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

[Brice]

Yeah, if we see sunshine peeking out , that will be a definite concern just how unstable the atmosphere might be.

 

[tennessee storm chaser]

If a HIGH RISK was needed, which is very slim, which ingredients would be needed to be stronger or more defined?

This has definitely got high risk potential for sure

 

[Equus]

Thermo/lapse rates always seem to be the big questions with events here. Have to really wait for obs to see what the warm sector is doing. If we get a slight EML as some models wanna show, and obs run higher than modeled (as always) high could be on the table IF things don't trend down. They're certainly far more conservative handing them out in the last several years though; MDT is a high enough outlook to draw alarm.

 

[Brice]

I agree, a moderate risk is more reasonable in my opinion.

 

[Kory]

I don't think lapse rates will be an issue. Those 8.0-8.5 C/KM along the coast are indicative of a pretty good cap, so no big convective blowup upstream to muck lapse rates...

 

[jmills]

High risk or not will have to be a Day 1 decision. However, this has high risk potential just based off of the synoptic setup.

Edit: Clarity

 

[Richardjacks]

18z NAM so far is trending slightly slower once again, if this keeps up, this will be a Sunday night threat for Alabama...gotta wonder since the Euro has slowed a bit too.

 

[Kory]

18z NAM so far is trending slightly slower once again, if this keeps up, this will be a Sunday night threat for Alabama...gotta wonder since the Euro has slowed a bit too.

End result looks about the same.

 

[Richardjacks]

18 NAM shows a well defined warm front moving past I20/59 around 2/3pm in Alabama, what follows after the warm front is VERY concerning. It looks like it is trying to show some junk convection over central Alabama, but it doesn't match up with what you usually see with an expanding warm sector in April. WOW, I was hoping this was going to have a different result with the slowing. JUST WOW lots of PDS soundings.

 

[Kory]

18 NAM shows a well defined warm front moving past I20/59 around 2/3pm in Alabama, what follows after the warm front is VERY concerning. It looks like it is trying to show some junk convection over central Alabama, but it doesn't match up with what you usually see with an expanding warm sector in April. WOW, I was hoping this was going to have a different result with the slowing. JUST WOW lots of PDS soundings.

I don't think that's junk convection...that's supercellular convection.

 

[Taylor Campbell]

30% hatched tornado probs are easily achievable with this setup and quite possibly higher. The models are looking apocalyptical! I can’t imagine what something like the HRRR, RAP, NAM 3km and higher resolution models will show. This is a big dog or should I say big bear. Woof woof. Roar roar. Prepare for chaos!

 

[Richardjacks]

I don't think that's junk convection...that's supercellular convection.

I wondered that... the problem is that the NAM is not a CAM

 

[Richardjacks]

I will say that the current outlooks are probably going to be have to be extended northward to the Tenn/AL border. The entire state of Alabama could be in play...especially the NW 3/4's.

 

[Isaac Williams]

I will say that the current outlooks are probably going to be have to be extended northward to the Tenn/AL border. The entire state of Alabama could be in play...especially the NW 3/4's.

I'd say this is a near certainty unless something massively changes in the evening guidance.

 

[TornadoFan]

When is the last time the SPC issued a Day 3 moderate risk?

 

[Equus]

There was a day three MDT for the May 20 outbreak last year I think but I can't remember many other times recently

 

[warneagle]

There have been 18 total Day 3 MDTs. 5/20/19 was the only one since 2014.

 

[Richardjacks]

BMX:



.LONG TERM...
/Updated at 0400 PM CDT Thu Apr 09 2020/

Sunday through Wednesday.

/Regional severe weather event probable on Sunday, with concern
for potential high-impact scenario. Persons should review their
severe weather safety plan & have multiple ways to stay up-to-date/

Early Sunday morning, a warm front should be aligned near the Gulf
coastline, tied to an upstream surface low. Aloft, an upper low
turned potent shortwave trough will be approaching with an axis that
steadily trends from positive to slightly negative. The wind field
will also be intensifying. The progression of this setup, through
Sunday, will result in (1) a steady northward advection of higher
theta-e/instability values during the afternoon & evening (2) an
upper-level divergence pattern & ~100 knot H5 jet streak atop a low-
level jet maxing ~60-70 knots (3) large, curved hodographs with a
high amount of shear through the 1km/3km levels. Related parameters
& composite indices support not only an organized severe threat,
but also occurrences of significant severe. All hazards will
result from deep, fast-moving convection.

Sunday morning, showers & elevated thunderstorms are expected in
the vicinity of the northward-moving warm front. This activity
could continue to affect our northern-tier counties to around
midday. Thereafter, to the south of the warm front/in the warm
sector during the afternoon, point soundings indicate some
presence of an elevated mixed layer, which would tend to result in
a precipitation lull in areas. In turn, as southerly advection
continues, SB-CAPE is modeled to exceed 1,000 J/kg. These are deep
profiles and are especially concerning given the background
shear. The initial question is how far north this unstable air
mass gets through the day. Guidance has had a westward/slower
trend regarding the arrival of the system, which would tend to
provide a longer window for destabilization to extend farther
north. This leads into the second question of warm sector
convection. Guidance is showing evidence of early height falls
across western Alabama as stronger flow also overspreads the
state. We`re not within range of the CAMs, but perhaps some
isolated activity could get going given enough lift. If that were
to happen, rapid growth into supercell structure/severe-caliber
convection should result with a threat of tornadoes, damaging
winds, and large hail. This part of the event isn`t certain, but
certainly something to consider.

Into the evening, even stronger dynamics & forcing arrive.
Supercells & convective lines, having blossomed to our west, will
yield a threat of tornadoes, damaging winds, and large hail as
they move across Central Alabama. Very strong low-level shear,
moderate instability (maintained by low-level warm advection), and
the fast forward motion of the storms brings concern of not only
a strong tornado threat, but also that an instance of a long-track
tornado could occur as well. Heavy rain will push rain totals
into the 2-3 inch range, locally higher, though flooding doesn`t
appear to be a mentionable threat at this time.

If current model trends hold, the `enhanced` risk area may need
to be expanded northward. Another thing to watch will be whether
or not new model runs continue the trend of slowing this system`s
arrival & if instability values manage to trend any higher.

 

[MichelleH]

This makes me sick to look at. Valid 10 pm Sunday night.

 

[Kory]

This makes me sick to look at. Valid 10 pm Sunday night.

And that is after we’ve had confluence bands of supercells out ahead of the dry line. It’s going to be a long afternoon and evening on Sunday.