Old Jamestown, Missouri 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

641
FXUS63 KLSX 091127
AFDLSX

Area Forecast Discussion
National Weather Service St Louis MO
527 AM CST Mon Nov 9 2026

.KEY MESSAGES...

- Above average temperatures will persist into Wednesday before
  colder temperatures arrive during the second half of the
  week.

- There is a 40 to 60 percent chance of showers, perhaps a
  thunderstorm or two, mainly along/east of the Mississippi
  River on Wednesday.

&&

.SHORT TERM /THROUGH TUESDAY NIGHT/...
Issued at 206 AM CST Mon Nov 9 2026

The Mid-Mississippi River Valley will be positioned beneath west-
northwesterly upper-level flow through Tuesday. A shortwave trough
within this flow will shunt the low-level ridge currently in place
eastward and force a weak cold front into the CWA tonight that
stalls thereafter. Weak forcing and limited moisture return will
preclude precipitation and even much in the way of cloud cover from
forming along the front. Accompanying these dry conditions,
southwesterly to westerly low-level flow and weak WAA will support
well-above average temperatures through Tuesday, including highs in
the mid-60s to low-70s F. Radiational cooling is expected to be
relatively efficient again tonight with a loose surface pressure
gradient near the front and little cloud cover. These conditions
could promote additional patchy river valley fog, but slightly
warmer temperatures will be less favorable for contributions
from steam fog formation processes.

&&

.LONG TERM /WEDNESDAY THROUGH SUNDAY/...
Issued at 206 AM CST Mon Nov 9 2026

A potent Alberta Clipper including an amplifying upper-level
shortwave trough will traverse the Mid-Mississippi River Valley on
Wednesday, facilitating the passage of a cold front through the CWA
trailing a deepening surface cyclone across the Midwest. As is
commonly the case with Alberta Clippers, the majority of associated
precipitation will be to the north of the surface cyclone`s track
and north of the CWA, but 40 to 60 percent of ensemble model
membership has scattered showers developing along/east of the
Mississippi River as moisture and forcing increase along the cold
front. There are still some timing differences, however, that will
influence how far west these showers develop. Steepening lapse rates
with mid-level cooling may sufficiently compensate for modest
moisture return to enable a few thunderstorms to develop as
deterministic model guidance projects a narrow corridor of 500 to
1000 J/kg of MUCAPE. Additionally, a tightening surface pressure
gradient will result in occasional wind gusts of 20 to 28 kt on
Wednesday.

Behind the cold front and initial shortwave trough, longwave
troughing will amplify across the eastern CONUS with deep cyclonic
flow ensuing over the Mid-Mississippi River Valley. As multiple
additional shortwave troughs and/or vorticity maxima rotate around
the parent longwave trough, the pattern appears favorable for
periodic light showers or drizzle through Friday. With uncertain
timing of each feature and a common dry bias in longer-rage model
guidance in these patterns, the forecast is currently dry after
Wednesday. Strong and persistent low-level CAA and increased cloud
cover will at least result in coldest temperatures we have seen so
far this season including highs in the 40s to low 50s F and lows in
the mid-20s to mid-30s F. Confidence is high in Friday being the
coldest day of the period with high temperatures falling to warm out
of the 40s F, followed by a nearby surface anticyclone favoring
widespread low temperatures in the 20s F if clouds clear enough.

Next weekend, global model guidance agrees that mid-level heights
will begin rising and the anticyclone will shift to the east of the
region as downstream longwave troughing progresses eastward. This
pattern change will lead to a warming trend but there are
significant disparities on how quickly this process takes place,
which determine how quickly temperatures will warm. Accordingly, the
NBM interquartile temperature range inflates to 15 F by next Sunday,
although even the 25th percentile still demonstrates an underlying
warming trend.

&&

.AVIATION /12Z TAFS THROUGH 18Z TUESDAY/...
Issued at 526 AM CST Mon Nov 9 2026

Fog will continue expanding in river valleys through sunrise,
threatening river valley terminals with MVFR to IFR flight
conditions including some visibilities even down to 1/4SM. Fog will
dissipate by 15z with VFR flight conditions prevailing thereafter.
At least patchy river valley fog is likely again tonight/Tuesday
morning, but confidence is low in impacts at this early juncture.
Light southwesterly to westerly or variable winds are forecast
through the TAF period.

&&

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

&&

$$

SHORT TERM...Pfahler
LONG TERM...Pfahler
AVIATION...Carney