Area Forecast Discussion

701
FXUS63 KLSX 140603
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
103 AM CDT Tue Oct 14 2025

.KEY MESSAGES...

- Warm and dry conditions prevail through at least Friday, when a
  cold front brings progressively cooler air and increased
  rain/thunderstorm chances (up to 70%) on Saturday.


&&

.SHORT TERM...  (Through Late Tuesday Night)
Issued at 232 PM CDT Mon Oct 13 2025

Fairly abundant cloud cover remains in place across most of the
region, representing the only sensible weather left from the weak
cold front this morning. In the absence of any appreciable mid-
level forcing, those in central/northeast Missouri and west-
central Illinois were teased with only a few hundredths of rain
this morning. Mid-level water vapor imagery from GOES-East now
shows a ridge gradually expanding from the southern Plains into
the Mid- Mississippi Valley. The resultant subsidence will bring
any threat for rain in our forecast area to an end, and shunt the
ongoing showers and thunderstorms in the central Plains to our
north and west. Clouds will gradually dissipate from south to
north overnight as the ridge expands, but will be thick enough to
keep temperatures areawide a few degrees warmer than last night.

The ridge is still forecast to expand northward and strengthen
further, approaching the 99th climatological percentile, on Tuesday.
This will further shunt what weak baroclinic zone that exists
further north out of the region, leaving us with widespread upper
70s to low 80s for high temperatures. What meager uncertainty that
did exist with respect to highs in northern Missouri and western
Illinois is now fairly low with most guidance agreeing on a
stronger, more northward ridge. Winds will generally be out of the
east on the backside of a retreating surface high in the Great
Lakes, but they will be very weak and thus negligible in promoting
any cooler air advecting into the bi-state region. With little
change in the synoptic pattern going into Tuesday night, expect
fairly similar sensible weather (perhaps a hair cooler south of I-70
where clouds will clear out).

MRB

&&

.LONG TERM...  (Wednesday through Next Monday)
Issued at 232 PM CDT Mon Oct 13 2025

The strong upper-level ridge will continue to dominate our sensible
weather in the Mississippi Valley through most of the week, leading
to warm and dry conditions each day. Confidence in a gradual warmup
to widespread low/mid-80s for daily high temperatures is fairly
high (80%) as most available ensemble members depict the ridge
axis settling directly overhead by mid-week. Meanwhile, a longwave
trough currently stalled in the western CONUS will be dislodged
by a potent northern stream shortwave descending from Alaska. This
feature will allow for our ridge to begin eroding through the day
on Friday as the stage is set for our best chance for widespread
rainfall in quite some time. Virtually all depictions of this
late-week system have the upper-level wave meandering east
starting on Thursday, leading to modest cyclogenesis lee of the
northern Rockies. Ensembles continue to vary somewhat on where
this main surface low develops, but all of them track in through
the northern Plains Thursday into Friday and drape a cold front
southwest through the central Plains. Friday will be our warmest
day of the forecast period when we are squarely in the expansive
warm sector of this low, largely due to the uptick in
southwesterly flow throughout the mixed boundary layer aiding in
more pronounced warm air advection.

The upper-level trough begins to split into two distinct waves in
most guidance through the night Friday: one ejecting into the Upper
Mississippi Valley and one drifting through the southern Plains.
From this point forward, considerable uncertainty in the speed and
amplitude of the synoptic wave plague the forecast and muddy any
potential impacts. The southerly winds will continue to stream warm
and gradually-more-humid air poleward into early Saturday, but
likely won`t surge north as most global-scale guidance tends to do
in the presence of the Ozark Mountains. Some deterministic guidance
that handles this moisture return a bit more realistically shows the
instability stunted near the Ozarks and wrapping around through
Kansas City/up the Mississippi River. Even if we do get the quick
surge of moisture and instability along and ahead of the approaching
front, there is increasing confidence that rain will be ongoing
across most of the region Saturday morning. Increased diffluent flow
ahead of the southern upper-level wave will provide modest lift and
interact with a nocturnal low-level jet and more abundant moisture
(integrated WV transport near the 90th climatological percentile) to
promote cloud cover and stratiform rain into Saturday morning. If
this rain is ongoing through Saturday morning, as an increasing
share of the ensembles show, convection rooted in the mixed layer
will be difficult and the severe weather threat will be limited to
our far southern counties at best.

If THAT limiting factor doesn`t occur, the orientation of the deep-
layer shear is also not promising for discrete strong convection.
While strong (40-60kts in most guidance), most depictions show the
vectors nearly parallel to the front, supporting quick upscale
growth and disrupted updrafts. All told, there is far more to
support widespread beneficial rain on Saturday than there is to
support a severe weather threat in the Mid-Mississippi Valley as it
currently stands. While rain chances are as high as 70% at some
point on Saturday areawide, the best chances for higher totals will
be closer to southeast Missouri and southwest Illinois where
moisture will be most abundant. NBM probabilities of 1.00 inches are
only about 30% in this area, but I`d say that`s a reasonable total
on a local scale. This wave won`t be a drought buster, particularly
because of its seemingly-progressive nature, but it`ll be a good
start for some. One aspect of the forecast that will be important to
monitor is the speed of the wave/front itself: if it`s slower, pre-
frontal rain may be more limited and instability would be more
abundant in the afternoon for stronger thunderstorms. Rain chances
jumped notably for Saturday (and dropped Sunday and late Friday)
with this forecast issuance as guidance comes into greater clarity
on timing.

While exactly how cool we get, and for how long, is not clear, this
front will usher in a reprieve from the unusual mid-October warmth
for Sunday and Monday. The envelope of ensemble members and NBM
distribution do show a modest warmup going into next week, but
abnormal warmth seems to evade us and we hover back to near-normal
(upper 70s). Depending on the evolution of the upper-level pattern,
we may remain under some threat for precipitation going into next
week as well.

MRB

&&

.AVIATION...  (For the 06z TAFs through 06z Tuesday Night)
Issued at 100 AM CDT Tue Oct 14 2025

Due to a combination of an early break in the clouds and calm
winds, LIFR fog has developed earlier than expected at JEF.
Visibilities are likely to fluctuate here through early morning,
and there is some VFR stratus to the west that may also move into
the area during the morning hours and affect the coverage of fog.
We may also see some brief visiblity drops and/or low stratus at
COU overnight and early this morning.

Otherwise, VFR conditions are expected at all other terminals
through the period, aside from a low probability of patchy fog at
SUS/CPS early this morning.

BRC

&&

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

$$

WFO LSX

NWS LSX Office Area Forecast Discussion