603
FXUS63 KLSX 211927
AFDLSX

Area Forecast Discussion
National Weather Service St Louis MO
227 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 WEDNESDAY/...
Issued at 220 PM CDT Tue Jul 21 2026

Cooler and drier air is moving into the Mid Mississippi Valley
behind a cold front this afternoon.  Dew point temperatures have
already fallen into the 60s for much of the area north of I-70.
There will still be a chance for thunderstorm development for the
next couple of hours across Reynolds, Madison, and Iron counties
just ahead of the front, but the front should move through by 22-23Z
which will bring the threat for storms to an end.  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 /WEDNESDAY 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 /18Z TAFS THROUGH 00Z THURSDAY/...
Issued at 1225 PM CDT Tue Jul 21 2026

A cold front is moving south across the eastern Ozarks this
afternoon. There is a chance for scattered thunderstorms in
southeast Missouri along and ahead of the front. Any storms that
develop will be capable of producing 40-50kt wind gusts and IFR
visibilities in heavy downpours. Otherwise, VFR flight
conditions and northerly flow is expected to prevail across the
area as high pressure builds in behind the cold front.

&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...Heat Advisory until 7 PM CDT this evening for MOZ041-047>051-
     059>065-072>075-084-085-099.
IL...Heat Advisory until 7 PM CDT this evening for ILZ069-074-079-
     100>102.

&&

$$

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