O'Fallon, Missouri 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

099
FXUS63 KLSX 042246
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
546 PM CDT Sat Jul 4 2026

.KEY MESSAGES...

- The threat for scattered showers and thunderstorms will continue through
  tonight. A few storms will be capable of damaging wind gusts
  for areas south of I-70 through early evening.

- Temperatures remain warm, but below dangerous heat levels
  through next week.


&&

.SHORT TERM...  (Through Late Sunday Night)
Issued at 359 PM CDT Sat Jul 4 2026

A potent MCS is ongoing across portions of eastern Missouri, and is
tracking along an outflow boundary from convection last night. While
deep-layer shear is a meager ~20 kts, MLCAPE is at 3,000 J/kg per
recent SPC mesoanalysis. With the MCS having a well-established cold
pool, the threat for strong to severe storms will continue through
early this evening east of the MCS for areas mainly south of I-70.

This MCS is scrubbing the atmosphere over much of the area of
instability, leaving a mostly stable environment in place ahead of a
cold front slowly sinking toward the area later this evening and
tonight. While I am keeping a low threat for isolated to scattered
showers and thunderstorms south of the front over much of the CWA
through tonight, a majority of locations will remain rain free once
the MCS clears the CWA to the southeast this evening.

Tomorrow, the front/effective boundary is expected to be south of
the CWA while a series of disturbances rotate around the base of
subtle trough over the Midwest. The positioning of the boundary will
mark the end of our recent stretch of heat, with most locations
topping out below 90 degrees. However, there will still be enough
warmth and moisture to produce SBCAPE around 1,500 J/kg, enough to
combine with the disturbances to produce a low chance (30%) for
isolated to scattered showers and thunderstorms through the day.
With this amount of instability and deep-layer shear forecast to be
around 20 kts, organized severe thunderstorms are not expected.

Elmore

&&

.LONG TERM...  (Monday through Next Saturday)
Issued at 359 PM CDT Sat Jul 4 2026

The axis of the upper-level ridge will be centered over the western
U.S. at the start of the workweek, with a weak trough/cutoff over
the Ohio Valley per guidance consensus. This will support the chance
for scattered showers and thunderstorms across portions of the area
during the afternoon. Cutoffs are notorious for being poorly
resolved by guidance, so while the majority of guidance has this
feature weakening and/or drifting eastward through Tuesday, there is
a low chance that it may still be close enough and strong enough to
produce another round of scattered convection on Tuesday.

Tuesday through Thursday, the upper-level ridge broadens across the
CONUS, but begins to deamplify as a shortwave moves through the
Northern Plains and Upper Midwest. For the Middle Mississippi
Valley, this will lead to a subtle upward trend in temperatures
Tuesday through Thursday. Through this stretch, a majority of
guidance is cooler than the period of heat the area has been
experiencing lately, with temperatures being closer to
climatological normals for highs (around 90 degrees).

The aforementioned shortwave pushes a cold front through the region
roughly late Thursday into Friday. A majority of guidance has upper-
level flow becoming increasingly northwesterly at this point in the
period, but to what degree and how quickly vary. This will have
impacts on the front`s progression, with a more progressive front
favoring a drier CWA late Friday, while a slower front will linger
rain chances into at least late Friday or Saturday morning. With
most global ensemble members showing some flavor of the latter, the
current forecast lingers rain chances into Saturday morning. At the
very least, ensemble membership shows a drop in temperatures with
the FROPA as we head into next weekend, with a 25% chance of highs
running just under seasonal normals.

Elmore

&&

.AVIATION...  (For the 00z TAFs through 00z Sunday Evening)
Issued at 543 PM CDT Sat Jul 4 2026

All showers and thunderstorms are expected to be clear of the
local TAF sites at the start of the forecast period. Winds will go
calm to light and variable early in the period. This may lead to
issues from firework smoke near KSUS, KCPS, and KSTL, but
confidence is low on impacts. There is a low chance for another
round of scattered showers and thunderstorms during the afternoon
tomorrow, but confidence in these directly impacting the local
terminals is very low at this time.

Elmore

&&

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

$$

WFO LSX