Severe WX April 19-20, 2020 Severe Weather Threat

[Justin Hindman]

Here we go SPCSWOD48 ALL
ACUS48 KWNS 160845
SPC AC 160845

Day 4-8 Convective Outlook
NWS Storm Prediction Center Norman OK
0345 AM CDT Thu Apr 16 2020

Valid 191200Z - 241200Z

...DISCUSSION...
Models have come into better agreement depicting a shortwave trough
moving quickly from TX Sunday/D4 morning to GA by 12Z Monday/D5. Low
pressure is forecast to develop over the Red River during the day
Sunday, shifting east toward the lower MS Valley by 00Z. Ahead of
the low, a warm front will lift north across the Gulf Coast states,
with upper 60s F dewpoints likely into central MS, AL, and GA, with
low 70s F along the coast. MUCAPE to at least 2000 J/kg is likely by
18Z from TX into AL, with strong westerly winds aloft and 500 mb
temperatures on the order of -10 to -12 C.

Storms are forecast to form relatively early over east TX, where the
environment will support large hail and damaging winds. Supercells
are possible initially, with an eventual MCS likely. A tornado or
two will be possible despite marginal low-level shear. Meanwhile to
the east, warm advection may support supercells well ahead of the
MCS across MS, AL, GA, producing a tornado or two along with hail.

The primary severe risk in terms of coverage will likely be an MCS
tied to the surface low as it quickly moves along the east-west
instability gradient across LA, MS, AL and into GA. Significant
convective feedback is present in the models, supporting the notion
of a well-defined MCS with damaging winds. While SRH will not be
very strong initially, it should increase after 00Z, with enhanced
wind and/or tornado potential. The corridor of maximum threat will
likely be adjusted in later outlooks.

For D5/Monday, there may be a small area of severe potential
remaining with this system over far eastern GA and the Carolinas
around 12Z. However, much will depend on how fast the system moves,
therefore predictability issues will preclude an area.

For the D6-D8 period, another shortwave trough will emerge out of
the southwestern U.S. with a narrow but robust moist axis developing
across the southern Plains. Quite a bit of model uncertainty exists
regarding the speed of this system, but severe outlooks will be
possible in later updates as the event nears.

 

[Taylor Campbell]

That’s an unstable warm sector. If the low were to deepen and the low level jet strengthen sooner that would help the threat.

 

[warneagle]

Still on the fringes of NAM range, but from looking at the global models, not nearly as much wind shear as Sunday. If those low-level winds were to back more, we would have a big problem though. Also looks like a model war shaping up over the placement of the warm front yet again.

 

[Kory]

NAM = Not A Model (you should use).

It was abysmal last system.

 

[Kory]

I disagree with the SPC's notion this will be fully an MCS threat. While I believe a complex of storms will ride along the warm front, south of it will pose a threat for some cellular development. Tricky situation with these westerly flow events that have complexes of storms racing from West to East is that timing is usually faster and that can certainly ruin some of the lower level wind fields. Case and point, the GFS is racing the SLP east with the convection. Is that purely feedback or is there some truth to that?

Euro brings that warm sector just north of 20/59 which is by far the more bullish. A lot hinges on the convection earlier in the day that will determine the northern spatial extent...where have we heard that before?

 

[mattdanielwx]

Definitely see potential for more instability with this system....but only if that warm front moves to the north. Shear is weaker compared to last Sunday, but still enough to support damaging winds and tornadoes. We shall see what happens.

 

[Kory]

There will be a sharp gradient with instability. The Saturday system sort of acts a primer getting low level flow going prior to Sunday and has potent instability lurking across Southern LA/Southern MS/Southern AL ready to come north as long as there is no large blocking mass of convection Sunday.

 

[mattdanielwx]

Euro and GFS showing a lot of moisture and rain falling during the morning and afternoon hours. Clouds and rain could keep the severe threat farther to the south. We just need to wait another 12-24 hours for our short range models to lock on to a better solution.

 

[Weatherphreak]

I know my area in South Huntsville got a good 3-4 inches of rain Easter so it'll be interesting to see how bad flooding gets if North Alabama gets another 3-4 inches. I had the worst flooding in my yard that I've had in 10+ years of living in my current location due to the short duration of the Easter event. The Tennessee River is already running a little high so another quick burst could cause issues.

 

[ARCC]

The NAM is scary for Sunday. Flagged 4/16/98 as a analog around BHM.

 

[Kory]

Euro looks like it really wants to get that warm sector pretty far into MS/AL by midday Sunday.

 

[Kory]

Euro looks like it really wants to get that warm sector pretty far into MS/AL by midday Sunday.

Oh yeah, it really escalated things for LA/MS/AL/GA for Sunday. Here we go again.

 

[andyhb]

Oh yeah, it really escalated things for LA/MS/AL/GA for Sunday. Here we go again.

And another next week (not this thread's focus I know).

 

[Bama Ravens]

Euro looks like it really wants to get that warm sector pretty far into MS/AL by midday Sunday.

 

[TCLwx]

Day 3 Outlook

 

[Kory]

The Euro continues to be the most potent solution with a 990s SLP skirting from Northern MS into TN. Much stronger SWerly LLJ enhances the low level shear as multiple waves of cellular convection move across MS/AL/GA. There continues to be a sharp cutoff with northerly extent, but the Euro and other shorter range guidance looks to bring that northerly cutoff to somewhere from Columbus > Jasper > Cullman > Gadsden.

 

[Bamamuscle]

The Euro continues to be the most potent solution with a 990s SLP skirting from Northern MS into TN. Much stronger SWerly LLJ enhances the low level shear as multiple waves of cellular convection move across MS/AL/GA. There continues to be a sharp cutoff with northerly extent, but the Euro and other shorter range guidance looks to bring that northerly cutoff to somewhere from Columbus > Jasper > Cullman > Gadsden.

US 278 corridor.. that’s pretty far N of the current enhanced region right?


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[Kory]

US 278 corridor.. that’s pretty far N of the current enhanced region right?


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If the Euro is correct, that risk might need to be expanded just a bit N.

 

[Kory]

We're going to have to see what models do (especially CAMs) in the coming few runs. There is no clarity given by the 00z and now 12z runs so far. Some guidance keeps the threat well south of 20/59. Others, encompass more northerly solutions.

 

[Equus]

At first honestly thought they had somehow accidently copied the D4 outlook from the first event into the D4-8 again. Much deja vu here