843
FXUS63 KLSX 091952
AFDLSX

Area Forecast Discussion
National Weather Service Saint Louis MO
252 PM CDT Sat May 9 2026

.KEY MESSAGES...

- A brief round of showers and thunderstorms is expected roughly
  along and just north of the I-70 corridor this evening and
  overnight, with the potential for lighting and gusty winds in
  the 40-50 mph range. Severe storms are not expected.

- Fluctuating temperatures are expected for much of the work week,
  but there is some potential for a more noteworthy and prolonged
  warmup Friday through early next week.


&&

.SHORT TERM...  (Through Late Sunday Night)
Issued at 247 PM CDT Sat May 9 2026

After a foggy, cloudy morning skies have cleared throughout the area
as we await a cold front and another round of showers this evening
and overnight. This is the focus of the short term forecast period,
as confidence is growing that we will see a few thunderstorms with
this evening`s activity, although the potential for strong/severe
storms remains low.

As of 1:30 pm, temperatures have risen anywhere from 5 to 15 degrees
higher than yesterday at the same time, thanks to the quick
resumption of southwesterly surface winds in the wake of yesterday`s
weak cold front. As mentioned previously skies are nearly clear
across the entire area, aside from a few stray cumulus clouds here
and there, owing to a relatively dry airmass.

However, another cold front is set to move through our area this
evening and overnight, driven south by a passing shortwave in the
Great Lakes. While moisture remains limited in most areas and Gulf
trajectories are poor, a narrow corridor of shallow moisture
pooling ahead of the advancing cold front will yield a pocket of
1000-1500 J/kg of SB CAPE by late afternoon (although this drops
to only 500 J/kg over the whole mixed layer). Meanwhile, effective
bulk shear is expected to increase to around 30-40kt by sunset,
and this combination of instability & wind shear should be enough
to support some marginally organized thunderstorm activity.

Over the past several iterations of CAMs, there is growing support
for the development of a compact thunderstorm complex right along
the cold front late this afternoon and evening, perhaps beginning in
southeast Nebraska/northwest Missouri over the next few hours. Storm
motions would likely carry this complex into northeast/central
Missouri near or slightly before sunset, and eastward across the
Mississippi River within an hour or two of midnight. While initial
projections for tonight`s activity suggested that this activity
would remain post-frontal and with little threat for
thunderstorms, more recent runs (notably the HRRR) maintain this
complex very near the boundary itself, which gives it a chance to
remain surface-based for a bit longer. If it can, this complex
will have access to at least some surface-based instability for
longer, although it will diminish quickly after dark. As such, we
can`t rule out some gusty winds with this complex, although the
potential for damaging winds (60mph or more) remains quite low.

Convection will weaken after midnight, and precipitation should end
prior to sunrise. A few additional showers will be possible across
the Ozarks in the afternoon, but guidance as continued to push this
activity further and further south, and it is not expected to
provide additional meaningful rain in our area tomorrow.

Otherwise, much cooler temperatures can be expected tomorrow behind
the cold front, with afternoon highs generally in the mid to upper
60s, and chilly Monday morning low temperatures in the low-mid 40s.

BRC

&&

.LONG TERM...  (Monday through Next Saturday)
Issued at 247 PM CDT Sat May 9 2026

Beyond tonight, aside from the potential for a few showers along
another cold front Tuesday night, a dry pattern will develop over
the course of the week, with a potentially significant warming trend
over the latter half of the week and next weekend.

After a chilly morning Monday, clear skies and low humidity at the
center of surface high pressure should allow temperatures to rebound
nicely into the low-mid 70s due almost exclusively to insolation and
deep mixing. This should make for an almost perfect day to head
outdoors, once temperatures warm from their morning lows.

By Tuesday, a surface low moving through the upper Mississippi River
Valley will tighten the local pressure gradient, and drive rather
breezy southwesterly surface winds across the area. As a result,
warm air advection will produce another bump in temperatures Tuesday
afternoon, with highs likely to reach at least the upper 70s to low
80s in most locations. While this will also yield a slight
increase in dewpoints, humidity will remain low overall, and this
will limit the potential for showers/storms along the next cold
front arriving sometime later that night. Once again, some modest
moisture pooling along/ahead of the boundary will support a 20-40%
chance for showers and thunderstorms late Tuesday, but given
these moisture limitations the potential for stronger storms or
widespread meaningful rain remains low.

Wednesday is expected to see another brief cooldown behind this next
cold front, but confidence is high that a building ridge will drive
a warming trend Thursday through the weekend, and possibly beyond.
The magnitude of this warmup remains in question, though, as this is
still a long ways in the future, and there remain considerable
differences in both the strength of the ridge, and the timing of a
departing trough across the eastern CONUS. A more slowly building
ridge (or lingering trough) would lead to a slower or more modest
warmup, while a larger ridge that spreads east more quickly would
produce temperatures on the warmer end of the ensemble envelope.

Meanwhile, there are considerable differences among both individual
ensemble members and entire ensemble suites when it comes to surface
temperature forecasts. The NBM is exceptionally warm, with ensemble
median temperatures reaching the low to mid 90s Saturday through
early next week, and 75th+ percentile outliers reaching and even
exceeding 100 degrees for several days in a row. While well above
normal temperatures are well within reason, this is 10 to 15 degrees
higher than LREF spreads, and we suspect that the NBM is
experiencing a bit of an early-season warm bias. As such, the
current operational NBM forecast is likely overdone on temperatures
later in the week, and may need to be manually adjusted in the
coming days if this discrepancy/warm bias continues to
significantly exceed other available guidance.

Finally, persistent southerly flow and improved Gulf moisture
trajectories will bring higher dewpoints back into the area late in
the week and over the weekend. While the signal for precipitation
remains relatively low, there is a notable increase in instability
projections, and also the potential to be grazed by additional
shortwaves/weak cold fronts depending on the strength of the ridge.
As such, we will need to keep an eye on the potential for
thunderstorms over the weekend.

BRC

&&

.AVIATION...  (For the 18z TAFs through 18z Sunday Afternoon)
Issued at 1131 AM CDT Sat May 9 2026

While VFR categories are expected to prevail for a majority of the
period, there are a few weather items to note. After mostly clear
skies and breezy west winds during the day today, a cold front
will bring a round of scattered showers and a few thunderstorms
this evening and overnight. While most terminals will see a brief
period of rain, confidence is lower that thunderstorms will occur.
The latter is most likely at COU/JEF, followed closely by St.
Louis area terminals.

Ceilings are more likely than not to remain above VFR levels, but
brief dips to MVFR are possible within stronger showers and behind
a passing cold front. This front will also switch winds to the
north this evening, and light north winds will persist through the
end of the period.

BRC

&&

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

$$

WFO LSX