461
FXUS63 KLSX 121127
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
627 AM CDT Tue May 12 2026

.KEY MESSAGES...

- The temperature peaks in the 80s today ahead of a cold front.
  Scattered showers and an isolated thunderstorm are possible as
  the front moves through this evening. The severe weather threat
  is low.

- After a brief cool down behind the front, even warmer weather
  arrives by this weekend.


&&

.SHORT TERM...  (Through Late Wednesday Afternoon)
Issued at 303 AM CDT Tue May 12 2026

Broad surface high pressure exists across the eastern US, with its
center moving east through the eastern Great Lakes this morning. Low
pressure near Fargo, ND this morning will also push eastward into
the western Great Lakes this afternoon, with a cold front moving
southeast through the Great Plains behind it. Ahead of the front,
southwesterly winds will advect warmer temperatures into our region
today, along with increased moisture. Afternoon highs will be about
5 degrees warmer than they were yesterday, largely in the low to mid
80s under a mostly sunny sky. Dewpoints rise into the low 50s, about
10 degrees greater than prior days. This increased moisture combined
with peak daytime heating will lead to modest instability developing
by this afternoon, especially across central and northeast Missouri.
Deep layer shear is pretty strong with surface to 6KM shear of about
50KT supportive of supercells if a storm were to get going. However,
warming temperatures aloft will also establish a capping inversion
which should prevent storms from forming this afternoon. So while we
cannot rule out a low end severe weather threat today, it is
contingent on storms forming this afternoon and that`s not currently
expected. Latest guidance puts those chances less than 15 percent.

The cold front moves through the region tonight. Low level moist
advection on a southwesterly low level jet will provide a chance for
some scattered showers and perhaps an isolated thunderstorm this
evening. However, latest guidance has come down considerably on the
rain chances. This is in part due to lower expected moisture, as
ensemble mean dewpoint forecasts have decreased by a few degrees
over the last 24 hours of forecasts. By tomorrow morning all areas
will be experiencing northerly winds and cold advection. But with
plenty of sunshine, we`ll still warm into the 70s, right around
normal for this time of year.

Kimble

&&

.LONG TERM...  (Wednesday Night through Monday)
Issued at 303 AM CDT Tue May 12 2026

Surface high pressure associated with the cooler post frontal air
mass will move quickly southeast on Thursday with low level winds
turning southerly and ridging nudging in from the west aloft. We`ll
see increasingly warmer temperatures and more humidity as we head
toward the weekend as the warmth from within the ridge becomes more
firmly established across the region. While this is the high
confidence trend, we are less confident on how extreme the warmth
will be as well as day to day rain chances.

Regarding the warmth, the NBM continues to be among the warmest
sources of guidance through this period. Its entire probabilistic
suite is affected by a bias correction which performs poorly during
the transition seasons. So while the latest NBM continues to
indicate several days of 90 degree heat across the region this
weekend, we remain skeptical of those near-record temperatures.
After collaboration with WPC, official forecast high temperatures
were lowered by about 2 degrees relative to NBM. For comparison, our
last bout of record heat was in late March when St Louis reached 93
degrees. Post analysis shows that 925mb temperatures that day were
+28C which aligns well with the observed high temperature assuming
good mixing. Ensemble mean forecast 925mb temperatures this weekend
are up to 23C indicative of surface temperatures in the mid 80s in
full mixing. There is some inherent uncertainty on those forecast
values, but only the maximum values in ensemble guidance reach close
to 28C on par with that late March warmth. This gives us low
confidence in reaching record hot temperatures this weekend. There
is also the complexity of clouds and rain chances that could prevent
strong heating or full mixing each day.

Speaking of rain chances, the southerly flow pumps Gulf moisture
northward into the region as early as Thursday night. By Friday
dewpoints rise into the 60s and stay that way through the weekend
and into next week. This anomalous moisture means that we`ll see
daily instability developing with the potential for passing waves to
trigger rounds of showers and thunderstorms. Timing and tracking
each of these waves is notoriously difficult further out in the
forecast, and that`s shown in the forecast rain chances of 20 to 50
percent every forecast period from Thursday night through Tuesday.
That doesn`t mean we expect rain every period, but it just reflects
the timing and location uncertainty on each of the triggering waves
while we remain in this warm and humid air mass. While we couldn`t
rule out a severe weather threat developing at some point, current
indications are surface to 6KM shear remains largely below 30KT
through the weekend, limiting the potential for storm organization.
A more robust trough early next week (around Monday or Tuesday) has
the potential to provide a more focused threat for storms with
better shear potential as well.

Kimble

&&

.AVIATION...  (For the 12z TAFs through 12z Wednesday Morning)
Issued at 625 AM CDT Tue May 12 2026

Winds become southwesterly today and gusty. The strongest gusts
will be up at Quincy where gusts to 30KT are possible. Winds
diminish this evening and then turn northerly behind a cold front
overnight. There`s a chance of a shower or an isolated
thunderstorm ahead of the front this evening, but the chances of
impacts at any terminal are too low to include in any TAF at this
time.

Kimble

&&

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

$$

WFO LSX