Severe WX April 22-24th, 2020 Severe Weather Threat (1 Viewer)


Equus

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Saragossa, AL
Aaaaand here we go again. Big ol enhanced hatched for east Dixie on the D3 likely to go Moderate on the D2...

day3otlk_0730.gif

DAY 3 CONVECTIVE OUTLOOK
NWS STORM PREDICTION CENTER NORMAN OK
0228 AM CDT TUE APR 21 2020

VALID 231200Z - 241200Z


...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS MAINLY OVER
ALABAMA...GEORGIA...AND NORTHERN FLORIDA INCLUDING THE PANHANDLE...

..SUMMARY


SIGNIFICANT SEVERE STORMS ARE EXPECTED OVER PARTS OF THE SOUTHEAST
ON THURSDAY, INCLUDING THE POTENTIAL FOR STRONG TORNADOES.

..SYNOPSIS


A POSITIVE-TILT UPPER TROUGH WITH 70-80 KT MIDLEVEL SPEED MAX WILL
MOVE FROM THE LOWER MS VALLEY THURSDAY MORNING TOWARD THE
APPALACHIANS BY 00Z, WITH A 50-60 KT LOW-LEVEL JET CORE SHIFTING
FROM AL TO THE MID ATLANTIC DURING THE DAY.

AT THE SURFACE, LOW PRESSURE WILL MOVE FROM NORTHEAST AR INTO KY
THROUGH 00Z, AND TOWARD THE DELMARVA BY 12Z FRIDAY. A DRYLINE WILL
EXTEND SOUTHWESTWARD FROM THE LOW, AND WILL MOVE ACROSS AL, MS AND
LA DURING THE PEAK HEATING HOURS. EAST OF THIS BOUNDARY, 65-70 F
DEWPOINTS WILL BE COMMON, BENEATH COOLING TEMPERATURES ALOFT.

MODELS DIFFER WITH HOW QUICKLY THE DRYLINE WILL MOVE, AND EARLY DAY
STORMS MAY DICTATE WHERE THE QUASI-STATIONARY WARM FRONT SETS UP
LATITUDINALLY. REGARDLESS, THE WARM SECTOR WILL BECOME VERY
UNSTABLE, WITH SHEAR FAVORING TORNADIC SUPERCELLS ALONG THE DRYLINE
AND WARM FRONT/OUTFLOW BOUNDARY. AT THIS TIME, IT APPEARS THE BULK
OF THE ACTIVITY WILL OCCUR OVER AL, GA, AND THE FL PANHANDLE INTO
NORTHERN FL.

THE NAM SOLUTION, IF TAKEN LITERALLY, SUGGESTS STRONG AND PERHAPS
VIOLENT TORNADOES WILL BE POSSIBLE OVER PARTS OF MS, AL, AND GA.

HOWEVER, THIS SOLUTION IS MUCH SLOWER WITH THE DRYLINE THAN OTHER
MODELS, AND MAY BE OVERDONE WITH INSTABILITY AND LOW-LEVEL SHEAR.
USING THE PREFERRED ECWMF SOLUTION, THE DRYLINE WILL EXTEND FROM
SOUTHERN MS INTO CENTRAL AL THURSDAY AFTERNOON, WITH EARLY DAY
STORMS REINFORCING A WARM FRONT ROUGHLY FROM BIRMINGHAM AL TO
ATLANTA GA. OUTFLOW COULD POTENTIALLY PUSH THE WARM FRONT FARTHER
SOUTH. SUPERCELLS WILL BE POSSIBLE BOTH ALONG THE DRYLINE AND ALONG
THE WARM FRONT, WITH STRONG TORNADOES POSSIBLE. THE STRONGEST LIFT
WILL OCCUR EARLY IN THE PERIOD RELATED TO WARM ADVECTION, AND THIS
WILL LIFT NORTHEAST DURING THE DAY. HOWEVER, SUBTLE LIFT ALONG THE
DRYLINE WILL FAVOR SUPERCELLS, AS OPPOSED TO A LINEAR STORM MODE.
MUCAPE OF 2000-3000 J/KG IS POSSIBLE, WITH MIDLEVEL LAPSE RATES OF
6.5 TO 7.0 C/KM AIDING UPDRAFT VIGOR. REGARDLESS OF PREFERRED MODEL,
THIS EVENT WILL BE ADJUSTED WITH TIME AS PREDICTABILITY INCREASES,
WITH CATEGORICAL UPGRADES POSSIBLE.


ELSEWHERE, LIFT ASSOCIATED WITH A LOW-AMPLITUDE SHORTWAVE TROUGH
MOVING INTO THE CENTRAL PLAINS MAY SUPPORT ISOLATED STRONG STORMS
ALONG A COLD FRONT OVER NE, KS AND OK WHERE WEAK INSTABILITY WILL
DEVELOP BENEATH A NORTHWEST FLOW REGIME.

..JEWELL.. 04/21/2020



So yeah. Surprisingly strong wording for a D3.

