067
FXUS63 KLSX 110237
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
937 PM CDT Sun May 10 2026

.KEY MESSAGES...

- The chance for a brief round of showers and thunderstorms has
  increased (30-70%) late Tuesday afternoon and evening. Severe
  storms are not currently expected, but there is a limited
  potential for small hail and gusty winds.

- Temperatures will fluctuate tomorrow through Wednesday, but a
  prolonged period of above average temperatures is expected
  Thursday through the weekend. However, Confidence is low
  regarding just how far above average these temperatures will
  reach, due in part to increasing weekend shower chances.


&&

.SHORT TERM...  (Through Late Monday Night)
Issued at 151 PM CDT Sun May 10 2026

While a few high-based and very light showers continue to linger
across parts of the area, largely dry conditions can be expected in
the short term period, with chilly morning lows tomorrow followed by
a short warm-up and breezy winds Tuesday. While a few showers and
thunderstorms may develop Tuesday night, this will be discussed in
more detail in the long term section.

As of early this afternoon, most areas have dried out in the wake of
last night`s cold front, although perhaps not entirely. The lone
exception to this is a narrow corridor of high-based and very light
showers moving along the I-70 corridor, which have amounted to
little more than a few sprinkles and virga. While these may persist
another couple of hours, and we may see a few sprinkles moving into
the Ozarks later in the afternoon/evening, dry conditions will be
the rule. Meanwhile, temperatures noticeably cooler today, and are
running about 5 to 10 degrees below yesterday`s values at the same
time. This is especially the case where clouds have persisted
through the day. Overnight, surface high pressure will settle into
the area, allowing skies to clear and winds to go nearly calm. While
low humidity is in place, we can`t rule out a few patches of shallow
fog in river valleys and low-lying areas. Expect chilly morning
temperatures in the 40s area-wide, with the lowest values in river
valley bottoms.

Surface high pressure will remain in place tomorrow, maintaining low
humidity and very light winds. Mostly clear skies will also allow
for almost full sun throughout the day, and will help temperatures
to rebound nicely into the low to mid 70s. This is very near the
average high temperature for this time of year, and this should make
for a very pleasant afternoon.

BRC

&&

.LONG TERM...  (Tuesday through Next Sunday)
Issued at 151 PM CDT Sun May 10 2026

On Tuesday, a shortwave and surface low will move into the upper
Mississippi Valley, which will increase our local pressure gradient
Tuesday afternoon and send a cold front southward and into our area
later in the evening. Ahead of the font, breezy southwest surface
winds will drive temperatures back upwards and into the upper 70s to
low 80s, and will also gradually transport increasing moisture
content northeast as well. While humidity will remain generally
low during the day, a narrow corridor of pooling moisture will
occur ahead of the advancing front, and is expected to produce a
correlated area of modest instability from southwest MO to west-
central IL. Model forecast soundings within this area suggest
rather high cloud bases and also a layer of warm mid-level
temperatures, neither of which is particularly conducive to
widespread rain. However, most models now initiate convection
along this boundary sometime between late afternoon and early
afternoon, and precipitation probabilities have risen to around
30-70%.

While there is growing evidence that a narrow corridor of showers
and perhaps a few thunderstorms will develop, it remains to be
seen whether this activity will be surface-based or largely
elevated. Model forecast soundings reveal the presence of
substantial convective inhibition, which suggests that the latter
is most likely and lowers the already-low ceiling for stronger
storms. On the other hand, there is fairly substantial bulk shear
(45-55kt 0-6km) and just enough projected MUCAPE (250-1000 J/kg)
to support a few organized thunderstorms, and we can`t rule out
some bouts of mostly small hail and gusty winds, the latter of
which will depend largely on whether storms are surface-based. If
they are, they may be able to tap into steep low-level lapse
rates, much like the storms last evening that produced wind gusts
in the 40 mph range. At the present time we don`t think that
severe-level storms (1" hail or 60mph winds) are probable, but
given the ample shear and increasing model signal for convection,
this will need to be monitored.

Overnight Tuesday, showers and lingering storms will move southeast
along with the cold front, and should end before sunrise Wednesday.
Another modest cooldown is expected Wednesday behind the passing
cold front thanks to breezy northwest winds, but guidance has
trended up slightly with afternoon temperatures in spite of this.
While there may be a wide spread of temperatures from southwest to
northeast, the average will likely fall somewhere near average
for this time of year.

From Thursday onward, the main forecast challenge continues to
revolve around the evolution of a building upper level ridge across
the central CONUS, as this will have implications for both
temperatures and precipitation chances. While confidence is high
that a prolonged period of above average temperatures will begin
Thursday and continue through the weekend thanks to 90th
percentile ensemble mean 850 mb temperatures, the magnitude of
this warmup remains highly uncertain due to a combination of high
ensemble spread, model bias, and the potential for
showers/thunderstorms on various days within that window. As noted
in previous forecasts, the NBM ensemble envelope remains much
warmer across the board than other guidance, in spite of a very
slight decrease (1-2 degrees) in outlier values. This is still a
full 10 degrees warmer than the LREF (which itself is already 5-10
degrees above average), and places the operational NBM very near
all-time daily high temperature records. We suspect that this is
due to the NBM bias correction, which tends to struggle with
early-season heat events.

Not only this, but cluster analysis reveals significant variations
in the timing and strength of the building ridge, which also has a
direct effect on shortwaves attempting to move across the central
CONUS sometime late in the weekend. While we do expect southerly
flow to resume and and moisture to increase late in the week and
over the weekend, opening the door to shower/thunderstorm chances,
the timing and nature of this activity remains very uncertain. We
could see anything from mostly dry and hot conditions, to perhaps
one (or more) rounds of showers and thunderstorms (even
strong/severe ones) that would likely reduce temperatures. It
should be noted that while there is very little timing agreement,
precipitation probabilities have risen considerably from Friday
onward in LREF guidance, and are considerably higher than NBM
precipitation chances.

Given all of the factors described above, there was a collaborative
effort to reduce forecast temperatures by roughly 5 degrees from the
operational NBM from Friday through the weekend. While we feel this
more closely fits the situation and at least partially mitigates the
(probable) NBM warm bias, there remains a wide range of
possibilities each day, and forecast confidence is unusually low
even at this extended time range. Even with this adjustment, the
resulting "official" forecast remains warmer than just about all
other available deterministic model solutions except the NBM.
Pending the evolution of the upper pattern and rain chances, this
may even need further adjustments in future forecasts.

BRC

&&

.AVIATION...  (For the 06z TAFs through 06z Monday Night)
Issued at 906 PM CDT Sun May 10 2026

Dry and VFR conditions are expected through the period with light
winds.

Britt

&&

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

$$

WFO LSX