955
FXUS63 KLSX 121043
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
543 AM CDT Sun May 12 2024

.KEY MESSAGES...

- After another mostly dry day today, showers and thunderstorms
  are expected, numerous at times, from early Monday morning into
  Tuesday evening.

- There is another opportunity for showers and thunderstorms
  around the Thursday-Friday timeframe.


&&

.SHORT TERM...  (Through Late Monday Afternoon)
Issued at 311 AM CDT Sun May 12 2024

An upper-level ridge, ahead of an eastward-ejecting closed low over
the Front Range, will dominate the Mid-Mississippi River Valley
through much of the day today with largely dry conditions. Departure
of a surface SLP ridge this morning and a transition to weak low-
level southerly flow/WAA with strong insolation will lead to a large
diurnal jump in temperatures today, from the upper 40s to mid-50s F
this morning to the 80s F this afternoon. Isolated showers and
thunderstorms are possible (20 percent chance) in northeastern MO
and west-central IL as early as late afternoon as low-level moisture
and modest diurnal instability increase, but large-scale forcing
remains nebulous until the crest of the ridge shifts east tonight.

The upper-level closed low will advance into the Central Plains
behind the ridge tonight, with showers and a few thunderstorms
gradually increasing across the CWA as southerly flow strengthens
and moisture and broad large-scale ascent increase. Showers and
thunderstorms are expected to become more numerous and pivot
northward through the CWA Monday morning into early afternoon as a
lead low-amplitude upper-level shortwave trough passes in tandem
with a band of mid-level WAA.

Confidence is high that showers and thunderstorms will persist
across portions of the CWA Monday afternoon into night as the closed
low slowly traverses the Mid-Mississippi River Valley, but coverage
and placement are a little less clear as forcing/ascent becomes
fairly broad. Some breaks in widespread clouds are expected during
the afternoon and evening, supporting a marginal increase in
instability, with the latest HREF indicating probabilities of SBCAPE
over 750 J/kg greater than 60 percent in a corridor through central,
southeastern MO into east-central MO. With 20 to 30 kt of deep-layer
wind shear also forecast in those areas, loose organization/
aggregation of thunderstorms into multicell clusters is possible
during afternoon and evening. Some deterministic model guidance
indicate localized MUCAPE just over 1000 J/kg, which could be
sufficient for stronger thunderstorms to produce small hail.
However, confidence in an overlap of instability and deep-layer
wind shear sufficient for severe thunderstorms is still greater
south and southwest of the CWA on Monday. Widespread clouds and
periods of precip will keep temperatures much cooler on Monday
than today and near to slightly below average.

Pfahler

&&

.LONG TERM...  (Monday Night through Saturday)
Issued at 311 AM CDT Sun May 12 2024

Late Monday night into Tuesday, a weak surface cyclone is progged to
traverse the CWA, but there is uncertainty in its exact speed and
track along with whether it will be deepening. Regardless,
lingering showers and a few thunderstorms are expected on Tuesday
mainly north and west of the the cyclone within weak large-scale
deformation. From this evening through the ending of showers and
thunderstorms Tuesday evening, ensemble model probabilities of
total rainfall over 0.5" are higher than 70 percent across the
entire CWA, suggesting that most locations will see at least 0.5"
of rainfall. Probabilities of over 1" are 50 to 60 percent
generally along the I- 70 corridor, but rainfall amounts over 1"
will be largely dependent on where thunderstorms and heavier
rainfall are most frequent. As with Monday, temperatures will
remain cool on Tuesday with clouds and precip.

Global model guidance have come into better agreement on the
evolution of the upper-level wave pattern across the CONUS, at least
through the end of the week. An upper-level ridge will briefly pass
over the Mid-Mississippi River Valley on Wednesday, providing short-
lived CWA-wide dry conditions, before an upper-level trough is
advertised to traverse the central CONUS Thursday into Friday. The
characteristics of this trough still differ across deterministic and
ensemble model guidance, especially its amplitude and tilt. These
differences aside, the majority of ensemble model members depict
showers and thunderstorms across the CWA between Thursday and
Friday. NBM interquartile temperature ranges Wednesday through the
end of the week show a slight warming of temperatures from around
average to slightly above average.

Pfahler

&&

.AVIATION...  (For the 12z TAFs through 12z Monday Morning)
Issued at 543 AM CDT Sun May 12 2024

Dry and VFR flight conditions will continue through this afternoon.
Isolated showers and thunderstorms are possible (20 percent chance)
near KUIN late this afternoon and evening, but confidence in any
direct impacts is very low. The chance of showers and a few
thunderstorms will increase late tonight into Monday morning as they
are expected to become more numerous in addition to lowering
ceilings. As a result, flight conditions will deteriorate to MVFR
around the end of the TAF period, with IFR flight conditions also
becoming possible.

Pfahler

&&

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

$$

WFO LSX