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Good morning!

Nothing going on this morning for us, so let's go ahead and look at Sunday/Monday at our severe weather potential.

Overnight the Storm Prediction Center has upped the risk for everyone in the southeast. For being 3 days out, this is not looking rosy.

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Day 3 Convective Outlook  
NWS Storm Prediction Center Norman OK
0244 AM CDT Fri Apr 10 2020

   Valid 121200Z - 131200Z

   ...THERE IS A MODERATE RISK OF SEVERE THUNDERSTORMS FOR
   CENTRAL/NORTHERN LOUISIANA... SOUTHEAST ARKANSAS... MISSISSIPPI... WESTERN/CENTRAL ALABAMA...

   ...SUMMARY...
An outbreak of severe thunderstorms appears likely Sunday into Sunday night, with the greatest threat expected from Louisiana east-northeastward through much of the Southeast and Tennessee Valley. Strong, long-tracked tornadoes and potentially widespread damaging wind are possible.

   ...Synopsis...
The ejecting shortwave trough initially over the southern High Plains Sunday morning is forecast to move quickly eastward and then northeastward through the period, as it moves around the periphery of a deepening longwave trough over the central CONUS. As this occurs, a surface low will move eastward to the Mississippi Valley by Sunday afternoon, and then rapidly deepen and move northeastward into the lower Great Lakes by Monday morning. Very strong mass response will draw rich low-level moisture northward into portions of the Southeast. 

   ...East Texas into the Southeast...
Ingredients for a potential severe thunderstorm outbreak still appear likely to come together Sunday. Moderate to locally strong destabilization in conjunction with an 80-100 kt midlevel jet and 50-60 kt low-level jet will result in a very favorable environment for organized convection. Ongoing storms across east TX Sunday morning will likely spread northeastward with time and become increasingly surface based as they encounter rapidly increasing low-level moisture, with additional development possible further south near the lower MS River valley by late morning. As the downstream airmass heats and destabilizes, long-track supercells may evolve out of the morning convection and track northeastward into  portions of MS/AL, with a corresponding risk of strong tornadoes, large hail, and damaging wind gusts. Some upscale growth is possible with time, which would result in a corresponding widespread damaging wind risk, given extremely strong wind fields. Some threat of damaging wind and a few tornadoes will likely reach portions of the Carolinas by 12Z Monday. 

 

And there is no reason to let a severe weather outbreak get in the way of other hazards

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Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
443 AM EDT Fri Apr 10 2020
 
Day 3
Valid 12Z Sun Apr 12 2020 - 12Z Mon Apr 13 2020 

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE TENNESSEE VALLEY TO PARTS OF THE SOUTHERN APPALACHIANS ON SUNDAY AND SUNDAY NIGHT...

Low pressure will form and deepen considerably during the day on Sunday as a shortwave ejects out of the Southwest United States and takes on a negative tilt by Sunday evening.  This will set up conditions favorable for intense rainfall rates and heavy to potentially excessive rainfall amounts over portions of the Tennessee Valley eastward into the Southern Appalachians.  

At the start of the period on Sunday morning, best inflow from the Gulf will be focusing over portions of Louisiana while the parent low pressure system will lagging over the western high plains.  By early Sunday evening, the mid level shortwave will have made its way into the southern/central Mississippi Valley.  Surface pressures fall in response to the approaching shortwave and by the coupling of a 130 kt to 150 kt upper level subtropical jet and 110 to 125 kt polar jet whose axis should extend from the eastern Great Lakes into New York/New England...leaving broadly 
difluent/divergent flow over the Tennessee Valley
.  H85 flow accelerates to between 55 kts and 70 kts from Alabama and Mississippi to the southern Ohio Valley... with maximum precipitable water values increasing to between 1.75 inches and 2 inches (some 2.5 to 3.5 standard deviations from climatology) and Integrated Vapor Transport values near at or above 1500.  

This should set the stage for cells to be highly efficient rainfall producers with some rainfall rates exceeding 1.5 inches per hour from parts of Alabama northward into central and eastern Tennessee.  The dynamic nature of the system suggests that the ECMWF may end up being to slow to propagate convection eastward... so the ECMWF was not given much consideration.  Despite the steady progression of cells, some rainfall amounts of 3 to 6 inches will be possible given the intense rainfall rates and the potential for training of cells or repeat thunderstorms.  Once the heavy rainfall reaches the Appalachians, the threat of flash flooding will be aided by low level flow being directed into and focused by the terrain.
 

