Severe WX April 3rd-5th, 2023 (South, Southeast, Ohio Valley)

[tennessee storm chaser]

Yeah I’m not questioning the potential, obviously this event has a very high ceiling, I’m just surprised at the level of confidence they’re expressing at this point given the potential failure modes (capping, LCLs, maybe VBV) that are still evident on the most recent model runs.

Yeah , they better hope cap can hold , because if not , it’s going get sideways pretty quick

 

[warneagle]

Yeah , they better hope cap can hold , because if not , it’s going get sideways pretty quick

Yeah I mean obviously you have the worst-case scenario where the cap holds long enough to suppress the crapvection during the day but eventually erodes enough to allow supercells in an untapped environment, which certainly seems plausible at least based on what the forecast soundings from the NAM are showing (i.e. the cap breaks along the pseudo-dry line after 0z). It makes intuitive sense even if the models don’t completely agree.

 

[ColdFront]

I plucked this from AmericanWX. LOT is putting out some really detailed and easy to understand outlooks, here is there’s for tomorrow:

Main focus then turns to the potential for severe weather mainly late Tuesday afternoon and evening. As we`ve been noting, this setup appears to be much more "conditional" than last Friday`s for a number of reasons. First is that large scale forcing will be more subtle/weaker as the parent 500 mb low is currently forecast to be well west across western/central Nebraska and South Dakota by early Tuesday evening. Second is that capping will be playing more of a central role as an expansive EML plume characterized by extremely steep mid-level lapse rates and attendant warm nose around 800 mb will likely result in considerably MLCIN through much of the afternoon hours. Whether or not the aforementioned subtle large scale forcing is able to overcome this is one of the main questions yet to be answered and will mean the difference between few to no storms in the local area versus a potential significant severe episode in the region.



From a conceptual standpoint, the surface warm frontwill make slow, but gradual northward progress through the region on Tuesday as more notable pressure falls develop across Wisconsin and Iowa. Seems like the front`s forward progress will probably get held up a bit by the cool lake with it having a harder time making progress north of the Wisconsin state line during the evening. Wide open Gulf trajectories will allow an expansive region of 60s dewpoints to surge northward although forecast soundings continue to indicate the moist layer will be somewhat shallow. Still, seeing signs of higher dewpoints pooling near the warm front, possibly in response to mixing into higher mixing-ratio air just off the surface (with this in mind drizzle and pockets of fog appear possible Tuesday morning and even into the afternoon north of the front). Even the WRF-ARW which utilizes a more aggressive PBLscheme and tends to readily "mix-out" shallow moist layers holds onto low 60s dewpoints up to the I-88 corridor.



The big question here is whether the base of the relatively warm EML plume can be lifted and cooled sufficiently by only very modest large scale forcing for ascent with neutral or even positive mid-level heighttendencies noted over us in a narrow window during the late-afternoon and early-evening. The ECMWF and GFSboth have been consistently saying "yes" to this question as the nose of a 60-70 kt 500 mb jet maxpushes across central Illinois during the afternoon hours, but it`s easy to find other guidance which says "no", leaving too much convective inhibition in place for thunderstorm initiation. That said, am somewhat concerned by potential trends noted in hires guidance which seem to be headed more towards the ECMWF/GFS camp in eroding the capping layer just enough to result in explosive thunderstormdevelopment somewhere across eastern Iowa/northeast Missouri/west-central Illinois, and seems like more than half of the EPS members do so as well based on a cursory glance.



IF convection develops, it would have access to an highly volatile cocktail of high instability and truly eye-popping kinematic parameters with large, sweeping/looping hodographs driving very high effective SRH and, importantly, significant low-level storm-relative inflows. While a cap bust scenario is still on the table (one where inhibition remains too high and storms struggle to develop or don`t develop at all), it`s hard (and probably foolish) to ignore the consistency of the ECMWF, GFS, and now recent runs of the extended HRRR. Corridor of most focused severe weather potential would seem to be setting across roughly the northwest half of the CWA late Tuesday afternoon into the mid-late evening with all modes of severe weather possible, but contingent on robust convective development materializing in the first place. Given the presence of at least some capping, it`s also possible that only a few storms develop or become severe, but any of these would have the potential to drop sig tors given the parameter space. It`s also conceivable that initial severe thunderstorms start to weaken a bit during the evening as CIN increases with the loss of daytime heating and as they get close to or cross the warm front. We`ll highlight these two scenarios in our graphical messaging today to try to better explain the vast chasm that exists between outcomes.

 

[ashtonlemleywx]

Seems like models have slowed down this system and have it farther West Tuesday Night/Wed morning.

I think this opens the door for higher severe potential in Mississippi on Wednesday evening.

 

[cincywx]

the spc took the unusual but arguably needed step of adjusting the day 3

Day 3 Convective Outlook AMEND 1
NWS Storm Prediction Center Norman OK
0302 PM CDT Mon Apr 03 2023

Valid 051200Z - 061200Z

...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS WEDNESDAY FROM THE
GREAT LAKES REGION AND OHIO VALLEY SOUTHWESTWARD INTO THE LOWER
MISSISSIPPI VALLEY...

AMENDED FOR SLOWING UPPER TROUGH, WESTWARD EXPANSION OF SEVERE
THREAT

...SUMMARY...
Severe thunderstorms are possible over a large area from the Great
Lakes region southwestward to the Lower Mississippi Valley
Wednesday. This may include cells capable of producing tornadoes and
extensive wind damage, primarily north of the Ohio River, with other
severe storms expected into the lower Mississippi Valley.