Large area of severe on Wednesday (Slight thus far) from Texas to Mississippi on Wednesday; main show in AL into GA/FL Panhandle on Thursday the 23rd into early Friday perhaps. Models had downtrended but now seem to be slowing the system down a bit putting E AL into play during peak heating. The NAM is really aggressive with this, no doubt overdoing it, with 4,000 j/kg SBCAPE across SE AL on Thursday evening and a dryline erupting plains-eqsue supercells in a favorable parameter space for intense tornadoes; Euro while trending up is showing more reasonable but still increasingly impressive values. GFS not convinced yet but seems to also be slowing the system down, and still shows a tornado risk along the Gulf Coast region from MS east to GA. Previously questionable wind fields also seem to be trending towards a more significant event. One to watch... yet again.

Not that I trust its aggressive solution at all unless GFS/Euro come more into line but just for what it's worth, here's the NAM over-enthusiastically throwing violent tornadoes across the Clanton-Prattville corridor Thursday afternoon
nam_2020042100_069_32.71--86.3`.png
 
Last edited:

Richardjacks

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Hoover, Al
The NAM has considerably overestimated the motherly extent of the warm sector on the last few systems.
True, but I am not ready to write this one off quite yet for as far north as Birmingham. Here is why- the surface low is further north than the last one--if it trends further south, then yes, it is overamped. Also, it is late April now, with gulf water temps above normal, it will take less time for the rich air to move further north. If we start to see the surface low being pulled south, and rainfall increasing along the boundary, then the threat more limited to just southern areas...if memory serves me correct, the NAM never showed numbers like this for Bham with the last event....the threat is a function of speed of this system...will be watching for trends.
 

mattdanielwx

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Birmingham, AL
True, but I am not ready to write this one off quite yet for as far north as Birmingham. Here is why- the surface low is further north than the last one--if it trends further south, then yes, it is overamped. Also, it is late April now, with gulf water temps above normal, it will take less time for the rich air to move further north. If we start to see the surface low being pulled south, and rainfall increasing along the boundary, then the threat more limited to just southern areas...if memory serves me correct, the NAM never showed numbers like this for Bham with the last event....the threat is a function of speed of this system...will be watching for trends.
Especially since the European model is showing a slower system and higher instability values too. Just a lot of uncertainty this far out. Hopefully know more by this evening and especially tomorrow. Lot will depend on what happens early Thursday morning.
 

Kory

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Tuscaloosa, Alabama
The Euro is interesting as overnight convection pushes into AL by early to mid-morning. We have rapid warm air advection again and those elevated thunderstorms go surface-based as we progress through the morning.

I find that the more likely solution versus the NAM which shows rapid destabilization behind the morning round. Not that we can't get rapid destabilization, but we've seen complexes mess up low-level winds as they push across the Southern portion of the state. And you just get a bunch of hailers with the second round.
 

rolltide_130

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Harvest, Alabama
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Of course, the NWS Birmingham graphic looks significantly different. Public confusion ensues.

They've been on the money lately and I'm good friends with one of their convective forecasters. He's not sold on the threat and is thinking the NAM is out to lunch with its solution. Could change but BMX is skeptical and they're traditionally money with these types of events.
 

rolltide_130

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I was a bit surprised to see the phrase "the NAM solution, if taken literally" in an SPC forecast given the NAM's uh, less than stellar record recently.
I've come to find out over the years that SPC forecasters aren't inherently better than anyone with a meteorology degree or even just a good foundational understanding of how forecasting works. They just happen to have the job with the fancy title next to their name...
 

Equus

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Saragossa, AL
Yeah I'm not really sure why they even put that section in the D3, with NAM being orders of magnitude more aggressive than more reliable/consistent models. Hopefully trends away from that extreme solution
 

warneagle

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I've come to find out over the years that SPC forecasters aren't inherently better than anyone with a meteorology degree or even just a good foundational understanding of how forecasting works. They just happen to have the job with the fancy title next to their name...
I feel like they've tended to go with the most aggressive model solutions in recent years, whether it's the NAM (usually) at this range or the HRRR in the short range. I get wanting to be better safe than sorry, but it's led to a lot of over-forecasting.
 

Kory

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The SPC is far too reactionary. They use the previous system and their outlook performance as a gauge on how to forecast the next event.

They could’ve easily went high risk and it verified for the Easter outbreak. They didn’t. They saw this past Sunday’s system and thought it would “over perform” in the same manner. It didn’t quite but they were going to go high risk until they were effectively shamed into not doing that by local WFOs.
 

warneagle

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The SPC is far too reactionary. They use the previous system and their outlook performance as a gauge on how to forecast the next event.

They could’ve easily went high risk and it verified for the Easter outbreak. They didn’t. They saw this past Sunday’s system and thought it would “over perform” in the same manner. It didn’t quite but they were going to go high risk until they were effectively shamed into not doing that by local WFOs.
Yeah, I can understand wanting to learn from your past mistakes, but there's way too much recency bias. When they miss on a forecast, they seem to assume that they're going to miss in the same way the next time. Or at least that's my thoroughly non-expert impression.
 

Equus

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Saragossa, AL
If the rumors of wanting to go high for Sunday were true, despite massive uncertainty and downtrending at issuance time, it's really alarming; hopefully the D2 will take into account the still less aggressive GFS and Euro trends
 

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