 

The analogs show a significant precipitation event

AVGPC48_gfs215F072.png.6b992424173905afe7142547d07f1a95.png

 

This is all serious stuff folks, not beating around the bush here. Here's the STP or Significant Tornado Parameter at 2 PM Sunday. Notice the scale goes to 10. This model only goes out 60 hours so this is the last frame available right now. 

nam-nest-conus-se-sig_tor-6714400.thumb.png.d9a63fb23df6d8ce9543cf982027a7b0.png

The other parameters are equally threatening:

Supercell Composite

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A multiple ingredient, composite index that includes effective storm-relative helicity (ESRH, based on Bunkers right supercell motion), most unstable parcel CAPE (muCAPE) and convective inhibition (muCIN), and effective bulk wind difference (EBWD). Each ingredient is normalized to supercell "threshold" values, and larger values of SCP denote greater "overlap" in the three supercell ingredients. Only positive values of SCP are displayed, which correspond to environments favoring right-moving (cyclonic) supercells.

nam-nest-conus-se-supercell_comp-6714400.thumb.png.f9c221e36df93985d8e95a31e40393c4.png

K-Index

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The K index is a measure of thunderstorm potential based on the vertical temperature lapse rate, and the amount and vertical extent of low-level moisture in the atmosphere.

nam-nest-conus-se-kindex-6714400.thumb.png.8d0f71b98b48390ba9c9489a406fb93b.png

 

Surface Helicity

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SRH (Storm Relative Helicity) is a measure of the potential for cyclonic updraft rotation in right-moving supercells, and is calculated for the lowest 1-km and 3-km layers above ground level. There is no clear threshold value for SRH when forecasting supercells, since the formation of supercells appears to be related more strongly to the deeper layer vertical shear. Larger values of 0-3-km SRH (greater than 250 m2 s-2) and 0-1-km SRH (greater than 100 m2 s-2), however, do suggest an increased threat of tornadoes with supercells. For SRH, larger values are generally better, but there are no clear thresholds between non-tornadic and significant tornadic supercells.

nam-nest-conus-se-hlcy_0-1000-6714400.thumb.png.4b1e499a09b37e6e89ebc9e995810d63.png

 

0-6 km Wind Shear

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The surface through 6-km above ground level shear vector denotes the change in wind throughout this height. Thunderstorms tend to become more organized and persistent as vertical shear increases. Supercells are commonly associated with vertical shear values of 35-40 knots and greater through this depth.

nam-nest-conus-se-shear_0v6000m-6714400.thumb.png.0dd61d7ce1927711a4f867710e0ea275.png

 

There are a couple of saving graces for us, but whether they can hold on long enough to minimize the severe potential, we aren't sure, but I know we would all be grateful. In order for thunderstorms to grow, they need food, and that food comes in the form CAPE. At the hour depicted in the image, you can see we are lacking that energy. Granted this is 2 PM so over the next day or two we're going to watch this. Other models show the CAPE increasing right before the storms get here, but nothing like the really high amounts you see in this image. 

Quote

SBCAPE (Surface-Based Convective Available Potential Energy) is a measure of instability in the troposphere. This value represents the total amount of potential energy available to a parcel of air originating at the surface and being lifted to its level of free convection (LFC). No parcel entrainment is considered. The CAPE and CIN calculations use the virtual temperature correction.

CIN (Convective INhibition) represents the "negative" area on a sounding that must be overcome before storm initiation can occur.

nam-nest-conus-se-cape-6714400.thumb.png.b49004f547888b9381f095c420cdabc9.png

 

Looking at dewpoints, you can see that there looks like a little wedge holding on across NE Georgia, and that can be a positive for some folks as the lower temps and dewpoints limit the thunderstorm intensity. 
nam-nest-conus-se-dew2m_f-6714400.thumb.png.8b99f37fb293a48656bf2c363b3ea0e7.png

 

These are the thoughts from the Atlanta NWS office:

Quote

Positioning of the strongest instability will track along a warm front which should move northward through the forecast area on Sunday morning. It's along this frontal boundary where the strongest combination of instability and

  • low-level helicity values will come together with 0-3km EHI values reaching up to 2-3 for the forecast area and over 5 in areas further west.
  • Along the front 0-1km SRH values will be in excess of 300 m2/s2, 

forecast hodographs showing nearly all of this helicity being focused into a streamwise component, which could be very favorable for strong, organized supercells with long-track tornadoes. Thankfully, the best combination of these parameters will likely be across MS/AL on Sunday afternoon but these storms have potential to track eastward into our forecast area through that time and it could still be a very favorable tornadic supercell environment with high Supercell Composite and SigTor values. By Sunday Night, these storms will grow upscale and move into Georgia
and the Carolinas resulting in a widespread wind threat with a 60-70kt jet at 850 mb
.
Furthermore, PWAT values over 2 inches will be focused along this line of convection which may result in several hours of heavy rainfall for the forecast area Sunday afternoon through the overnight. Forecast rainfall values are around 2-3 inches for North Georgia, but it is very likely to see localized higher amounts, due to training convective storms in several locations. As a result there is also a Moderate Risk for excessive rainfall leading to potential flash flooding for far North Georgia through this event. With the shortwave ejecting quickly the line of storms will sweep through and clear the area by Monday morning.

