061
FXUS63 KLSX 202308
AFDLSX

Area Forecast Discussion
National Weather Service St Louis MO
608 PM CDT Thu Aug 20 2026

.KEY MESSAGES...

- Another round of fog (some potentially dense) is favored
  tonight/Friday morning, mainly in river valleys.

- Near to slightly below average temperatures will persist
  through the weekend into early next week.

- There is a low (20 percent) chance of showers and a few
  thunderstorms Friday night into early Saturday afternoon.

&&

.SHORT TERM /THROUGH FRIDAY/...
Issued at 225 PM CDT Thu Aug 20 2026

The passage of a subtle upper-level shortwave trough and a weak
boundary across southwestern IL/southeastern MO has been sufficient
for isolated showers to develop this afternoon, dissipating/drifting
southeastward of the CWA by this evening. Otherwise, a high pressure
center across the Midwest and Great Lakes will continue building
into the CWA tonight into Friday morning, contributing to conditions
favorable for efficient radiational cooling and fog with clearing
skies and residual moisture. Fog will be most widespread and dense
in river valleys with low temperatures around 60 F providing enough
air-water temperature difference for steam fog development in those
locations as well.

Fog will dissipate within an hour or two of sunrise Friday morning.
As the surface high pressure center begins retreat to the northeast,
weak low-level WAA and strong insolation will lead to warmer high
temperatures on Friday, closer to average in the mid to upper
80s F.

&&

.LONG TERM /FRIDAY NIGHT THROUGH THURSDAY/...
Issued at 225 PM CDT Thu Aug 20 2026

Late Friday evening into night, an upper-level trough entering the
Great Lakes will force a weak cold front into the CWA overnight that
slowly advances southward Friday night through early Saturday
afternoon. Showers and thunderstorms are expected to be ongoing
Friday evening along the front across IA into IL, but conditions
will be less favorable for precipitation along the front overnight
this far southwest with strong large-scale forcing passing to the
northeast, a weak capping inversion, and weak frontal convergence.
However, another opportunity for showers and thunderstorms comes
with weak mid-level FGEN and upper-level diffluence extending from
the Central Plains into MO later in the night into Friday morning.
Effective deep-layer wind shear of 35 to 50 kt, steep mid-level
lapse rates, and 1500 to 2500 J/kg MUCAPE suggest at least a low
threat of large hail could exist with these thunderstorms, but
significant questions regarding the coverage and large-scale forcing
mechanisms to support thunderstorms locally preclude any messaging
of threats at this point. With the front lingering near southeastern
MO into early Saturday afternoon, the potential for thunderstorms
also lingers across that area. However, CAMs have widely different
scenarios for the Friday evening-Saturday afternoon timeframe with
HREF probabilities of measurable rainfall mainly 25 percent or less.

Behind the cold front, anomalous upper-level longwave troughing
across the Great Lakes will facilitate arrival of a relatively more
substantial, seasonably cool and dry airmass into the Mid-
Mississippi River Valley Saturday into Monday with below average
high temperatures mainly in the upper 70s to low-80s F. By Monday
through mid-next week, global model guidance is in better agreement
that longwave troughing will loosen its hold on the Great Lakes
allowing a wavering front to lift northward toward the Mid-Missouri
River Valley. This front will be the favored corridor for rounds of
showers and thunderstorms around an upstream ridge in response to
passing shortwave troughs and/or nocturnal LLJs. There is an east-to-
west gradient in ensemble model probabilities of measurable rainfall
across MO through the period indicating uncertainties in the
northeastward progression of the front toward the CWA, which also
impacts how much temperatures warm next week. In the NBM, there is
at least an underlying warming trend toward average after Monday
with airmass modification.

&&

.AVIATION /00Z TAFS THROUGH 06Z SATURDAY/...
Issued at 602 PM CDT Thu Aug 20 2026

Fog remains a concern tonight at most of the local terminals,
especially those in river vallies. KJEF, KSUS, and KCPS are
expected to see the worst of the conditions, with a low chance
for dense (1/4 mile visibility or less) at these three
terminals. There is a low chance for fog at KSTL, though
confidence in impacts occurring is low enough that it has been
left out of the TAF for now. Once fog dissipates tomorrow
morning, VFR flight conditions are then expected through the
day.

&&

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

&&

$$

SHORT TERM...Pfahler
LONG TERM...Pfahler
AVIATION...Elmore