323
FXUS63 KEAX 211122
AFDEAX

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

...Updated 12z Aviation...

.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 /12Z TAFS THROUGH 12Z WEDNESDAY/...
Issued at 617 AM CDT Tue May 21 2024

Cold front is approaching the area. Winds will pick up in speed
through the afternoon. Storms are expected to develop, but may
hold off until the cold front is past the terminals. For now,
have just placed VCTS in the TAFs and will monitor trends
through the afternoon. Ceilings expected to remain VFR. If
storms develop prior to the cold front moving through, IFR
visibility could become possible with heavier rain.

&&

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

&&

$$

DISCUSSION...CMS
AVIATION...Krull