Severe Weather Threat 3/17-3/19, 2022

[Taylor Campbell]

A thread to discuss the 3/17-3/19, 2022 severe risk.

 

[Clancy]

D4 outlook.

 

[MattW]

CIPS Analog-Based Severe Probability Guidance

The analogs are liking this one.

CIPS Analog Threat Guidance

www.eas.slu.edu

 

[Taylor Campbell]

The models are trending worst each run with this setup and seem unusually agreeable for a nasty severe weather threat to transpire Friday. I suspect we will see a substantial increase to the risk category by the SPC if things keep up.

 

[MattPetrulli]

Think this is a big sleeper considering next week's hype overlapping but we could get a sneaky significant event here
Some uncertainty regarding how far north instability gets and how much, but trends from NAM/GFS here are showing a decent aerial overlap of instability. NAM on 00z gets instability much further north and involves BMX and ATL metro at points.18z GFS is a little more to the south with lesser instability. Don't really have full access to 12z Euro but it seems to be a median between GFS/NAM with leaning closer to NAM but lesser instability.

Working with decent kinematics, going off NAM/GFS working with about area wide 250-350 m effective SRH 0-3 km with 35-45 knt LLJ.

That sounding is from southern Alabama, there are much better soundings in terms of greater tornado potential at 21z/18z but picked this to show the divide between GFS/NAM. Overall, could be a significant sleeper event and would not be surprised to see a day 3 enhanced from SPC tonight but would expect a sight for now.

 

[MattW]

Um...I'm seeing discrete cells on the NAM. Too early to read anything verbatim, but an ominous sign this far out on the non hi-res NAM.

 

[Clancy]

D3 for Friday.

 

[CheeselandSkies]

And the infamous pink hazard type has cropped up on the NAM forecast soundings. A couple of things my amateur eye notes about this one. Turning with height above 700mb isn't ideal, but there's 300 j/kg of cape in the lowest 3 kilometers (3CAPE). That is very substantial and presents a similar setup to the one over Iowa on the 5th.

 

[Clancy]

NAM seems to be depicting cells forming after the MCS moves out of the area. Those cells look like troublemakers, if they do indeed form.

 

[CheeselandSkies]

NAM seems to be depicting cells forming after the MCS moves out of the area. Those cells look like troublemakers, if they do indeed form.
View attachment 12578View attachment 12579

I was just about to edit my post to mention the 12Z 3KM NAM at the end of its run. It has a several hour window with no precip over MS/AL followed by those discrete cells developing and tracking into west AL by 00Z Saturday.

 

[Clancy]

I was just about to edit my post to mention the 12Z 3KM NAM at the end of its run. It has a several hour window with no precip over MS/AL followed by those discrete cells developing and tracking into west AL by 00Z Saturday.

I would guess that it's a bit of an open question if they form with the usual caveats of a worked-over atmosphere, but the conditions behind the MCS look suitable for them if the air recovers. Obviously, will defer to the more experienced for a more concrete say on that matter.

 

[CheeselandSkies]

12Z GFS has mostly straight-line hodographs across MS/AL on Friday evening. The system is a little too fast, which veers the low-level winds across the warm sector.

 

[Fred Gossage]

The type of setup shown for Friday afternoon over the northern half of MS/AL into TN is one that usually doesn't actually work out. The low-level jet focus and low-level convergence gets shifted away with the morning activity, and it's hard to get convection to fire again in the instability axis ahead of the front. However, there is occasionally an exception to that, and one such exception is January 21, 2010... and there was an EF2 in Huntsville with that setup. This trough is more open instead of being a closed upper low, but the overall synoptics aren't that dissimilar. Still, this type of setup for our immediate area here has a very high bust rate.

 

[MattPetrulli]

Seems like shear kinda downtrended but instability up on latest runs. Instability part is pretty interesting because some models like NAM get a lot of CAPE in for a Mid-March setup with morning convection. Also HRRR showing somewhat of an EML which could cause issues. Kind of a weird/kinda sloppy setup at this point, interested to see what SPC says at 2 AM.

 

[Clancy]

Discussion on Friday's threat from BMX's morning AFD.

