Area Forecast Discussion

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FXUS63 KLSX 182324
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
624 PM CDT Sat Oct 18 2025

.KEY MESSAGES...

- Rain and thunderstorms continue this evening but exit to the
  east overnight as a cold front slides through from the
  northwest.

- Much cooler and drier air is expected for the coming week. In
  fact, there is the potential for some frost Wednesday morning.


&&

.SHORT TERM... (Through Sunday Night)
Issued at 217 PM CDT Sat Oct 18 2025

Our long awaited rainfall has finally arrived. Moisture surging
northward from the Gulf is interacting with a robust shortwave
trough exiting the Southern Plains producing widespread showers
and storms across southern Missouri and Illinois. The initial line
of storms that arrived late this morning are serving as an
effective northern boundary of the surface based instability, and
thus the severe weather potential, this evening. We believe the
severe weather threat decreases significantly to the north of this
line, and once this line exits our forecast area the severe
weather threat is largely over. However, as the shortwave
continues crossing the region, additional showers and elevated
storms continue to expand across the region this evening before
exiting eastward overnight. Most areas will see between 0.5 and
1.5 inches, but localized swaths of 2 to 4 inches, perhaps a bit
more, are possible in areas that get more persistent heavy
downpours from thunderstorms. With the recent dryness, we do not
expect any significant impacts on rivers, though if it falls fast
enough it could cause local flash flooding issues where storms
linger.

A cold front currently stretched from eastern Iowa to central
Oklahoma will push southeast tonight, shifting winds to the
northwest and advecting much drier air into the region. Dewpoints
fall from the 60s currently to the 30s by midday tomorrow. With
good mixing, temperatures warm into the 60s despite strong post
frontal advection. Those 60s will feel colder, though, after the
recent warmth and with the stiff northwest breeze. The surface
high associated with this air mass moves across the Mississippi
delta region tonight, so our winds decrease somewhat overnight,
but not enough for strong cooling. We`ll largely fall into the
40s despite dewpoints lower than that.


Kimble

&&

.LONG TERM...  (Monday through Next Saturday)
Issued at 217 PM CDT Sat Oct 18 2025

Brief ridging aloft coincides with the surface high shifting east
and a return southwest flow in the low levels on Monday. That will
warm our temperatures up back into the 70s before the next front
crosses through Monday night into early Tuesday. The surface high
to our south will block moisture return ahead of this front, so
there are no significant rain chances despite the frontal passage.
This front is being driven by a trough moving through the Upper
Midwest into the Great Lakes, so it will have a cooler and even
drier air mass behind it. The surface high associated with it
moves out of the Central Plains into the Mississippi Valley on
Wednesday representing our coldest stretch of this forecast. If
conditions line up for a clear sky and light winds either Tuesday
night or Wednesday night then we could see widespread 30s for
lows. This represents our first real chance of frost this season.
NBM probability of 35 or colder temperatures has remained roughly
the same over the past few runs for Tuesday night/Wednesday
morning, but it has increased for Wednesday night/Thursday
morning. This is because there`s a bit more uncertainty on which
night will have the better radiational cooling conditions. If the
surface high crosses our area during the day on Wednesday then
neither night will achieve peak cooling, but if it crosses
primarily on one of those two nights then temperatures will be
even a bit colder, closer to an actual freeze in the typical cold
spots. Urban areas will be substantially warmer.

As we go later in the week we`ll see a return flow on the back
side of the high and ridging in the northern stream allowing for a
modest warm up. But a southern stream trough moving out of the
Southwest US and into the Plains will draw Gulf moisture northward
ahead of it and provide our next best chance for rain. Long range
models have struggled a bit on the timing of this wave, but this
variability has improved a bit. Just yesterday we were looking at
sometime between Thursday and the weekend for this wave`s passage,
but now the time frame has centered more on Friday or perhaps into
Saturday. As a result, our NBM PoP has increased a bit during this
period and decreased on the other end reflecting the better timing
consensus. Based on the track of this wave largely to our south I
would expect more of a winter time setup with a synoptic driven
rainfall rather than a strong thunderstorm setup. Because of this,
rainfall amounts will be relatively light especially compared to
today`s heavier amounts.

Kimble

&&

.AVIATION...  (For the 00z TAFs through 00z Sunday Evening)
Issued at 624 PM CDT Sat Oct 18 2025

As of 23z, thunderstorms have moved east of the metro terminals
with latest observations showing a significant decrease in
lightning production behind the line. The greatest impacts extend
along and just behind the line with conditions ranging from IFR
(due to low ceilings/visibility) to VFR further behind the line.
VFR is interspersed with MVFR ceilings, but visibilities are
improving as heavier rain moves east along with stronger
thunderstorms.

For the rest of this evening, -SHRA/SHRA will be the theme.
Rainfall potential tapers from west to east with improvement over
central Missouri terminals between 04z-08z and the remainder of
the area just prior to daybreak Sunday. Spotty showers are
possible through early Sunday morning, which could extend a few
hours beyond 12z. However, little/no impact should stem from
showers with much drier air filtering in from the northwest.

Gusty winds wrap in behind the exiting system late tonight.
Considering that skies will clear Sunday, deeper mixing will
result in northwesterly winds gusting near 30 knots at times.
The core of the higher winds gradually pull east through the day
Sunday with VFR expected through the remainder of the period.

Maples

&&

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

$$

WFO LSX

NWS LSX Office Area Forecast Discussion