012
FXUS63 KLSX 011719
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1219 PM CDT Wed May 1 2024

.KEY MESSAGES...

- An active shower and thunderstorm pattern will persist through
  next week, although much of this period will be dry rather than
  wet.

- Currently, the highest chance (70 to 90 percent) of showers and
  thunderstorms of the period is Thursday evening into night. An
  isolated strong to severe thunderstorm is possible across
  central, northeastern MO and west-central IL during the
  afternoon and evening.


&&

.SHORT TERM...  (Through Late Thursday Afternoon)
Issued at 321 AM CDT Wed May 1 2024

Isolated to widely scattered showers and thunderstorms have
continued across portions of the area within a 40-kt southwesterly
LLJ and near a cold front stretching from west-central IL through
west-central MO as of 07z. However, showers and thunderstorms are
diminishing early this morning as the LLJ weakens and frontal
moisture convergence decreases.

Short-term model guidance indicate that the weak cold front will
become stationary today near the I-70 corridor, although largely dry
conditions are anticipated with meager moisture convergence along
the front and mid-level heights slightly rising through the day. In
the absence of appreciable sources of ascent/forcing, decreasing
convective inhibition with daytime heating and destabilization will
be the main (limited) catalyst for isolated showers and
thunderstorms during the afternoon and evening. Deep-layer wind
shear will be marginal (15 kt or less) but MLCAPE of 1500 to locally
2500 J/kg near and south of the front may be sufficient for a brief,
strong thunderstorm with gusty winds and small hail. Above average
temperatures will also persist with high temperatures ranging from
the mid-70s F north of the front, to mid-80s F south of the front.
However, exact temperatures today will be sensitive to how pervasive
upper-level cloud debris from areas of showers and thunderstorms in
the Central Plains is.

At least a temporary increase in shower and thunderstorm coverage
will take place across central, northeastern MO and west-central IL
late this evening as a LLJ develops and associated forcing increases
along with a quick northward movement of the front as a warm front.
But the strongest LLJ forcing will be focused to the northwest of
the CWA and quickly shift even further northward tonight.

On Thursday, an upper-level trough will pivot over the Central and
Northern Plains, forcing a cold front toward the Mid-Mississippi
River Valley late Thursday. Within upper-level southwesterly flow
ahead of this trough, there has also been a consistent signal for an
MCV or convectively induced perturbation to also progress
northeastward through the Mid-Mississippi River Valley. These two
features should be the main focii of showers and thunderstorms
Thursday afternoon and night across the CWA. Deep-layer wind shear
will be in the 15 to 20 kt range and supportive of loosely organized
multicell thunderstorm clusters, with the strongest wind shear
closer to 20 to 30 kt over central, northeastern MO and west-central
IL and to the west. With MLCAPE of 750 to 1500 J/kg depicted by most
model guidance, the aforementioned area has the greatest potential
for a strong to severe thunderstorm with damaging winds and
marginally severe hail during the afternoon and evening. The
development of cold pools could also allow showers and thunderstorms
to grow upscale and become widespread in some areas. Ahead of
showers and thunderstorms, strong low-level south-southwesterly flow
and deep BL mixing will contribute to Thursday being the warmest day
of the week with high temperatures in most of the area reaching the
low to upper 80s F.

Pfahler

&&

.LONG TERM...  (Thursday Night through Tuesday)
Issued at 321 AM CDT Wed May 1 2024

Showers and thunderstorms will continue through much of Thursday
night, depending on how significant subsequent coverage and
overturning of instability becomes. On Friday, the highest chance of
showers and thunderstorms will be in the morning in association with
any lingering showers and thunderstorms. Thereafter, the cold front
passing the CWA will become increasingly diffuse and large-scale
forcing will be minimal. Instead, the main highlight of Friday will
be the arrival of cooler air behind the front but still warm enough
for temperatures to be near to slightly above average.

Global model guidance align in time-mean upper-level southwesterly
flow persisting into next week over the central CONUS, navigated by
a parade of troughs varying in track and amplitude. This upper-level
configuration will support a continued active shower and
thunderstorm pattern. That being said, most of the time will be dry
rather than wet, with the greatest support for dry conditions late
Friday and late Saturday into early Sunday owing to either upper-
level ridging or minimal large-scale ascent. Early to mid-next week,
deterministic and ensemble model guidance spread increases, but some
of the membership includes patterns more favorable for severe
thunderstorms somewhere across the Mississippi River Valley next
week, which bears monitoring going forward. In terms of
temperatures, a slight warming trend from the near to slightly above
average temperatures on Friday is captured by the NBM interquartile
temperature ranges over the weekend into next week.

Pfahler

&&

.AVIATION...  (For the 18z TAFs through 18z Thursday Afternoon)
Issued at 1215 PM CDT Wed May 1 2024

VFR conditions will prevail at the terminals through Thursday
afternoon. A weak cold front that passed through the region this
morning will lift back north gradually this afternoon and evening,
sending the current northerly winds back out of the south.
Convergence along the front is very weak, and despite sufficient
instability for convection, this coupled with a lack of upper-
level forcing will result in isolated to widely- scattered storms
at best. There is little confidence in direct terminal impact
(even "vicinity" wording") and as such thunder was kept out of the
TAFs.

There will be another threat for showers and thunderstorms
tomorrow morning and afternoon from west to east, but the threat
is outside of the current TAF period (except KSTL, where VCSH was
added at the very end of the TAF to suggest that potential).

MRB

&&

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

$$

WFO LSX