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FXUS63 KLSX 021138
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
638 AM CDT Thu May 2 2024

.KEY MESSAGES...

- An active weather pattern with rounds of showers and
  thunderstorms will continue through next week, although a
  majority of the time will be dry.

- There is a potential for isolated strong to severe thunderstorms
  today, generally within a window from 12 to 7 pm and across
  central, northeastern, and east-central MO as well as west-
  central IL. Gusty to briefly damaging winds and marginally
  severe hail are the main hazards.


&&

.SHORT TERM...  (Through Late Friday Afternoon)
Issued at 349 AM CDT Thu May 2 2024

A warm front just to the north of the I-70 corridor as of 08z will
lift quickly northward early this morning. The strongest forcing
associated with a 40 to 50 kt south-southwesterly LLJ extending over
the front, sufficient for isolated to scattered showers and
thunderstorms, will remain focused just west and northwest of the
CWA. However, a few showers and thunderstorms could develop across
central, northeastern MO and west-central IL early this morning.
Patchy fog has also developed where yesterday evening`s cluster of
showers and thunderstorms tracked and skies have cleared across
southeastern MO and southwestern IL.

Two MCSs are ongoing early this morning:  one in NE and northern KS
along/ahead of an approaching cold front and one in OK and northern
TX associated with an MCV. These MCSs and their parent forcing
mechanisms will be the drivers of showers and thunderstorms across
the CWA today and tonight as they arrive. It is uncertain whether
reintensification or entirely new redevelopment of thunderstorms
will occur, but short-term model guidance depict 750 to locally near
2000 J/kg of MLCAPE downstream during the afternoon along with 10 to
20 kt of deep-layer wind shear. This marginal instability-wind shear
parameter space supports thunderstorms to be loosely organized into
one or more northeastward-advancing multicell clusters or MCSs
containing sporadic gusty to briefly damaging downburst winds and
marginally severe hail. Relatively greater deep-layer wind shear
closer to 25 kt is expected across portions of central, northeastern
MO and west-central IL which may allow for slightly greater,
transient organization of thunderstorms, perhaps a short-lived
supercell. Therefore, the threat of isolated strong to severe
thunderstorms is relatively greatest across those areas, decreasing
with southeastward extend. The associated window is from 12 to 7 pm
but could end up much shorter if widespread thunderstorm development
occurs and cold pools begin undercutting thunderstorms. Ahead of
showers and thunderstorms, strong low-level south-southwesterly flow
and deep BL mixing will lead to high temperatures reaching the low
to upper 80s F. Cooler temperatures in the 70s F are expected in
northeastern MO, where showers and thunderstorms will arrive earlier.

Thunderstorm coverage and intensity is anticipated to quickly
decrease this evening as instability is overturned and nocturnal
stabilization occurs in the absence of a strong LLJ, leaving a
transition toward light rain/showers and a few rumbles of thunder
overnight. The vast majority of this precip will depart to the east
and/or dissipate by Friday morning. Redevelopment of isolated to
scattered showers and thunderstorms is possible Friday afternoon,
primarily in southeastern MO and southwestern IL. However, the
aforementioned cold front will become increasingly diffuse and ill-
defined by that time across those areas and large-scale ascent will
be minimal, decreasing the shower and thunderstorm potential.
Instead, the highlight of Friday will be a cooler airmass arriving
behind the front, although with temperatures still slightly above
average.

Pfahler

&&

.LONG TERM...  (Friday Night through Wednesday)
Issued at 349 AM CDT Thu May 2 2024

Saturday through early Monday, a pair of migratory upper-level
shortwave troughs within time-mean southwesterly flow will traverse
the Mid-Mississippi River Valley, maintaining the ongoing pattern of
episodic showers and thunderstorms. There are some differences among
model guidance in the timing of these troughs, but the general
timeframes of highest shower/thunderstorm probabilities in ensemble
model guidance are Saturday during the day and then Sunday afternoon
into night. Much of the weekend will actually be dry, though, and
accompanied by near to above average temperatures, given the quick-
moving nature of the associated troughs.

Upstream of the upper-level shortwave troughs, global model guidance
portray an upper-level closed low tracking from the West Coast and
Rocky Mountains over the weekend before shifting into the
Northern/High Plains next week, providing a significant
strengthening of upper-level southwesterly flow over the central
CONUS. There are still notable differences of the position and
amplitude of the closed low captured in cluster analyses applied to
the height patterns of ensemble model membership, but this overall
pattern appears supportive of strong to severe thunderstorms across
portions of the central CONUS sometime next week. That being said,
the specifics of this pattern are nearly impossible to glean at this
juncture, including the exact threat of strong to severe
thunderstorms to the CWA. Persistent low-level southerly flow
associated with the pattern will also favor temperatures warming
further above average, with exact values depending on
showers/thunderstorms, cloud cover, and the position of an
oscillating front.

Pfahler

&&

.AVIATION...  (For the 12z TAFs through 12z Friday Morning)
Issued at 637 AM CDT Thu May 2 2024

Showers and thunderstorms currently across northwestern and western
MO as well as northwestern OK and southwestern MO this morning are
anticipated to redevelop and/or reintensify this afternoon while
sweeping northeastward through the evening. Thunderstorms will be
capable MVFR to IFR flight conditions and gusty winds, especially at
KCOU, KJEF, and KUIN. At St. Louis metro terminals, thunderstorms
may begin decreasing in intensity during their arrival. Following
thunderstorms, a period of showers/light rain, perhaps with a few
rumbles of thunder, is also expected in addition to MVFR to IFR
stratus. IFR stratus is most likely at KSTL and KCPS. Flight
conditions will improve Friday morning from northwest to southeast
as stratus scatters and lifts.

Pfahler

&&

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

$$

WFO LSX