Sunrise 5:58 AM   Sunset 8:31 PM Moonrise 5:59 PM   Moonset 3:40 AM

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FXUS63 KEAX 211739 AFDEAX

Area Forecast Discussion National Weather Service Kansas City/Pleasant Hill MO 1239 PM CDT Tue May 21 2024

KEY MESSAGES

- Severe thunderstorms capable of producing all hazards will develop this afternoon in northern and eventually central Missouri. Main forecast uncertainty is if storms will initiate before the cold front moves through the Kansas City metro area.

- The main time frame for severe weather is 2 to 9 pm today.
Storms should move south and east of the region by late evening.

- Unsettled weather returns to the area late this week into the holiday weekend.

DISCUSSION
Issued at 201 AM CDT Tue May 21 2024

Objective 00z upper-air analysis indicates a deep trough in the western U.S., with a 130+ kt 250-mb jet steak in the Four Corners region, ejecting east-northeastward into the Front Range. Mid- and upper-ridging was nosing northward in advance of this trough into the central Plains. A midlevel speed max was present in the Desert Southwest, with a broad stretch of southwesterly flow downstream into the Great Lakes region.
Southerly flow at 850 mb stretched from the Gulf of Mexico to Kansas and Missouri, pooling moisture along a deformation axis extending from western Nebraska to the Wisconsin/Illinois area.
At the surface, a cyclone was developing in the lee of the Rockies in southeast Colorado, with a baroclinic zone stretching northeast and then eastward through the Corn Belt. Multiple areas of convection had developed along the baroclinic zone during the evening and overnight hours, with a substantial MCS moving through southwest Nebraska and far northwest Kansas as of 2 am.

Convection-allowing models (CAMs) have struggled with the exact axis by which the MCS will track this morning. Recent HRRR simulations have trended north, taking the MCS northeastward through northeast/east-central Nebraska and northern/central Iowa this morning. Suspect this trend is a little overdone, especially as convection tends to develop farther south into the available instability versus typical model guidance projections. Nevertheless, the strength of the upper ridging would suggest the MCS will most likely stay north of the CWA this morning, keeping any severe potential out of our area.

There are a couple of implications with this anticipated morning evolution, however. First, it means the CWA will diurnally destabilize with little mitigation from cloud cover/precipitation. And the presence of a very strong elevated mixed layer and associated capping will allow for strong instability to be present by afternoon (i.e., MLCAPE > 3000 J/kg). The second implication is that convection may be hard- pressed to initiate unless/until large-scale lift is strong.
Such lift will be provided by the aforementioned speed max and attendant shortwave trough racing northeastward through the Rockies into the northern/central Plains by late afternoon. The trajectory of this vorticity maximum is displaced north of our region, and the favorable left-exit region dynamics will be mostly to our north as well, both suggesting that convective initiation will first occur in Nebraska and Iowa as the surface cyclone ejects northeastward today. CAMs are considerably more variable regarding convective initiation southward along the progressive cold front into Kansas and Missouri later today. The general consensus this morning is that storms will initiate southward into northern Missouri early-to-mid afternoon (2-4 pm), then rapidly develop farther south as the speed max glances the region around/after 21z. Timing of the front will be critical here, as it could very well be east of the Kansas City metro by the time initiation occurs. Thus, although the entire region is in either an enhanced or moderate risk of severe weather today, the threat is increasingly conditional the farther west in the CWA you go. It is quite possible (40 percent chance) little or no convection occurs west of the U.S. Highway 65 corridor.

With the above caveats in mind, the preconvective environment is, in scientific vernacular, off the chain. As mentioned, very large MLCAPE will be present (> 3000 J/kg), in deep layer shear of 50-70 kt and effective SRH > 150 J/kg. With very steep lapse rates and considerable CAPE in the hail-growth layer, any convection that initiates and remains semi-discrete will be capable of producing very large hail. With dry midlevels, downward momentum transport may be quite efficient with strong downdrafts that develop, suggesting a considerable damaging wind threat as well. The tornado threat is a little more conditional, with at least some modest limiting factors present.
First, near-surface will veer in advance of the front, which does appear in the somewhat modest-looking hodographs compared to significant tornado events (although far from prohibitive).
Second, rapid upscale growth looks probable, since (1) strong large-scale lift will likely be required to initiate convection (and this tends to occur, per the adjective used, at fairly large scales), (2) upper flow becomes more parallel to the progressive front with time, and (3) because of strong antecedent capping, the time window for that upper flow to be more orthogonal to the front (permitting longer-lasting discrete convection) will be small. Thus, convection will mostly likely become banded or clustered quickly, perhaps developing quite rapidly into a lengthy squall line. Nevertheless, with strong dynamics and thermodynamics in play, a QLCS with embedded mesovortices is a plausible outcome, with a tornado threat in such a scenario.

The speed of the ejecting system and attendant cold front suggests that convection that initiates will move rapidly east out of the CWA by late evening. Guidance trends clear the storms of the region by 9 or 10 pm, with any heavy rainfall threat as storms on the southern flank of the developing line should stay safely south of the CWA

Not much change was made to the rest of the forecast. A reinforcing vort max will move through the Midwest on Wednesday, which may keep storm chances in our southern fringes during the afternoon/evening. More likely, though, is that storms will stay south of the region Wednesday and Wednesday night (70 percent chance). After the warm weather today, temperatures will be much more seasonal on Wednesday in the wake of the system.

The pattern late this week into this weekend remains unsettled, with general quasi-zonal or southwesterly low-amplitude flow in place. Multiple perturbations will eject from a broad large- scale trough in the West, leading to multiple chances for convection through the holiday weekend. Ensembles are gradually converging on a relatively higher threat Thursday night and Friday as well as Sunday. With an open Gulf and relatively fast flow in proximity to the perturbations, at least some severe threat exists with each of these passing systems. Notably, the CSU machine-learning probabilities provide nearly daily chances of severe weather from Thursday through Monday for the region.
Sounds about right.

AVIATION /18Z TAFS THROUGH 18Z WEDNESDAY/
Issued at 1232 PM CDT Tue May 21 2024

Isolated to scattered thunderstorms are expected to develop after 20Z quickly developing into a line of thunderstorms. There is a question as to whether storms will be developed in the KC metro, so went with a few hours of vicinity. Gusty winds are expected to persist through the afternoon hours, subsiding with sunset this evening.

EAX WATCHES/WARNINGS/ADVISORIES
MO...None.
KS...None.

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