619
FXUS63 KLSX 281925
AFDLSX

Area Forecast Discussion
National Weather Service St Louis MO
225 PM CDT Mon Sep 28 2026

.KEY MESSAGES...

- Above normal temperatures are forecast through Thursday ahead of a
  slow-moving cold front.

- Widespread rain with a few embedded thunderstorms is forecast
  late Wednesday night through Friday, with the heaviest amounts
  (2+") expected in northeast Missouri and west-central
  Illinois.

&&

.SHORT TERM /THROUGH TUESDAY NIGHT/...
Issued at 224 PM CDT Mon Sep 28 2026

A weak surface ridge of high pressure is currently located near
the Mississippi River and is forecast to drift eastward
overnight. Some weak return flow is expected in its wake, mainly
across portions of central and northeast Missouri. This return
flow should help keep lows up slightly in these regions, in the
mid to upper 50s. Elsewhere, light/variable winds combined with
a clear sky should lead to favorable conditions for radiational
cooling. Widespread low 50s are expected, with some spots likely
dropping into the upper 40s.

The warmup will continue on Tuesday as low-level warm air advection
strengthens. A mostly sunny sky is also forecast, which should help
high temperatures climb into the low to mid 80s, or about 6-8
degrees above normal for the date. Even milder conditions are
forecast Tuesday night as mid/high level clouds increase from west
to east along with stronger southerly surface winds. Lows are only
expected to drop back into the 60s.

&&

.LONG TERM /WEDNESDAY THROUGH MONDAY/...
Issued at 224 PM CDT Mon Sep 28 2026

(Wednesday - Friday)

Mostly dry weather is expected on Wednesday along with what should
be the warmest day of the week. Temperatures at 850 hPa approach
+18C, or above the 95th percentile of climatology. At least partial
sunshine is also forecast, though there probably will be more clouds
in northeast Missouri and west-central Illinois closer to the
surface cold front. Highs may be closer to 80 degrees in these
locations, but in the upper 80s to near 90 degrees for most of the
rest of the area. While these values should fall several degrees
short of daily records, they would be about 10-15 degrees above
normal for the date and a fitting way to end what will be the
warmest September on record at both KSTL and KCOU.

The chances of rain showers begin to increase on Wednesday in our
far northern counties, but ramp up considerably late Wednesday night
into Thursday as a slow-moving cold front begins to move through our
region. The period of heaviest rainfall is likely to be on Thursday
as a midlevel shortwave trough moves across the region. Strong, deep
ascent is expected along the frontal zone, which should mean
widespread showers along with a few embedded thunderstorms. The
rainfall likely also will be pretty efficient given precipitable
water values over 1.8" (>99th percentile of climatology) and very
deep warm cloud depths (>12 kft AGL). The anomalous moisture is due
at least in part to Hurricane Polo, as its remnants get captured by
the anomalous troughing ejecting out of the desert southwest.
Limited convection should be a deterrent to rainfall rates high
enough to cause flash flooding, but river flooding is definitely
possible. The northern part of the Mississippi and western portion
of the Missouri rivers have the best chance (30-50%) of seeing
flooding from this event, with even northern tributaries (i.e.,
Fabius rivers) all below 20% on the latest Hydrological Ensemble
Forecasting Service (HEFS) outlooks. If the rainfall is heavier
upstream than forecast, there is even a low chance (10-30%) for
moderate flooding along these mainstem rivers. The cold front will
continue to slowly move southeast through the area through Friday
morning, but the mid/upper level forcing is considerably weaker late
Thursday night into early Friday along/behind the boundary.
Therefore, there should be a substantial gradient across the region
in terms of rainfall totals. Probabilistic information from the LREF
shows this gradient fairly well for chances for 1+" (70-40%) and 2+"
(30-5%) from northwest to southeast. Even parts of southeast
Missouri and southwest Illinois though do have about a 70% of seeing
at least 0.50" of rain. This is where dry conditions have been for
~6 weeks, and appreciable rainfall is needed most.

Temperature uncertainty increases on Thursday, as the exact frontal
location and speed will be critical for afternoon highs. Ahead of
the front, another day in the mid to possibly upper 80s are
expected. Temperatures behind the front however will be
significantly cooler, with upper 60s and low 70s most likely. The
front should be southeast of the entire area by late Friday morning,
so much cooler temperatures are expected areawide. Highs in the mid
60s to mid 70s are forecast from northwest to southeast.


(Friday Night - Next Monday)

Seasonably cool temperatures along with mostly dry conditions are
forecast for the weekend into early next week. There is increasing
confidence that the front should push far enough south of the area
to yield these dry conditions. There are some lingering chances for
rain, mainly early this weekend across our southern counties, but
these likely will continue to drop over the next 24-48 hours as
uncertainty with the evolution of the front decreases. Confidence is
pretty high that the front will make it well south of our area due
to the strong (~1026 hPa; >90th percentile) surface high pushing
southeastward out of the northern Plains this weekend. Climatology
also favors cold fronts in early October to push at least into the
mid south.

&&

.AVIATION /18Z TAFS THROUGH 00Z WEDNESDAY/...
Issued at 1205 PM CDT Mon Sep 28 2026

Dry/VFR conditions with light winds and a mostly clear sky are
expected through Tuesday morning.

&&

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

&&

$$

SHORT TERM...Gosselin
LONG TERM...Gosselin
AVIATION...Gosselin