Friday morning:

Strong forcing for ascent and upper-level divergence from the
negative tilt trough will result in the development of an MCS
across the ArkLaMiss tonight. A 50-60 kts LLJ will maintain it as
it moves eastward, moving into West Alabama after 4 AM, and
reaching the Georgia state line by around midday. This MCS will be
north of the warm front/elevated, but with steep mid-level lapse
rates, 500 to 1000 J/kg of MUCAPE, and 70 kts of 0-6km bulk shear,
it will potentially produce the first round of severe storms
mainly across the southern half of Central Alabama. Potential AM
threats would include quarter size hail from any elevated
supercells and damaging wind gusts wherever stronger downdrafts
can make it through a near-surface stable layer. This would most
likely not include tornadoes. The exception would be if some of
the slower solutions or the solutions that are less organized on
the southern end of the convection verify. In that case, there
would be a risk of storms becoming surface-based in our far
southeastern counties by late morning which would result in a
more substantive tornado threat. However, at this time the most
likely scenario is that outflow from the MCS would keep this from
occurring. Additionally, localized flooding could occur, mainly in
areas that received heavy rainfall yesterday.

Friday afternoon and evening:

Forecast confidence decreases regarding what happens by afternoon
behind the MCS. It can often be difficult to get air mass recovery
behind a morning MCS in March. Synoptic scale subsidence and
weak height rises will be present behind the shortwave and mid-
levels dry out with PWATs falling to 1 to 1.2 inches. At the same
time, however, low-level southwesterly flow brings in high theta-e
air at low-levels, beneath the mid-level dry slot. Models are in
good agreement on increasing SBCAPE values to around 2000 J/kg,
with 0-6 km bulk shear values still around 60 to 70 kts, west of a
outflow-reinforced warm front draped somewhere along/east of I-65.
There is an upper-level jet max approaching from the west
bringing a little divergence, and some guidance does indicate a
subtle 700mb speed max/vort max which could provide a subtle
trigger. Current thinking is a least a couple strong to severe
supercells will re-develop Friday afternoon, but overall storm
coverage would be scattered in nature. This would provide a
conditional threat of quarter to golf ball size hail, and damaging
winds due to the steep low-level lapse rates/dry air aloft. The
threat for tornadoes is conditional as well but potentially
impactful. Non convectively-contaminated forecasting soundings in
the warm sector generally indicate fairly straight hodographs due
to veered low-level flow. However, there will be some localized
backing of the winds near the warm front, perhaps along the I-65
corridor, and some models also show some sharpening to the cold
frontal pressure trough. In this corridor a axis of enhanced STP
values may evolve, but placement of this area is uncertain. Stay
tuned for updates as we get closer to the event.

 

[MattW]

The HRRR and NAM are both definitely showing high SigTor values behind the MCS, but little to no convection. It looks like the HRW model wants to show some storms behind the MCS.

 

[Taylor Campbell]

Storm initiation does not look like a problem. Simulated reflectivity products show supercell formation within the maxed parameter set across the northern half of Alabama most likely for the Huntsville and northern Birmingham WFO area. Large hail and damaging winds are your greatest threats; however a tornado shouldn’t be ruled out.

The MCS looks to provide a damaging winds and hail threat with elevated instability and an increased probabilities for a tornado further south for Louisiana, the Florida Panhandle, southern MS, and southern Alabama where storms could be surface based.

 

[Taylor Campbell]

@Lori @WesL @KoD can we please get this thread pinned to the top. Thank you.

 

[CheeselandSkies]

NAM and GFS both show little problem recovering the instability behind the MCS...with how dramatically they underdid the instability on 3/5 in Iowa at medium range, it would be ironic if they overdid it this time.

 

[Fred Gossage]

In setups like this, you will usually hear that the afternoon threat just ahead of the front is all conditional based on how much the atmosphere recovers behind the morning activity, and then when things fail to fire, you hear people blindly say that the atmosphere just did not grow unstable again. More often than not, that is actually not true. The thing that usually fouls up afternoon redevelopment on days like this is a focus for low-level convergence is taken away from the frontal zone by the morning action and how it shifts the low-level jet away from the area by the afternoon. Because of this, you often get winds that veer too much just above the surface, and you are left with not enough low-level lift along the front to get sustained updrafts to form in the mid-level dry air. It's highly unsurprising that the models are recovering CAPE values by afternoon the way they are. They usually do in setups like this. Mid-level dry air will allow the sun to break out, and portions of north Alabama may be in the mid 70s by 3:00 or 4:00 tomorrow afternoon. The question is where there will be enough lift to get storms to refire. An awful lot of the CAMs think that at least low-topped discrete storms refire in the afternoon. You're not looking for big discrete storms with massive UH tracks in a setup like that.

Particularly telling to me is that the 12k (non-CAM) NAM fires off a few discrete updrafts in that environment, and its whole convective parameterization scheme is explicitly designed, intentionally, to not initialize convection in mid-level dry air (the reason why we see it underdo convective coverage in a lot of EML tornado setups). It's also telling me that the surface winds stay backed to the SSE ahead of the afternoon front/pressure trough even on the models that often underdo low-level backing and isallobaric responses (GFS and NAM).