285
FXUS63 KLSX 220227
AFDLSX

Area Forecast Discussion
National Weather Service St Louis MO
927 PM CDT Tue Jul 21 2026

.KEY MESSAGES...

- Cooler and less humid weather is expected Wednesday and
  Thursday under the influence of a Canadian high pressure
  system.

- The next chance for rain will be late Thursday night through
  Friday as an upper level disturbance moves across the area.

&&

.SHORT TERM /THROUGH THURSDAY/...
Issued at 919 PM CDT Tue Jul 21 2026

A surface cold front has moved through the region and has even
cleared the state of Missouri. Dew points have fallen into the
60s across most of the area, with KUIN approaching the upper
50s. Some high clouds will be passing overhead tonight, but am
expecting lows mostly in the lower to mid 60s.

On Wednesday, surface high pressure will approach from the
northwest with winds out of the north. Dew points will likely
fall into the upper 50s to lower 60s across most of the region.
One item that was noticed on forecast soundings was the robust
cu field forecast on the RRFS and GFS. The HRRR isn`t as bullish
on cloud cover, but taking a look upstream in Iowa on what
happened this afternoon skies will likely be at least partly
cloudy (especially over eastern Missouri and west-central
Illinois. Expect high temperatures in the upper 70s to lower
80s.

The remainder of the short term through Thursday will be quiet
with highs falling to 5-10 degrees below normal in the low to
mid 80s. Dew point temperatures will also be 10+ degrees lower
Wednesday and Thursday in the mid 50s to low 60s in most
locations. This will produce much more comfortable weather in
the wake of our recent heat.

&&

.LONG TERM /THURSDAY NIGHT THROUGH TUESDAY/...
Issued at 220 PM CDT Tue Jul 21 2026

Global guidance brings the upper level low which is currently near
the New Mexico/Arizona border northward around the upper level high
and then into the central Plains Thursday night.  The wave is picked
up in the mean northwest flow aloft and it moves southeast into the
Mississippi Valley by late Friday.  Models are showing some weak to
moderate lo level cyclogenesis over the central Plains in response
to the upper forcing.  The gradient between the high over the Ohio
Valley and the developing low over the Plains produces a 30-40kt low
level jet late Thursday night that shifts into the lower Missouri
and Mid Mississippi Valley Friday.  This results in what is likely
be the best chance for precipitation for our area over the next 7
days.  The highest chance for rain remains along and west of the
Mississippi as high pressure to the east will continue to funnel
drier air into the region which undercuts the moisture convergence
produced by the low level jet.  Clouds and precip also limit daytime
heating on Friday with most locations struggling to reach the the
mid and upper 70s.

The short wave continues to move southeast away from the area Friday
night, and the upper ridge builds east into the Mississippi Valley
Saturday.  This will bring warmer temperatures and higher humidity
back to Missouri and Illinois for the weekend, although there`s some
question as to how fast the warmup will occur.  A little over 60% of
the LREF members keep max temperatures below 90 on Saturday. However
some of the warmer 40% are very warm with highs over 95.  These are
the outliers...though the operational GFS is among them with 850mb
temps around 22C which mixes down to 95F at the surface.  NBM temps
in the mid 80s to around 90 look reasonable given the uncertainty
for Saturday`s highs.  The upper ridge continues building eastward
through the weekend into next week, although deterministic guidance
shows another weak short wave moving across the Mississippi Valley
through Iowa into central Illinois.  This attenuates the ridge
somewhat, but it doesn`t look strong enough to spare our area from
another period of mid 90s temperatures along with low to mid 70s dew
points.  This would put us squarely back into Heat Advisory
territory for most of the area for at least Sunday and Monday if
these conditions are realized.

&&

.AVIATION /00Z TAFS THROUGH 06Z THURSDAY/...
Issued at 611 PM CDT Tue Jul 21 2026

This evening and tonight: A cold front has pushed south of all of
the terminals this evening with VFR conditions in place. A few
transient wind gusts will be possible for the next hour or so as
weak cold air advection persists, but the PBL will likely quickly
decouple with north winds relaxing.

Wednesday: Surface high pressure will move southeast out of
North Dakota with winds from the north around 10 kt. Latest
forecast soundings from the GFS, RRFS, and HRRR all show a
rather robust cumulus field forming in the afternoon and this
makes sense as 850 mb moisture never fully clears Missouri.
Either way, cigs will be VFR.

&&

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

&&

$$

SHORT TERM...BAH/Carney
LONG TERM...Carney
AVIATION...BAH