329
FXUS63 KLSX 061956
AFDLSX

Area Forecast Discussion
National Weather Service Saint Louis MO
256 PM CDT Mon May 6 2024

.KEY MESSAGES...

- Late tonight into early Tuesday morning, there is a threat ofstrong
  to severe thunderstorms with damaging winds and brief tornadoes
  the main hazards.

- On Tuesday afternoon, there is a conditional threat of a few
  strongto severe thunderstorms in southeastern, far eastern MO
  and southwestern IL.

- On Wednesday, the greatest chance of severe thunderstorms
  exists. Severe thunderstorms will be capable of all hazards
  including very large hail and a few strong tornadoes.


&&

.SHORT TERM...  (Through Late Tuesday Night)
Issued at 255 PM CDT Mon May 6 2024

The CWA is currently beneath an upper-level shortwave ridge
sandwiched between a departing trough over the Ohio Valley and a
deepening, occluding closed low over the Northern Plains. Quiet
conditions under the shelter of the shortwave ridge will come to an
end tonight as a series of shortwave troughs eject northeastward
from the base of the closed low within southwesterly flow, helping
to establish the much-advertised active weather pattern that will
linger across the region through Wednesday.

The first shortwave is now entering the Central Plains, assisting
the development of strong to severe thunderstorms this afternoon and
evening ahead of a cold front as increasingly warm, moist, and
unstable air is transported northward via low-level southerly flow
in the warm sector. CAMs unanimously depict the thunderstorms in the
Central Plains congealing an eastward-moving QLCS tracking quickly
through MO tonight. This QLCS is expected to reach central and
northeastern MO around 12 to 2 am TUE. By that time, instability
will be relatively lower than earlier to the west, but the general
consensus in short-term model guidance is that SBCAPE will still be
over 1000 to locally near 2000 J/kg accompanied by 35 to 45 kt of
deep-layer wind shear, supporting only a gradual weakening of the
QLCS. In addition, 30 to 40 kt of 0 to 3 km wind shear and 0 to 1 km
wind shear will be available. Therefore, a threat for damaging winds
and brief tornadoes will accompany the QLCS, especially any
northeastward surging bowing segments orthogonal to the 0 to 3 km
wind shear vector. As the QLCS continues eastward through the
remainder of the CWA very late tonight into Tuesday morning,
instability will decrease slightly with both time and eastward
extent due to increasing BL stability and decreasing moisture.
Exactly how much this instability decreases is uncertain and will
impact if the QLCS weakens more quickly and if the corresponding
damaging wind and brief tornado threat decreases more quickly. The
weakening QLCS will depart the CWA through 7 to 8 am TUE.

The depth and southward extent of the QLCS`s cold pool Tuesday
morning will have implications on how much recovery of instability
can occur by Tuesday afternoon ahead of an approaching Pacific cold
front. However, even the 10th percentile of SBCAPE in the latest
HREF reaches 1000 to 1500 J/kg in southeastern MO and southwestern
IL, increasing confidence in instability recovering. Large-scale
ascent will be rather nebulous during that time, but weak moisture
convergence along a Pacific cold front passing through southeastern
MO and southwestern IL, perhaps far eastern MO during early
afternoon is the main potential catalyst for thunderstorm
initiation. Although not all CAMs initiate thunderstorms before the
front departs the CWA. IF thunderstorms develop, a few could be
strong to severe with deep-layer wind shear projected to be 50 to 60
kt with a few supercells possibly merging into small bowing
segments. The main hazards with any severe thunderstorms will be
damaging winds, large hail, and possibly a tornado, with the threat
ending by late afternoon as the front clears the CWA. Low-level CAA
behind the Pacific cold front will be counteracted by strong
insolation and downsloping of flow off the Ozark Mountains, leading
to widespread high temperatures in the low to mid-80s F near and
south of I-70 and in the upper 70s F northeastern MO and west-
central IL.

Pfahler

&&

.LONG TERM...  (Wednesday through Next Monday)
Issued at 255 PM CDT Mon May 6 2024

Nearly all indications in model and analog guidance point to
Wednesday being the summit of the active weather pattern. As another
upper-level shortwave trough preceding an eastward ejection of the
Northern Plains closed low occurs from the Central Plains to
Mississippi River Valley, Tuesday`s cold front will lift back
northward into the CWA as a warm front on Wednesday ahead of a weak
surface cyclone. Along and south of the front, a strongly unstable
warm sector is anticipated to exist with model guidance advertising
MLCAPE of 2000 to 4000 J/kg during the afternoon amidst 50 to 70 kt
of deep-layer wind shear. There are still differences in how far
north the warm front reaches by afternoon, which will serve as both
the northward delineation and possibly an enhanced corridor of
significant severe thunderstorms, being an area of enhanced low-
level wind shear and SRH. Latest model guidance has this front
anywhere from southeastern MO to south-central IL by the afternoon,
significantly diminishing confidence in its effective afternoon
position. These discrepancies aside, confidence continues to
increase in the instability-wind shear parameter space being
supportive of severe thunderstorms capable of all hazards, including
very large hail (2" or greater in diameter) and a few strong
tornadoes (EF2 or greater). The exact timing and evolution of
thunderstorms is not entirely clear, but a preliminary window of
severe thunderstorms is from midday through evening on Wednesday
when a cold front finally pushes to the east of the CWA.

Most attention in the forecast was obviously given to the next 3
days, but confidence is high in at least a quieter weather pattern
after Wednesday as a period of time-mean upper-level northwesterly
flow through the weekend. This flow pattern will provide cooler,
near to below average temperatures as periods of low-level CAA
follow multiple fronts passing through the region. Some
opportunities for showers and thunderstorms will accompany these
fronts as well; however, not all ensemble model guidance contain
rain through the period and have timing differences related to
frontal timing and associated upper-level trough timing and
amplitude disparities.

Pfahler

&&

.AVIATION...  (For the 18z TAFs through 18z Tuesday Afternoon)
Issued at 1245 PM CDT Mon May 6 2024

MVFR stratocumulus will continue to scatter and lift through early
afternoon with improvement to VFR flight conditions at all
terminals, latest at KUIN. Late tonight into Tuesday morning, a line
of showers and thunderstorms is expected to traverse the region from
west to east, likely weakening some with time. Thunderstorms will be
accompanied by periods of IFR flight conditions and gusty winds.
Confidence is higher in relatively more intense thunderstorms at
KCOU, KJEF, and KUIN, with the highest threat of stronger wind gusts
and lower visibilities. Timing in the current TAF is reflective of
the general timing in latest forecast model guidance.

Behind the showers and thunderstorms MVFR stratus is expected to
linger several hours Tuesday morning, until around the time that
winds veer to southwest and start gusting to 20 to 25 kt. There is a
20 to 30 percent chance of showers and thunderstorms redeveloping in
the vicinity of St. Louis metro terminals early Tuesday afternoon.

Pfahler

&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...None.
IL...None.
&&

$$

WFO LSX