 

I went and look at the top 15 analogs for this setup, and here is what that looks like. This image shows the % of the 15 analogs that have a severe weather report within 100 km of a given point, and that is pretty much all 15.  PRALLC01_gfs215F072.png.7f356829390ddd3f2698b574f088922d.png

 

This is the top analog from March 2, 2007 and this shows how that worked out. 

2007030206_024_ptsvr.png.2f4214dabb1b60c9f128537cb04aaff3.png

 

These next two are the Supercell Composite and SigTor. Again, these are averages of the 15 analogs.

SCPmeangfs215F072.png.2ae81c81a107c12ec89236803a0fb54e.pngSTPmeangfs215F072.png.8adce8bdcbaec68c5cba05f70459c28e.png

 

That's enough for now, things will change between now and then and hopefully not for the worse. 

I hope everyone has a great Friday!

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nam-nest-conus-se-dew2m_f-6714400.png

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 Am I reading this right we are not in the bulls eye..looks bad for the panhandle of Florida.

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6 minutes ago, MNW said:

 Am I reading this right we are not in the bulls eye..Correct.  looks bad for the panhandle of Florida. Looks can be deceiving. Central MS/AL will get the brunt.

 

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In the post, you see references to the H85 or 850 mb winds, and I shared some pictures of that yesterday, but I'll share again to refresh you. The height of any of the levels I post, whether it's 925, 850, 700, 500, etc, vary depending on the pressures. Lower pressures mean lower heights, high pressures higher heights. On this map you'll see some numbers, as an example, look at southern Alabama and you'll see a 147.  That is the height in decameters (the top of the image tells you the height unit of measure). To get meters just add a zero on the and of the number and that 147 becomes 1470 meters or about 4823 feet. The reason I bring this up is that as a strong low pressure system approaches, those heights lower. The 1470 could be 1320 I see over Iowa (as an example) and that would mean the 850 MB level is now 500 feet lower. That also means the WINDS are 500 feet lower. That matters, especially in a thunderstorm as downdrafts can bring those winds down toward the surface. I've been showing these winds for several days now because I think they are significant enough to cause some major damage, and one reason the Atlanta office highlighted that threat. 

These winds will be pushing into our area as the system progresses east.

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We can look out further on the Euro and see the 925 mb winds around 2 AM Monday morning. Keep in mind, 925 mb is pretty low and in this case it is getting close to 2300 feet, and MANY areas of north Georgia are 2300 feet or higher. The max winds are 74 knots or 85 mph. If you even take 20% off of that, you're still looking at winds in the 50-65 mph range at 2300 feet and above. 

ecmwf-deterministic-se-z925_speed-6757600.thumb.png.a1cce51c1fc678f9ad9e6afce784f553.png

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We definitely looked wedged in right now which is hopefully our saving grace. But if long track super cells are established to our west I worry about their tendency to track well into less favorable conditions before starting to succumb to them. 

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Posted (edited)

I am hoping and praying Low-ish CAPE values, and the Wedge, save us, but am also ready, just in case

Edited by Asperman1
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27 minutes ago, KingOfTheMountains said:

We definitely looked wedged in right now which is hopefully our saving grace. But if long track super cells are established to our west I worry about their tendency to track well into less favorable conditions before starting to succumb to them. 

If with that in mind I have read discussion to where they could merge and form a QLCS and with all of that wind aloft no thanks.

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Steve with 3 days out have the models come to an agreement on the location of the Low pressure, strength and movement.  In other words at 1st the thought was for the southern part of the Gulf coast states, and now the threat continues to march North.  Just wondering because at the rate the low keeps getting pushed North we here in deep s east Georgia look to be dealing mainly with a linear line as far as severe?  Looks to me the further south the low is, the more effects in the whole south instead of basically Dixie Alley( esp the strong tornadoes)?

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Just now, RickyD said:

Steve with 3 days out have the models come to an agreement on the location of the Low pressure, strength and movement.  In other words at 1st the thought was for the southern part of the Gulf coast states, and now the threat continues to march North.  Just wondering because at the rate the low keeps getting pushed North we here in deep s east Georgia look to be dealing mainly with a linear line as far as severe?  Looks to me the further south the low is, the more effects in the whole south instead of basically Dixie Alley( esp the strong tornadoes)?

That is always a possibility, but this close in time it won't move far. 