This amendment is primary due to the slowing upper trough, and the
westward expansion of severe probabilities. Severe storms may be
ongoing as far west as the St. Louis area at 12Z Wednesday, with
favorable conditions remaining for tornadoes and damaging winds
expending mainly northeastward. Farther south, lower 70s F dewpoints
and heating ahead of a cold front is expected to support scattered
storms during the day into the lower MS Valley, where mainly hail
and damaging gusts will be possible. This westward demarcation of
the severe threat will likely be adjusted further incoming updates.

 

[brianc33710]

C AL has had several occasions with severe weather threats in recent years where marginally severe thunderstorms converted what mightve been a significant severe weather outbreak to instead substantial flash flooding. Also at least 2× in 2022 we had 3-4/5 threat levels due to models forecasting warm & humid air moving into C AL. Instead the 50 mph/80 kmph S inflow was warm but dry with 50-55F/10-12.5C dewpoints. We actually had wildfires because the winds downed numerous powerlines & the rain arrived hours later than forecast.

Either of these scenarios could lessen Tuesdays threat & both have shown as potential limiting threats in the MOD Risk bullseyes. But I was wrong when thinking the HIGH Risk was an overreaction Friday so I'll keep my fingers from typing my expected at least a substantial underperformance again & instead just hope for it.

 

[TH2002]

Me after seeing the current D2 outlook

 

[CheeselandSkies]

The amendment to the Day 3 outlook helps confirm my suspicion that with more favorable timing this could have been a Palm Sunday 1965 redux.

 

[jiharris0220]

While there are many failure modes with this system, the caveat that needs to be said here is that the cap is really the only substantial FM. The other modes like LCL height and VBV are much more minor.
And with the thing with VBV, people In my opinion put a bit to much stock into this being a tornado killer. While VBV can induce -helicity into cells, this really only causes supercells to become cyclic and produce short lived tornadoes. But this does in no way prevent them in becoming strong.
Again, the most major failure mode is the cap, if it breaks, expect a strong tornado or more. As I don’t think LFC and like I explained with VBV is going to be that much of an issue.

 

[andyhb]

I do not envy SPC and the local WFOs for tomorrow's threat. Conditional/moderate probability but potential high impact scenarios are the most difficult to forecast for. I think given the reluctance of the global models from backing down along with some CAM support, we should see a couple of supercells develop in the warm sector as that subtle shortwave induces cooling in the 850-500 mb layer tomorrow afternoon. HRRR and NAM/NAM 3 km have their biases with overmixing and undermixing, respectively, which will play a big role in getting robust convective initiation.

And that's ignoring the nocturnal threat. SPC mentioned the front acting like a dryline. Well, one of the higher end analogs for this setup – 3/12/2006 – did exactly that. Not saying that will occur, but there is reason to believe it won't be an "auto-linear" setup when the forcing arrives post-00z.

 

[ColdFront]

What I have found fascinating is on our analogs for tomorrow and last Friday, a lot of them are pre-2010s types, even early 2000s. It seems it’s been over a decade or more since we have had patterns like this.

 

[brianc33710]

Anyone else notice that the very tippy NW AL is 2/5 & the rest of the area is 1/5? Did Huntsville beg to have just a littlest bit of its CWA Slight lol ?

 

[MattPetrulli]

Extremely surprised with the 1730z moderate risk down south. They're the experts of course but there really isn't a strong convective signal at all down there. The parameters are there of course, but there is a weak convective signal. Honestly thought even the 10% hatch was a bit of a stretch but would've been fine. If convective signal down there increases, then obv moderate risk is justified. Def think more initiation along the warm sector is possible given some weak capping in some areas, but I think the only area that deserves a moderate atm is the northern/warm front area. Still a decent convective signal from CAMs though. Would like to see more confidence.

 

[andyhb]

Extremely surprised with the 1730z moderate risk down south. They're the experts of course but there really isn't a strong convective signal at all down there. The parameters are there of course, but there is a weak convective signal. Honestly thought even the 10% hatch was a bit of a stretch but would've been fine. If convective signal down there increases, then obv moderate risk is justified. Def think more initiation along the warm sector is possible given some weak capping in some areas, but I think the only area that deserves a moderate atm is the northern/warm front area. Still a decent convective signal from CAMs though. Would like to see more confidence.
View attachment 19809

That's valid only until 23z. The threat further south is primarily nocturnal.

 

[MattPetrulli]

That's valid only until 23z. The threat further south is primarily nocturnal.

Should've clarified that I was showing that for the northern moderate.

Not the weakest signal but not the strongest signal. 10% hatch would've sufficed if a bigger signal doesn't come up.

 

[andyhb]

The problem is that models tend to have issues with nocturnal initiation for various reasons, which are often tied to their boundary layer parameterizations, and this applies even moreso east of the Plains. I would not be surprised to see a greater coverage of storms than the HREF indicates there.

 

[MattPetrulli]

The problem is that models tend to have issues with nocturnal initiation for various reasons, which are often tied to their boundary layer parameterizations, and this applies even moreso east of the Plains. I would not be surprised to see a greater coverage of storms than the HREF indicates there.

I mean I wouldn't deny the entire dryline unzipping due to CAMs initiation in subtle flow problems. Still think they should've stayed 10 hatched though, but im just being picky tbh.

 

[Lake Martin EF4]

The amendment to the Day 3 outlook helps confirm my suspicion that with more favorable timing this could have been a Palm Sunday 1965 redux.

How so?

 

[atrainguy]

Not sure if this outlook is outdated yet or if this guy is leaving out some discrepancies, but I thought I should share here just in case.

 

[andyhb]

This is relevant in terms of HRRR forecasts for tomorrow and its degree of mixing + high LFCs/LCLs.