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May 20-21 Severe Threat


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***Edit: Added tomorrow (Monday) to the thread title as there is broad agreement among the CAMS in an MCV-driven setup along the IL/WI border region tomorrow (SPC added a small slight risk to the updated SWODY2 to account for this). These events are subtle with generally modest parameters depicted beyond Day 1, but if everything comes together just right they are capable of producing some prolific localized :twister:outbreaks in this area, such as 8/9/21 and 7/12/23.***

Guidance has been in good agreement for several days now that a compact, negatively tilted shortwave will impinge upon the IA/MN/WI/NW IL region on Tuesday with strong southwesterly flow at 500mb. At the surface, a low will deepen throughout the day with warm sector dewpoints well into the 60s, if not upper 60s by 00Z Wednesday. Basic pattern recognition suggests a very high ceiling and I'm frankly baffled why SPC's day 3 outlook does not at least include an all-hazards hatched area for wind potential if nothing else.

That said, strictly in terms of:twister:chase potential, there are several potential flies in the ointment as it seems there always are. One of them is, exactly how many rounds of storms initiate, where and when? For example, models have EXTREME levels of SRH along the warm front in southern Wisconsin, but suggest late-morning/early afternoon convection preventing that area from destabilizing. Other models suggest supercells might initiate in western Iowa, but might not be colocated with the best tornado parameters until they have begun to congeal into a QLCS (of course, this was also a concern with 3/31/23).

Regardless, I think this day will end up a noteworthy regional severe weather event and worthy of its own thread.

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ACUS02 KWNS 201759
SWODY2
SPC AC 201758

Day 2 Convective Outlook  
NWS Storm Prediction Center Norman OK
1258 PM CDT Mon May 20 2024

Valid 211200Z - 221200Z

...THERE IS A MODERATE RISK OF SEVERE THUNDERSTORMS ACROSS SOUTHERN
IOWA...NORTHWEST ILLINOIS AND FAR NORTHEAST MISSOURI...

...SUMMARY...
Severe storms are expected Tuesday across the Midwest, especially
including portions of Iowa and Missouri into Wisconsin and
western/northern Illinois. Tornadoes (a few strong), damaging wind
gusts, and large hail are expected.

...Lower/Middle Missouri Valley to the Upper Great Lakes...
A notably strong belt of cyclonically curved westerlies (70+ kt 500
mb) will extend from the south-central Great Plains to the Upper
Midwest and Upper Great Lakes on Tuesday, with a lead/convectively
augmented shortwave trough likely to spread from the central High
Plains toward the Lake Superior vicinity late Tuesday night. An
elongated surface low over eastern Nebraska Tuesday morning will
deepen and lift northeast to northern Wisconsin/Upper Michigan by
evening, with a surface warm front lifting northward ahead of the
surface low, although its effective position will likely be
influenced by the early day precipitation. Meanwhile, a trailing
cold front will surge east across Iowa/eastern Kansas/northern
Missouri from mid-afternoon into the evening.

Remnant clusters of storms will likely be ongoing Tuesday morning
across central/eastern Nebraska, with the possibility of additional
storms farther east across southern Iowa/northern Missouri in
association with a warm-advection regime. Some key uncertainties
persist about the influences of this early day convection, but
trends will be for quick air mass recovery given a
progressive/highly dynamic pattern.

Very steep mid-level lapse rates will overspread the increasingly
moist warm sector, supporting aggressive diurnal destabilization
especially on the southern fringe of lingering convection/cloud
cover. Significantly strengthening mid-level winds and a diurnally
persistent strong low-level jet will support initial supercells
capable of large hail and a few tornadoes, including strong (EF2+)
tornado potential. But a mixed/transitional mode is expected
overall, as upscale growth occurs given the degree of forcing for
ascent, with likely evolution into a well-organized/fast-moving QLCS
capable of increasingly widespread/intense damaging winds and a
continued QLCS-related tornado risk. This scenario currently appears
to be most probable across southern/eastern Iowa into northwest
Illinois and nearby far northern Missouri. The severe risk will
persist into the Upper Great Lakes region and middle Mississippi
Valley overnight.

...Southern Plains...
Convective coverage will likely decrease with southward extent as
large-scale ascent focuses northward toward the Great Lakes.
Nevertheless isolated storms are possible into northeast
Oklahoma/northwest Arkansas by late afternoon/evening, and more
conditionally with southwest extent near a dryline from central
Oklahoma into central Texas. Large hail and severe gusts will be
possible with this activity.

...Northeast States...
A weak upper shortwave trough will move across southern Quebec and
northern New England. This will result in a band of enhanced
westerly deep-layer flow from northern New York into Maine. A
seasonally moist airmass will be in place with surface dewpoints
into the low 60s F, with strong heating and steepening low-level
lapse rates. MLCAPE to around 1000 J/kg and effective shear
magnitudes around 35 kt will support organized cells. Elongated
hodographs and favorable vertical shear suggest marginally severe
hail will be possible along with locally damaging wind gusts.

..Guyer.. 05/20/2024

spcd2four_panel.us_mw.png

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Yeah, based on all the solutions I've seen so far, southeastern Iowa is going to only get wave three, whereas northeastern and southwestern Iowa are scheduled to get hit by waves 1 and 2. If that happens and it's a linear threat, that's all well and good, but what happens if a cell develops ahead of that line? That's going to be interesting to watch. We'll need to see more info on this, but I think if there's going to be a strong tornado, based on the models I've seen, areas south of the QCA out to Galesburg and back into the Mt. Pleasant are going to be under the gun. If a cell develops there ahead of that line, I'd be concerned.

Untitled11.thumb.png.78c1c9e5b26e4df2eb9dc7416feb0d78.png

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Thinking about targeting the Waterloo-Webster City area. Multiple HRRR runs and the 06Z 3K NAM suggest slightly better backing of the surface winds in north-central to northeast Iowa, with a relative maximum of 3CAPE/EHI and some discrete-ish cells making their way through there in the afternoon. Besides, along and north of the US-20 corridor is the best chase terrain in Iowa. Almost as flat and open as western Kansas. B) I got lucky that Keota was in a fairly open spot along Highway 92 but encountered issues with hills and trees while trying to chase it toward Wellman/Kalona.

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Models have not handled the morning convection well at all.  The CAMs had (and still have) all this stuff quickly weakening and lifting into northern Iowa, but that is not happening.  Instead, one line has moved through my area and a bigger line is moving into central to south-central IA.

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Really unsure where to target today. Part of me wants to get on the storms near low before they line out but afraid they may get undercut. Part of me is afraid something will go east along a remnant boundary and shear vector orientation and hodograph curvature is better east. Tricky forecast today

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1 hour ago, A-L-E-K said:

hrrr keeps shitting out a dry fropa here and i would prefer not

A lot more CAMs than HRRR doing that now. I feel like anecdotally that always happens and the squall will actually survive past the Lake.

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32 minutes ago, SluggerWx said:

PDS Watch issued for almost the entire state of Iowa and part of Southern MN. 90/80 probs.

I almost wonder if threads need to be merged since it's on the edge of C/W as well. Regardless, some extremely strong wording coming from the SPC today. Looks like a lot of CIN still in the region, definitely need that to erode and instability.

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Really not expecting too much of intertest around here with this.

Environment really tails off with east/southeast extent. So much of the guidance that is showing activity quickly weakening as it moves through is probably is correct.

SPC is way too overzealous with the enhanced risk.

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13 minutes ago, jwalsh1 said:

Forgive me, but are you saying you anticipate weakening as it enters LOT area?

Expectation should be that it'll be in a weakening phase as it passes through the CWA.

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