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Posted (edited)

*takes a rag and lysol and tries to scrub the orange off Georgia*

Well, it works for the virus 🤷‍♀️

*Note: I'm taking this very seriously. Sleeping downstairs on Sunday night and remaining very aware.

Edited by audpogue
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9 minutes ago, NorthGeorgiaWX said:

That is always a possibility, but this close in time it won't move far. 

So it is now not a question will it happen and where it will happen, but the details of it all as far as timing 

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13 minutes ago, audpogue said:

*takes a rag and lysol and tries to scrub the orange off Georgia*

Well, it works for the virus 🤷‍♀️

*Note: I'm taking this very seriously. Sleeping downstairs on Sunday night and remaining very aware.

Needed that laugh.😂😂😂

I’ve already told my kids we’re having a living room sleepover Sunday night (their rooms are the upstairs ones)... be safe!!!

 

 

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17 minutes ago, Asperman1 said:

Any positives to share about the situation?

Yes. It’s Friday. Nothing to worry about right now as it’s a ways off 

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2 minutes ago, LoveSnow said:

Yes. It’s Friday. Nothing to worry about right now as it’s a ways off 

True 🙂

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I love the Birmingham AFD's, always very detailed. Plus you need to read these when severe weather is around, because our severe weather is coming from them. Anything bolded are their words, just my bold. 🙂 

Quote

/// SIGNIFICANT SEVERE WEATHER EVENT ON SUNDAY ///

Saturday night:

A lead shortwave will move over the area Saturday night, along
with a mid-level warm front, while the surface warm front remains
along the coast. Strong moist isentropic lift aloft and upper-
level diffluence will allow the atmosphere to become saturated
shortly after midnight, with widespread showers expected.
Additionally, the NAM indicates around 1000 J/kg of MUCAPE, so
expect some elevated thunderstorms as well with small hail
possible due to steep lapse rates aloft. Isolated severe hail may
be possible after midnight Saturday night as well, but will keep
the HWO focused on Sunday`s severe event for now.

Sunday/Sunday night:

The upper low currently over southern California will round the
base of an amplifying trough over the Plains Sunday and Sunday
night, taking on a negative tilt at upper levels and a slight
negative tilt at mid-levels by Sunday night
. Very strong wind
fields will be in place with this system, e.g. jet maxes of
155 kts at 250mb, 115 kts at 500mb, 75 kts at 700mb, 65 kts at
850mb, and 50 kts at 925mb
.
In response to strong upper-level
forcing, a surface low will lift northeastward from the Southern
Plains to the Great Lakes by Monday morning, rapidly deepening
from 992 mb to 978mb during this time frame and resulting in
backed surface winds across Central Alabama
. A weak shortwave
moves across the area Sunday morning as the surface warm front
lifts northward. Most activity during the morning will probably be
elevated, but will have to watch for any surface-based
development along the warm front
. Current thinking is the morning
activity will not inhibit afternoon destabilization, as
warmer/drier air will move in at 700mb associated with an EML by
midday, allowing for daytime heating.

A very volatile environment will be in place across Mississippi
and Alabama during the afternoon and evening hours, as rich Gulf
moisture lifting northward results in CAPE values around 1500-2500
J/kg
. Dew points/instability have trended upward in our northern
counties from previous forecasts. 80-90 kts of 0-6km bulk shear
will be present, with 400-600 m2/s2 of 0-1km SRH and 200-400 m2/s2
of 0-500m SRH and large curved low- level hodographs
.
Southwesterly shear vectors in the warm sector becoming more
southerly close to the front and will be supportive of both an
intense QLCS and supercells developing ahead of the line. The
strong 0-6km and 0-8km shear will help these supercells stay ahead
of the line, supporting the threat of strong (possibly violent)
long-track fast-moving tornadoes. The QLCS will also be capable of
producing widespread damaging winds and embedded tornadoes with
embedded supercells, which could be strong as well, not just brief
spin- ups.

As always, the severe threat will come down to mesoscale details
as the event gets closer and difficult to forecast complex storm
interactions, which ultimately will determine how significant the
threat ends up being. But the parameter space is certainly
supportive of a moderate risk. The threat appears slightly higher
west of I-65 than east of I-65, however want to emphasize that all
of Central Alabama needs to take this seriously no matter the
color of the risk area.

 

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2 minutes ago, NorthGeorgiaWX said:

It's a beautiful day today! 🙂 

Take care as you're driving around, though. The wind's strong enough to topple trees and limbs...and large vehicles!

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14 minutes ago, Tanith said:

Take care as you're driving around, though. The wind's strong enough to topple trees and limbs...and large vehicles!

It is a little breezy today! 🙂

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