O'Fallon, Missouri 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

503
FXUS63 KLSX 141710
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1210 PM CDT Wed May 14 2025

.KEY MESSAGES...

- Near daily record high temperatures are forecast on Thursday,
  the warmest day of the period.

- There is a threat of strong to severe thunderstorms Thursday
  evening and Friday with all hazards possible. However, there is
  considerable uncertainty whether or not thunderstorms actually
  develop on Thursday.


&&

.SHORT TERM...  (Through Late Thursday Afternoon)
Issued at 312 AM CDT Wed May 14 2025

Enough residual BL moisture and light winds could result in some
patchy river valley fog this morning, but the current airmass is
relatively drier than 24 hour ago and conditions are overall less
favorable. However, a warm front is expected to lift northward
through the CWA today, marking the onset of strong low-level WAA.
With increasing WAA and greater insolation than Tuesday through
diurnal cumulus and some mid-level clouds, high temperatures will be
warmer and around 80 to the mid-80s F. Although an overhead upper-
level ridge and arriving EML plume will be present, up to 20 percent
of HREF membership has isolated showers and maybe a thunderstorm
along/east of the Mississippi River during the afternoon and evening
with moisture convergence along the front where the inversion will
be weaker/more elevated at the periphery of the EML.

On Thursday, a potent upper-level trough will eject across the
Northern Plains into Upper Midwest with a weak, trailing "Pacific"
cold front passing eastward through the CWA during late morning
through evening. Strong, veering low-level WAA preceding the front
with some downsloping off the Ozark Plateau and 850-hPa temperatures
nearing the maximum of seasonal climatology will support well-above
average to near daily record high temperatures in the upper 80s to
low-90s F. Even behind the front, deeper BL mixing may offset weak
CAA to maintain these warm temperatures through the day.

The overhead EML will aid strong instability (2500 to 4500 J/kg
MLCAPE) along/ahead of the front Thursday afternoon/evening but also
maintain a strong capping inversion between 900 and 750 hPa. In the
absence of robust large-scale forcing with the parent trough passing
north of the CWA, there is still considerable uncertainty on whether
surface-based convective initiation will occur. Around 20 to 30
percent of HREF membership has showers and thunderstorms developing
during afternoon and evening across southeastern MO/southwestern IL,
but some of this convection could be high-based well above the
capping inversion and unable to fully utilize the instability. A few
CAMs have shown signs of sufficient moisture convergence and BL
mixing/heating sufficient to overcome the inversion and allow
isolated surface-based thunderstorms to develop across southeastern
MO and southwestern IL during the evening. If this were to occur,
strong deep-layer wind shear of 45 to 55 kt and hodographs with some
low-level curvature will support supercells capable of all severe
hazards:  very large hail, damaging winds, and tornadoes.

Pfahler

&&

.LONG TERM...  (Thursday Night through Tuesday)
Issued at 312 AM CDT Wed May 14 2025

The weak cold front is forecast to lift back northward on Friday as
a warm front ahead of an upper-level shortwave trough and its
associated cold front, acting as catalysts for a better opportunity
for thunderstorms. There is uncertainty in how far northward the
effective warm front will advance into the CWA, but ensemble model
guidance have trended toward a farther northward advancement with
probabilities of 2000+ SBCAPE increasing to 30 to 60 percent
along/south of I-70. Morning precipitation can often slow the
northward advancement of effective warm fronts, but model guidance
indicate that most showers and thunderstorms during the morning in
southern MO/northern AR will quickly depart to the east adding some
merit to a quick northward advancement. Probabilities of measurable
rainfall increase through the day to 50 to 80 percent by evening
along/south of I-44 in MO and I-70 in IL, but the exact initiation
time and coverage of developing thunderstorms is unclear and
sensitive to weakening of the capping inversion and arrival of the
shortwave trough. With 50 to 60 kt of deep-layer wind shear,
organization of thunderstorms into supercells potentially growing
upscale into clusters, all severe hazards appear possible. However,
forecast soundings indicate that any supercells have the potential
to produce very large hail with steep mid-level lapse rates and a
large amount of CAPE in the hail growth zone.

Brief upper-level quasi-zonal flow and Friday`s cold front south of
the CWA on Saturday will yield dry, relatively cooler conditions.
But global model guidance continue to point to the CONUS flow
pattern becoming increasingly amplified late Saturday into early
next week that allows a warm front to gradually lift northward
across the Ozarks and the Mid-Mississippi River Valley, leading to
increasing opportunities for showers and thunderstorms through early
next week. Exactly how quickly this process occurs will likely be
influenced by how soon a building upper-level ridge gives way to
southwesterly flow, but ensemble model guidance depicts a peak in
the probabilities of measurable rainfall around Tuesday when the
trough in this amplified pattern finally ejects northeastward across
the central CONUS. Most of the NBM temperature distribution remains
above average through early next week but with an up to 8 F spread
capturing uncertainties with the front and rounds of showers and
thunderstorms.

Pfahler

&&

.AVIATION...  (For the 18z TAFs through 18z Thursday Afternoon)
Issued at 1204 PM CDT Wed May 14 2025

Scattered to broken cumulus this afternoon is expected to
dissipate by late this evening. Cannot completely rule out a
shower or weak thunderstorm through this evening, but chances of
anything hitting a terminal are quite low and not enough to
mention a PROB30 or VC group. Southeasterly winds and weak gusts
will subside by this evening and veer slowly with time. By late
Thursday morning, winds will become more out of the
south/southwest and increase in speed with gusts to near 20 knots.

Gosselin

&&

.CLIMATE...
Issued at 227 PM CDT Mon May 12 2025

Record maximum temperatures and warmest minimum temperatures
for May 15.

                MAY 15

ST Louis:
MAXIMUM         94 1944
WARMEST MIN     72 2013


Columbia:
MAXIMUM         90 1944
WARMEST MIN     71 1941


Quincy:
MAXIMUM         93 1944
WARMEST MIN     69 1962


&&

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

$$

WFO LSX