437
FXUS63 KLSX 171139
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
539 AM CST Mon Nov 17 2025

.KEY MESSAGES...

- A brief period of breezy winds and low humidity will support
  erratic fire behavior in some areas for a few hours this
  afternoon.

- Shortly afterward, confidence is high (60 to 80%) that a round
  of showers and a few thunderstorms will move through the area this
  evening and overnight, especially in northeast Missouri and
  Illinois. Severe storms are not expected, but small hail is
  possible.

- Widespread rainfall is a near certainty (90+%) Wednesday night
  through Friday night. Rain amounts are likely to be substantial
  but largely beneficial, with a 40 to 80% chance of at least 1
  inch area-wide.


&&

.SHORT TERM...  (Through Late Tuesday Afternoon)
Issued at 246 AM CST Mon Nov 17 2025

A slowly approaching low pressure system will bring some notable
changes to area weather over the next two days, including the
potential for briefly elevated fire weather conditions this
afternoon, followed shortly by a round of showers and a few
thunderstorms overnight.

1) Elevated Fire Weather Potential

While moisture is on the way later today, a very dry airmass remains
in place early this morning, and it will be difficult to dislodge.
Over the course of the day, a compact but potent shortwave will move
through the Central Plains. While both this feature and its
associated surface low will be in a slow weakening phase as they
arrive, breezy southeasterly flow will develop during the day
nonetheless. This will begin to draw increasing humidity into the
area, but this process will be very slow, and much of the initial
moisture advection will occur in the mid and upper levels before it
actually impacts the boundary layer. As such, there is increasing
confidence that there will be a 2 to 5 hour overlap of sufficient
wind speeds (10-14 kt sustained) and low relative humidity (20 to
30%) to support erratic fire behavior. There will be a couple of
mitigating factors, including relatively cool surface temperatures
(mid 50s to low 60s) and increasing high clouds, but ultimately the
degree of dryness and increase in wind speeds has tilted us in favor
of messaging a brief window of elevated fire weather conditions this
afternoon.

2) Showers and Thunderstorms

Near sunset, a robust low level jet will spread northeastward ahead
of the advancing low, rapidly increasing both moisture transport and
isentropic ascent. As this occurs, showers and likely a few
thunderstorms are expected to develop along the nose of this
feature, and quickly move northeast through the area during the
evening and overnight while expanding in coverage. While all areas
may see some rain, highest rain probabilities remain across
northeast Missouri and much of Illinois, where there is a 50% or
greater chance of seeing a "wetting" rain (1/10 inch or more). In
other words, expect a brief period of rain this evening, but not a
washout. We may also see some lingering drizzle through the night
behind the initial round of showers.

Meanwhile, model guidance continues to advertise a plume of elevated
instability developing this evening, with MUCAPE values remaining in
the 500-1000 J/kg range and modest but sufficiently steep mid-level
lapse rates in the 6 to 7 C/km range. Meanwhile, plenty of effective
wind shear will be in place to support organized updrafts, although
the elevated nature of storms should limit their ceiling. All things
considered, it would not be surprising to see some stronger cores
that are capable of producing small hail.

3) Quieter Tuesday

By Tuesday morning, the surface low will be knocking on the door of
central Missouri, and will slowly move from west to east roughly
along the I-70 corridor throughout the day. Ahead of this low,
surface frontogenesis will produce a sharpening warm front, with
very warm temperatures south of the boundary and relatively mild
temperatures to the north. Model guidance continues to lift this
boundary north of St. Louis during the day, although the
predictability of such features is notoriously low. A few
redeveloping showers and thunderstorms will be possible during the
afternoon along this boundary, although model soundings indicate
that a stout cap will make this difficult. If this occurs, it will
be most likely across southwest Illinois. Otherwise, expect a warm,
humid, and mostly dry day overall Tuesday.

BRC

&&

.LONG TERM...  (Tuesday Night through Sunday)
Issued at 246 AM CST Mon Nov 17 2025

After a brief lull in precipitation Tuesday through Wednesday
afternoon, additional rounds of rain are expected between Wednesday
night and Friday night thanks to the arrival of a much more dynamic
weather system. Not only is widespread rain a near certainty (90+%),
but many areas are in line to see a beneficial soaking, and have at
least some potential for more impactful totals as well.

Between Wednesday and Friday, a shortwave trough will slowly pivot
northeastward from the Southwest and into the Central Plains / Mid-
Mississippi River valley. Ahead of this trough, a deepening surface
low will develop and move along a stalled frontal boundary, with a
steadily strengthening low level jet ahead of it. The latter feature
will drive a prolonged period of robust moisture transport and
isentropic ascent, with ensemble mean precipitable water values
reaching or exceeding the the 97th percentile (per the NAEFS/ECMWF)
by Thursday. As such, a broad area of precipitation is expected to
develop to our southwest Wednesday and both expand and strengthen as
it spreads northeast and into our area. Latest model trends suggest
that the onset of rainfall is drifting later into Wednesday night,
with the heaviest rain falling between Thursday afternoon and Friday
morning when the low level jet is at its strongest. Ensemble data
then suggests that precipitation rates will diminish through the day
Friday, and come to an end by Saturday morning. There remains non-
trivial variability in the timing among members, but this window has
been slowly narrowing over time.

As far as amounts, NBM precipitation probabilities remain relatively
steady in latest runs, with a roughly 40 to 80% chance of at least 1
inch of rain area-wide, and the highest probabilities over the
southern half of Missouri and southwest Illinois. However, we also
note that the probabilities of 2 inches of rain or more has dropped
slightly to around 20 to 40%, which is likely reflective of the
delayed precipitation onset and lower rain totals early on in the
event. It should also be noted that these probabilities appear to be
a bit washed out, and there are considerable differences in rain
totals among various LREF clusters. All told, there is still quite a
bit of uncertainty regarding where the heaviest rain will fall, even
though most areas will see a soaking, beneficial rain. Meanwhile, we
can`t completely rule out some localized flooding, but latest
guidance has trended slightly away from this possibility. We also
continue to monitor the potential for thunderstorms, although
surface-based CAPE probabilities remain very low and this remains
a lesser threat at the moment.

Finally, temperatures are likely to remain seasonable to above
normal in spite of the persistent cloud cover and precipitation.
This isn`t the focus of this forecast, but is noted to rule out any
meaningful potential for wintry precipitation. A cold front will
drive a modest cooling trend over the weekend, which continues to
trend dry as well.

BRC

&&

.AVIATION...  (For the 12z TAFs through 12z Tuesday Morning)
Issued at 532 AM CST Mon Nov 17 2025

VFR conditions with breezy southeast winds are expected from the
start of the period through mid to late afternoon. VFR stratus
will overspread all terminals from late afternoon through the
overnight hours, with a round of showers expected as well. These
showers will be most prevalent at UIN and St. Louis area
terminals, with the heaviest activity over a brief period near and
shortly after sunset. A few thunderstorms will also be possible,
with short bursts of heavy rain and even some small hail in the
strongest cores.

Showers will weaken overnight, but may linger through the end of
the period. Some drizzle is also possible near the end of the
period, along with some MVFR to IFR stratus.

&&

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

$$

WFO LSX