Beaucoup Illinois Weather Forecast Discussion

418
FXUS63 KLSX 082001
AFDLSX
Area Forecast Discussion
National Weather Service Saint Louis MO
301 PM CDT Wed Oct 8 2025
.KEY MESSAGES...
- Mostly dry weather will persist through at least this weekend.
- A warming trend will kick off Friday with highs in the 80s
  beginning Saturday.
&&
.SHORT TERM...  (Through Late Thursday Night)
Issued at 300 PM CDT Wed Oct 8 2025
Surface high pressure currently centered over the Upper Midwest will
push into New England by Thursday night. Despite the high never
centering directly over the CWA, this feature will still have enough
influence to keep both high and low temperatures dampened through
the short term. With cold, dry air settled into the Mid-Mississippi
Valley and efficient radiational cool conditions expected tonight,
lows will tank into the upper 30s to low 40s. Low-lying and
sheltered areas will see the brunt of the cold and can expect to
fall a degree or two cooler than our forecast lows. Conditions will
be more favorable for at least steam fog tonight given the cooler
air and position relative to the surface high. Expect patchy fog
again, especially in river valleys. Similar conditions are expected
for most of the area tomorrow night. The exception will be west-
central Illinois through central Missouri as increased cloud cover
will impede radiational cooling.
In the interim, a cool start to the morning and ongoing cold air
advection will keep high temperatures near average again tomorrow
afternoon. Expect highs in the low to mid-70s with very little
humidity.
Jaja
&&
.LONG TERM...  (Friday through Next Wednesday)
Issued at 300 PM CDT Wed Oct 8 2025
A mid-level closed low will shunt southeastward through the Great
Lakes late this week, dragging a cold front through the Upper
Midwest. Ahead of this feature in the Mid to Upper Mississippi
Valley, southerly low-level flow, warm air advection, and meager
moisture return will establish itself Thursday night into early
Friday morning. The increase in warmth and moisture may provide
enough instability for elevated showers and thunderstorms Friday
morning in portions of west-central Illinois southwestward through
central Missouri. The mitigating factor to any precipitation will be
moisture return. Without sufficient moisture, 1) instability will be
lacking and 2) anything that does develop will be quickly snuffed
out by the dry environment they`ll exist in. The latest ensembles
suggest there may be a middle ground in which there is enough
moisture to promote elevated convection but not enough to sustain it
for long. This is represented by an increase to a 45 - 80%
probability of measurable rain on Friday with a relatively tight
gradient decreasing from west to east. If it does rain, these same
probabilities only call for a 20 - 30% chance of >0.1" of 24 hour
precipitation, so it will not be beneficial. This is our greatest
chance of rain through at least this weekend.
The same warm air advection promoting rain chances on Friday will
aid in our return to above normal warmth Friday through early next
week. Mid-level ridging will dominate our sensible weather, allowing
us to heat without the threat of precipitation at least through the
weekend. Precipitation chances do increase slightly again early next
week as shortwaves ride the ridge into the Mid-Mississippi Valley,
but differences in the timing, strength, and location of these
features preclude high confidence in any one solution.
Jaja
&&
.AVIATION...  (For the 18z TAFs through 18z Thursday Afternoon)
Issued at 1145 AM CDT Wed Oct 8 2025
Both steam fog and radiation fog are expected tonight, so
included fog at all river valley terminals. With fog being as
thick as it was at KSUS this morning and even more favorable
conditions for it expected tonight, I think IFR to LIFR
visibilities are justified. Outside of fog, sub-10 kt winds will
persist through the TAF period with northeasterly winds veering to
east-southeasterly by 18z Thursday.
Jaja
&&
.LSX WATCHES/WARNINGS/ADVISORIES...
MO...None.
IL...None.
&&
$$
WFO LSX

NWS LSX Office Area Forecast Discussion