O'Fallon, Missouri 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

949
FXUS63 KLSX 200412
AFDLSX

Area Forecast Discussion
National Weather Service St Louis MO
1112 PM CDT Wed Aug 19 2026

.KEY MESSAGES...

- More showers and thunderstorms are expected across portions
  of the area this evening, including a conditional threat of
  severe thunderstorms with a main hazard of damaging winds.
  This treat is greatest across southeastern MO/southwestern IL
  and northeastern MO/west-central IL.

- Near to slightly below average temperatures will continue
  through at least the weekend.&&

.SHORT TERM /THROUGH TONIGHT/...
Issued at 235 PM CDT Wed Aug 19 2026

A rather complex and low-confidence forecast is apparent through
this evening regarding the potential for severe weather, owing to
uncertainties and nuances in the recovery of sufficient instability
amidst considerable lingering cloud cover and scattered showers.

The first area of interest involves a weak surface low pressure
center across southwestern MO with a remnant outflow boundary/warm
front extending south/southeastward, which are both progged to lift
east-northeastward and be catalysts for thunderstorm development
through this evening. Given the consistency of latest CAMs there is
growing confidence that these features will facilitate thunderstorm
development at least across southeastern MO this evening. Deep-layer
wind shear of 35 to 45 kt favors a mixed mode of supercells
transitioning to line segments with time, but it is still uncertain
if instability will increase enough (CINH decrease enough) for more
robust, surface-based thunderstorms locally. Instability has been
increasing across southwestern MO, but this process has been much
slower to occur across southeastern MO so far, thanks to lingering
cloud cover. If thunderstorms can become surface-based, damaging
winds are the main hazard, followed by marginally severe hail as
well as a brief tornado or two considering enhanced low-level wind
shear and ambient vorticity near the surface low and warm front.
With time, these thunderstorms would propagate east-southeasward out
of the CWA later this evening.

Another, secondary area of interest is associated with a compact MCS
tracking southeastward out of southeastern NE that is forecast to
track more eastward with time as it navigates broadly cyclonic mid-
level flow and follows another weak cold front drifting southward
into northern MO. Downstream of this MCS, the back edge of thicker
cloud cover is advancing eastward which should help destabilization
occur across at least northern MO, supporting maintenance of the MCS
into northeastern MO this evening. However, this MCS may become
slightly elevated and begin weakening through that time as it
encounters lesser instability to its southeast and with overall
nocturnal stabilization after sunset. The main hazard with this MCS
would be damaging winds, if it can remain organized and surface-
based when it reaches the CWA. Some additional thunderstorms may
develop along its trailing outflow, but those would be even more
likely to be elevated.

A gradual decrease in showers and thunderstorms is anticipated this
evening as coverage decreases and most activity shifts to the east
and southeast where the strongest forcing associated with the LLJ is
focused. With clouds also expected to decrease and moist boundary-
layer with weak flow to exist there is conditions will be favorable
for the development of fog in some areas. However, the exact cloud
cover evolution and preference for fog versus low stratus is not
completely certain. Dense fog cannot be ruled out, especially if
clearing occurs over locations that receive rainfall this evening
and have had limited sun today.

&&

.LONG TERM /THURSDAY THROUGH WEDNESDAY/...
Issued at 235 PM CDT Wed Aug 19 2026

Any fog Thursday morning will dissipate within a few hours of
sunrise and as a drier post-frontal airmass slowly infiltrates the
region as a surface high pressure center enters the Mid-Mississippi
River Valley. Although the airmass will also be seasonably cool,
greater insolation will lead to much of the CWA seeing high
temperatures warmer than today and in the 80s F. The high pressure
center will dominate the region into Friday morning, providing
favorable conditions for at least additional patchy fog in river
valleys late Thursday night/Friday morning.

In prevailing upper-level northwesterly flow, a shortwave trough
will force a weak cold front through the Mid-Mississippi River
Valley sometime in the Friday evening-Saturday morning timeframe.
Given limited moisture return, weak frontal converge, and frontal
passage out of phase with peak daytime heat; only scattered showers
and a thunderstorm or two is forecast. Accordingly, ensemble model
guidance has 20 to 30 percent probabilities of measurable rainfall
with the front. The main impact of the front will instead be another
seasonably cool post-frontal airmass arriving in its wake, leading
to continued near to slightly below average temperatures at
least through the weekend.

Sunday through early to mid-next week, upper-level northwesterly
flow will continue, but global model guidance have slight
differences in the position of longwave troughing over the Great
Lakes and ridging across the Rocky Mountains into Southern Plains
and which feature has greater influence on the Mid-Mississippi River
Valley. Between these features, there will be a wavering baroclinic
zone separating a hot airmass to the southwest and a seasonably cool
airmass to the northeast, resulting in the NBM interquartile range
spanning below average to above average temperatures (5 to 8 F
difference). To further complicate the temperature forecast,
episodic showers and thunderstorms are favored to develop/track
along the baroclinic zone at times from Sunday night into early
Tuesday as multiple shortwave troughs pass.

&&

.AVIATION /00Z TAFS THROUGH 06Z FRIDAY/...
Issued at 1112 PM CDT Wed Aug 19 2026

A remnant mesoscale convective system is pushing eastward from
Missouri to Illinois with thunderstorms currently impacting KUIN.
This activity is expected to exit KUIN before 06z. Behind and to the
south of this system, clearing is ongoing across central/east-
central Missouri, which has allowed patchy fog/low stratus (<1kft)
to begin forming. As clearing continues to expand eastward, the
expansion of fog/low stratus is expected at all terminals. Guidance
indicates the formation of a scattered to broken LIFR stratus deck
around 200-500 ft AGL along with widespread and somewhat dense fog
resulting in LIFR visibilites, potentially below airport minimums.
This is forecasted to impact the region from roughly 06-12z with
gradual clearing  after sunrise resulting in improving cigs/vis with
return to VFR expected at all terminals by the late morning or early
afternoon at the latest. Surface winds will be calm and variable
through the morning with northerly winds slowly veering to easterly
by tomorrow afternoon.

&&

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

&&

$$

SHORT TERM...Pfahler
LONG TERM...Pfahler
AVIATION...Peine