320
FXUS63 KLSX 180404
AFDLSX

Area Forecast Discussion
National Weather Service St Louis MO
1055 PM CDT Mon Aug 17 2026

.KEY MESSAGES...

- There is a high chance (80%) for scattered showers and
  thunderstorms Tuesday night. The chance for severe
  thunderstorms is very low.

- Another round of showers and thunderstorms (80% chance)
  impact the region Wednesday night.

- Temperatures remain seasonable through this weekend.

&&

.SHORT TERM /THROUGH TUESDAY NIGHT/...
Issued at 153 PM CDT Mon Aug 17 2026

Current water vapor imagery shows that subtle northwesterly flow
stretches from the Northern Plains, through the Midwest, and across
the East Coast. Low-level ridging centered over the Great Plains has
flow closer to the surface more northwesterly, providing weak cold
air advection over the Middle Mississippi Valley. This pattern has
provided the first seasonable day for the CWA in a while, with
temperatures this afternoon topping out in the low to mid 80s for
most locations.

Tonight into Tuesday, the low-level ridge will pivot eastward and
deamplify. This will push the Monday`s post-frontal air mass
eastward and return low-level southerly flow to the region. The
deamplification of the ridge will keep the focus for heat southwest
of the CWA, though portions of central and southeastern Missouri will
see enough warm air advection to push afternoon temperatures to
around 90 degrees.

A series of subtle shortwaves will move southeastward within the
upper-level flow starting Tuesday night. With a warmer air mass in
place across the broader region, instability will have built through
the day ahead of these waves. Deep-layer shear will be seasonably
high, with values around 40 kts favorable for discrete and/or
clusters of supercells over the Central Plains. With the CWA being
on the periphery of the more favorable air mass, confidence is low
in these storms making it into the CWA. A majority of deterministic
soundings show poor low-level lapse rates and/or an inversion that
would inhibit surface-based convection and a severe thunderstorm
threat, so we are not publicly messaging the SPC Day 2 SWO.
Currently, the most likely scenario is that we scattered showers and
weak thunderstorms Tuesday night as the low-level jet veers into the
CWA ahead of a cold front moving southward into the region.

&&

.LONG TERM /WEDNESDAY THROUGH MONDAY/...
Issued at 153 PM CDT Mon Aug 17 2026

Rain chances will decrease during the day on Wednesday as the
leading shortwave pushes into the Ohio Valley. A cold front sinking
southward through the CWA will keep a low chance (20-30%) for rain
going through the day as subtle disturbances ripple overhead.
Another more prominent wave moves through the Middle Mississippi
Valley late Wednesday into Thursday, though guidance varies on the
amplitude of the wave and the position of the front. Both will
impact the chance for a round of widespread rainfall. A front in our
immediately south of the CWA will lead to the focus for rainfall
being mainly along and south of I-70, while if the front is able to
drift well south of the CWA by Wednesday evening, then the focus for
rainfall will be south of the CWA. Currently the common solution
among guidance favors the former, with the wave spawning widespread
convection along and south of I-70. This is reflected in the
forecast, which calls for a 60-80% chance for rain in this part of
the CWA.

For Thursday through the weekend, flow will become increasingly
northwesterly in the wake of Wednesday`s wave, pushing the cold
front well south of the region and ushering in another round of
relatively cool air. Upper-level ridging over the western CONUS and
troughing over the East Coast will keep the CWA beneath
northwesterly flow through the weekend, continuing our stretch of
cooler temperatures. While there is not a consensus on anything
right now, we`ll have to watch for embedded disturbances within the
flow that may bring rain to portions of the area through this part
of the period.

&&

.AVIATION /06Z TAFS THROUGH 12Z WEDNESDAY/...
Issued at 1055 PM CDT Mon Aug 17 2026

VFR flight conditions and light northerly flow are expected prevail
across most of the area for the remainder of the night.  Still
looking at the possibility of MVFR fog across the eastern Ozarks
into southern Illinois, however the probabilities have decreased
somewhat since 00Z this evening.  Still could see visibilities
around 3SM primarily in river valleys and other low/protected
spots.  Any fog that develops should dissipate quickly after
sunrise.  Could see a few showers late in the afternoon across
central Missouri.  Current indications are that any rain which falls
won`t be heavy enough to impact visibilities.  More rain is expected
after 06Z Wednesday, and this looks more likely to impact
visibilities and ceilings.

&&

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

&&

$$

SHORT TERM...Elmore
LONG TERM...Elmore
AVIATION...Carney