statements regarding burns. Many of
the counties in this region have ongoing burn bans. Problems on
Tuesday afternoon should be more limited thanks to precip and the
front slowing as it moves through the area.

Severe Threat:

Models are doing the usual thing where they want to quickly remove
the Wedge/CAD on Tuesday as the secondary shortwave approaches the
CWA due to the strong WAA/LLJ. This is a challenging case, as the
forcing for the wedge will be disappearing as the sfc high across
the northeast begins to move out into the Canadian maritime and
we lose ageostrophic/barrier jet forcing. However, the surface
reflection of the wedge will often stick around for some time and
resist mixing out as the models always want to do... but, in this
case, there are some reasons to believe it could happen given very
strong LLJ. My forecast keeps the wedge hanging around through
the day Tuesday, but confines in to far NE GA in and near the
Athens area. The implications are that everywhere to the south
that remains ahead of the cold front will still be in the warm
sector, and open for severe potential.

In that warm sector, even accounting for the early eroding of the
wedge, dewpoints in the HREF surge well into the 60s across the
CWA and enough sunlight looks to break through to bring sfc temps
into the upper 60s and even 70s across central GA. This should be
more than enough to allow for surface based convection. HREF means
bring SBCAPE of 300+ J/kg through most of the warm sector, with
it getting higher the further south you go. Upper level support
will arrive in the form of the shortwave diving into Arklatex
region during day...though it will be somewhat removed from the
region in terms of the support it will actually provide. Still,
broad jet streak will be located overhead. Strong LLJ will provide
from some very strong speed shear in the 0-1 and 0-3 km, and
hodographs in the low level are pretty fish hooked as a result.
SRH values will be in the 300- 400 m2/s2 range. Put together, this
means that if you can get a decent updraft going, it could
definitely spin. The question remains as to how easy it will be to
get an updraft going, especially out ahead of the cold front that
could cut off the surface from said updraft. Current SPC outlook
reflects well the possibility that if something gets going, it
definitely could spin. An isolated tornado or isolated damaging
wind gust is definitely possible, and will say that there exists a
somewhat higher potential given the environment, especially if
things aren`t as worked over in the upper atmosphere from the
earlier morning convection/rain.

[NorthGeorgiaWX]

 

 

[Jeff9702]

What a nothing burger no thunder nothing. i get it was for the south but man i was looking forward to at least something in north ga

[NorthGeorgiaWX]

1 minute ago, Jeff9702 said:

What a nothing burger no thunder nothing. i get it was for the south but man i was looking forward to at least something in north ga

Both the HRRR and MAN bring a final line of thunderstorms between 1 pm and about 4 pm this is the line that could have severe weather, but if the warm front doesn't arrive in time, we won't see any.

[NorthGeorgiaWX]

I don't think that the warm front will make it in time, so that will reduce the chance for severe weather.

[NorthGeorgiaWX]

Wind gusts in the Smokies

 

 

 

[Weatherdude]

 

[TheJohnnyDee]

Humm? 🫣🤔

 

[Weatherdude]