Severe WX April 11th-13th, 2020 Severe Weather Threat

[Kory]

And CIPS has 4/27/11 as the #2 analog. A very high chance we don't come close to that (15 violent tornadoes is obscene). But if we get 3 or 4, that is a massive outbreak. Don't get caught up in hyperbole, but take this threat seriously. We have been lucky (sans Lee County) since 4/27/11 and tornadoes are a fact of life in Alabama in April.

 

[warneagle]

Odd that 12/16/00 didn't show up at all given how frequently it's shown up in the SARS analogs. I'm sure it's just down to differences in how SARS and CIPS work and it's probably way over my head.

 

[Richardjacks]

This is an excellent first, more focused outlook. It encapsulates the most favorable parameter spacing for the most significant risk, but it also just shows how much AREA this event will cover.

Yes and agrees with my idea that generally Jan to Bham will be sweet spot with maximized instability and low level shear. This is a frightning setup.

 

[Kory]

06z Euro continues to show supercells firing off in the warm sector Sunday afternoon across MS and West TN. Those head ENE/NE through the evening into AL and Central TN. I can count several distinct QPF streaks through MS/TN/AL.

 

[Kory]

The Sunday morning rain prior to the warm front blasting through looks like a nonissue. No organized cold pool. Deep layer flow WSW/SW doesn’t favor back building of convection. It looks very minimal on the Euro and NAM.

 

[brianc33710]

day 3 moderate


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James Spann said the SPC rarely issues a Moderate Risk for day 3. However, on 27 April 2011, temperatures climed into the 80s. Right now, the Bham NWS has bumped up Sundays his from 73 to 76. I guess if the warm front does get thru here fast enough and any am weather issues don't cool the air, we might hit 80. However, I truly believe 27 April 2011 was a once in a generational/meteorologist career event. I could see maybe an 08 April 1998 type of event with multiple strong EF-2-3 tornadoes and an EF-4/5 as well But I can't imagine having another setup where multiple violent tornadoes ravage the S Central/SE. I hope I'm right at least.

 

[Kory]

Outbreaks don’t work on a timer. Just because Alabama’s are spaced roughly 40 years apart doesn’t preclude an outbreak of that magnitude Sunday or next year or in 5 years. Nature doesn’t sit there with a calendar checking off days to the next one. This looks like a dangerous synoptic set up, but mesoscale stuff will be determined Saturday night/Sunday morning.

As I have said, models are running too cool. BMX will continue to adjust highs upward. MEX guidance had BMX at 79 for a high Sunday. I bet we get 80/low 80s in some of the western counties of Alabama and into MS Sunday afternoon.

I’m not sure it matters if we have 2000 J/KG or 3000 J/KG CAPE.

 

[skelly]

You know more than me for sure but as it stands it looks that highs will depend on the warm front. Without it we won’t be in the 80’s.

 

[jmills]

You know more than me for sure but as it stands it looks that highs will depend on the warm front. Without it we won’t be in the 80’s.

Right, if you’re north of the warm front in general you’re not going to have a severe threat.

The question a couple of days ago was how far north the warm front would get, but that’s pretty much been answered. With the kind of LLJ we’ll have and if we keep a broken cloud deck, then 80 is easily achievable.

 

[MichelleH]

I don't think whether an area hits 80's temps is an issue. If you are in the warm sector Sunday, you're looking to have a bad day, whether it's 78 degrees or 82.

 

[Kory]

Oh my. The NAM. There are perfect violent tornado soundings along and ahead of the band of supercells it develops in MS and pushes into AL. Holy cow.

 

[Brice]

Any idea of how quick this potential squall line with embedded supercells will move?

 

[gangstonc]

Any idea of how quick this potential squall line with embedded supercells will move?

I’ve seen mean storm motions as high as 70-80 mph.

 

[rolltide_130]

Any idea of how quick this potential squall line with embedded supercells will move?

This is not going to be a squall line with embedded supercells unless you're referencing the early morning stuff (which does not look significant at this time). This will be multiple discrete to semi-discrete supercells over an open warm sector with potentially more than one round of severe weather.

As for timing, anytime between noon and midnight needs to be watched closely.

 

[NorthGaWeather]

Outbreaks don’t work on a timer. Just because Alabama’s are spaced roughly 40 years apart doesn’t preclude an outbreak of that magnitude Sunday or next year or in 5 years. Nature doesn’t sit there with a calendar checking off days to the next one. This looks like a dangerous synoptic set up, but mesoscale stuff will be determined Saturday night/Sunday morning.

Thank you!

 

[Brice]

This is not going to be a squall line with embedded supercells unless you're referencing the early morning stuff (which does not look significant at this time). This will be multiple discrete to semi-discrete supercells over an open warm sector with potentially more than one round of severe weather.

As for timing, anytime between noon and midnight needs to be watched closely.

Okay, thanks!

 

[bwalk]

The Sunday morning rain prior to the warm front blasting through looks like a non-issue. No organized cold pool. Deep layer flow WSW/SW doesn’t favor back building of convection. It looks very minimal on the Euro and NAM.

There are still more than a few folks thinking there is a good chance the warm sector could be washed out by morning convection. Warm sectors can recover from pre-event convection (4/27/11 being the most famous exception).

You make a solid argument for pre-event convection not being a threat to warm sector quality. The role of crapvection will need to be watched but for now, looks to be minimized by the factors you mention.

Morning EML is a factor that should inhibit warm sector convection and give the severe dynamics time to organize.

 

[Equus]

BMX AFD


The upper low currently over southern California will round the base of an amplifying trough over the Plains Sunday and Sundayn 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 Plainsto the Great Lakes by Monday morning, rapidly deepening from992 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 Jkg. Dew points/instability have trended upward in our northern counties from previous forecasts. 80-90 kts of 0-6km bulk shear willbe present, with 400-600 m2/s2 of 0-1km SRH and 200-400 m2/s2 of0-500m 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 ofthe 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 asthe 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 westof 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.

Besides the threat of severe storms, the strong LLJ and deepening surface low will be supportive of strong gradient winds reaching wind advisory criteria. PWATs around 2 inches will also be supportive of localized flooding, however this potential will be limited by fast storm motions, relatively dry conditions over the past couple weeks, and springtime vegetation.

 

[Jetstream]

Looks like I will be cleaning out the storm shelter in the garage that we bought from Valley a few years back.So, either that or spot since I have been house bound since early March. Hope everyone formulates a plan to keep your family safe!

 

[Kory]

Yeah, when the GFS is showing widespread 2000-3000 J/KG CAPE, this is going to be a problem. That warm sector is surging north after unorganized morning convection across Northern MS and Northern AL and TN....leaving all kinds of thermal boundaries...

The cap is about to blow here...