963
FXUS63 KLSX 181734
AFDLSX

Area Forecast Discussion
National Weather Service St Louis MO
1234 PM CDT Tue Aug 18 2026

.KEY MESSAGES...

- Widespread showers and thunderstorms are forecast late tonight
  into Wednesday morning. Storms are likely to weaken as they
  head into the area late tonight, but one or two may contain
  gusty winds in central/southeast Missouri.

- Additional showers and thunderstorms are possible (30-50%)
  Wednesday afternoon and evening. There is a conditional threat
  for severe thunderstorms capable of producing damaging winds
  in southeast Missouri, but better chances are to our south.

- Near to slightly below normal temperatures are forecast
  through this weekend.

&&

.SHORT TERM /THROUGH WEDNESDAY NIGHT/...
Issued at 321 AM CDT Tue Aug 18 2026

A 1016+ hPa surface high is currently centered in eastern
Missouri with surface observations showing light/variable winds.
Latest GOES- 19 nighttime microphysics RGB imagery shows a
mostly clear sky, though some increasing cirrus is coming in
from the northwest. This cirrus debris comes from ongoing
convection in eastern Nebraska, which is moving southeast. The
nighttime microphysics RGB also shows expanding river valley
fog, with most basins now entrenched in fog in central/east
central/southeast Missouri. This fog may continue to expand in
the next few hours to additional river valleys. Any fog should
be quick to evaporate this morning, likely within an hour or so
after sunrise. The remainder of the day should see increasing
mid/high level clouds from the northwest downstream of the
aforementioned convection. This activity is expected to weaken
and likely dissipate completely later today due to a waning low-
level jet (and associated moisture convergence) and a much more
stable environment downstream. That being said, a few isolated
showers (or even sprinkles) may make it as far southeast as
parts of northeast Missouri and west-central Illinois this
afternoon.

Convective initiation is forecast across eastern Kansas by mid-late
evening as a midlevel shortwave trough approaches from the mid-
Missouri Valley and moisture convergence strengthens on the nose of
a 30-35 knot southwesterly low-level jet. Widespread showers and
thunderstorms are then expected to move east/southeast across parts
of central, east central, and southeast Missouri after midnight.
Instability continues to be a limiting factor in our area,
with mean MUCAPE values from the EPS only near 1000 J/kg. In
addition, model soundings show a pretty good inversion near the
surface with convection based near 700-800 hPa. Effective shear is
relatively modest, only in the 15-20 knot range. That likely is too
low to keep convection organized as storms head east/southeast into
parts of central Missouri just after midnight. I cannot completely
rule out a marginally severe storm or two in parts of
central/southeast Missouri late tonight, but better chances are
across eastern Kansas and far western Missouri. IF any storm does
manage to be strong to severe in our area, small hail and gusty
winds are the main threats.

Locally heavy rainfall is also a concern late tonight with
precipitable water values expected to be around 1.8" (>90th
percentile of climatology), anomalously strong v (southerly)
component of the low-level jet (~90th percentile), and the
possibility of some training. The main limiting factors are concerns
on how deep and organized the convection will be in our area along
with a somewhat limited duration of showers and storms (~5-6 hours)
in any given location. The LPMM of the HREF and RRFS both show a WNW-
ESE axis of 1-2+" amounts, with some spots up to 3". This sounds
reasonable, but even the higher end possible amounts would be below
6-hr flash-flood guidance. Therefore, no flash flood watch has been
issued but I cannot rule out an isolated instance of flash flooding
somewhere in central/southeast Missouri.

There should be widespread ongoing showers and thunderstorms across
east central and southeast Missouri, as well as southwest Illinois
Wednesday morning. The coverage and overall strength should wane at
least slightly as the low-level jet abates. However, scattered
showers may percolate through the afternoon across southeast
Missouri and southwest Illinois north of a quasi-stationary front.
This is where low-level convergence is strongest with another
midlevel shortwave trough approaching from the west. This
continuation of shower activity and associated cloud cover is very
likely to help keep instability down. Morning convection also should
help keep the surface frontal boundary to our south. There are some
uncertainties with the exact position of the frontal boundary, but
CAMs likely have the right idea being more toward northern
Arkansas/TN given the widespread convection expected Wednesday
morning. That surface frontal zone and vicinity is where there is a
severe threat during the afternoon/evening hours, but this is more
likely to occur to our south. The convective mode may include
supercells as well as bowing segments given anomalously strong deep-
layer shear (~40 knots). Damaging winds are the most likely threat,
though large hail and even a tornado or two may be possible as the
frontal zone should have some enhanced low-level shear/helicity.

&&

.LONG TERM /THURSDAY THROUGH TUESDAY/...
Issued at 321 AM CDT Tue Aug 18 2026

(Thursday - Friday)

The frontal boundary should push further to the south by Thursday,
with a weak ridge of surface high pressure moving into the mid-
Mississippi Valley. This should help promote dry conditions with
near to below normal temperatures to end the work week.


(Friday Night - Monday)

Ensemble guidance remains in pretty good agreement on the general
synoptic pattern across the CONUS, with anomalous ridging across
the Rockies/south central Plains and a strong trough moving across
the Great Lakes early this weekend. There are still some subtle
differences amongst membership of the LREF, but it seems likely that
our area will be on the western periphery of a cooler air mass
moving in behind a cold frontal passage Friday night/Saturday. This
front will bring a low chance (20%) for showers and maybe a rumble
of thunder, but temperatures behind it are more uncertain. The
spread between the 25th/75th percentile for 850-hPa temperatures on
the LREF is quite high Sunday into Monday, generally 5-6C. Not
surprisingly, the inter-quartile range of the NBM also shows a large
spread in high temperatures, with most locations in the 6-10 degree
range. For reference, that is roughly the difference between well-
below normal temperatures to slightly above normal values. Monday`s
temperatures may also be impacted by an increase in mid/high level
clouds as well as the potential for showers and thunderstorms as a
midlevel shortwave trough comes across the northeastern periphery of
the ridge. The exact timing and track of this feature of course are
big question marks nearly 7 days out.

&&

.AVIATION /18Z TAFS THROUGH 00Z THURSDAY/...
Issued at 1234 PM CDT Tue Aug 18 2026

Dry and VFR flight conditions will continue through at least this
evening, until showers and thunderstorms develop and track eastward
through the area overnight, mainly along and south of the I-70
corridor. Therefore, confidence in impacts is highest at KCOU, KJEF,
and St. Louis metro terminals; however, KCOU/KJEF is preferred for
relatively stronger thunderstorms that could contain gusty winds.
Thunderstorms will also be capable of MVFR to IFR flight conditions
but flight conditions will generally be improving Wednesday morning
into afternoon as showers and rumbles of thunder gradually decrease
in coverage. Winds will be light outside of thunderstorms.

&&

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

&&

$$

SHORT TERM...Gosselin
LONG TERM...Gosselin
AVIATION...Pfahler