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FXUS63 KLSX 140340
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1040 PM CDT Wed Aug 13 2025

.KEY MESSAGES...

- After a a cooler and slightly less humid day on Thursday, heat
  and humidity build back into the area Friday through the
  weekend.


&&

.SHORT TERM...  (Through Late Thursday Night)
Issued at 223 PM CDT Wed Aug 13 2025

Isolated to widely scattered showers and thunderstorms have
impacted portions of southeast Missouri and southwest Illinois
this afternoon. The latest SPC mesoanalysis does show some steep
low- level lapse rates and high DCAPE values. While there is very
little (~10-15 knots) effective shear, one or two storms may get
tall enough to produce some gusty winds via microbursts if they
collapse.

This activity has been mostly tied to the surface cold front, which
is gradually progressing to the southeast. The front should move out
of the area late this evening. Dry weather is favored behind it,
though cannot totally rule out a few showers/sprinkles along the
trailing 850-hPa cold front. In general though, look for decreasing
cloudiness from northwest to southeast overnight. At least partial
clearing and very light winds should give way to some fog
development, mainly in river valleys. Some of the fog may be locally
dense late tonight into early Thursday morning before conditions
quickly improve within an hour or two after sunrise.

Seasonable temperatures with slightly lower humidity values are
forecast on Thursday as a weak surface ridge remains entrenched
across the mid-Mississippi Valley. There is a weak midlevel
shortwave moving across the area and some CAMs suggest maybe an
isolated shower or two, but moisture is more limited and the overall
environment does not look too conducive for measurable
precipitation. This is a signal however that suggests scattered-
broken diurnal stratocumulus/cumulus development however. If this is
widespread enough and/or forms early enough, it could have at least
some impact on afternoon high temperatures. Speaking of which, highs
are forecast to range from the low 80s to near 90 degrees from north
to south.

Thursday night should be a near carbon copy of tonight, with
decreasing clouds and light/variable winds. Low temperatures may be
ever so slightly warmer as the air mass becomes more stagnant. Lows
are forecast to be mainly in the mid to upper 60s, or pretty close
to normal for the middle of August.


Gosselin

&&

.LONG TERM...  (Friday through Next Wednesday)
Issued at 223 PM CDT Wed Aug 13 2025

(Friday - Monday Night)

The well-advertised return of the mid/upper level ridge remains on
track as it builds into the mid-Mississippi Valley. This feature
will help steer any thunderstorm chances away from the area, and
also bring a return to the heat and humidity. The mid/upper level
anticyclone itself is pretty strong for late summer (594+ dm
: >95th percentile of climatology), but the air mass itself is
less atypical with 850-hPa temperatures slightly above +20C (<90th
percentile). Humidity levels, while still seasonably high, also
don`t look like previous rounds of heat we have seen this summer.
Dewpoints in the upper 60s to mid 70s are a bit closer to the
75th percentile of climatology. These lower dewpoints anticipated
with this round also make sense given a bit more dryness observed
over parts of the area the past several weeks and soil moisture
percentiles are lower than a month ago. Peak heat index values
near or slightly above 100 degrees are forecast each day, though
the weekend looks a hair warmer than Friday. We will have to keep
an eye on the potential for heat headlines, but at this point it
appears the threat for widespread magnitude (105+) criteria is low
at this time. It is also now mid August, so tolerance for heat is
higher than earlier in the season. However, vulnerable
populations such as those working outside and living without air
conditioners will still be more prone to heat-related illnesses.
There could be a duration component though, with ~100 heat index
values potentially continuing into Monday for parts of the area.
That will also be something we have to watch over the next few
days.


(Tuesday - Next Wednesday)

Medium range guidance is in pretty good agreement that the mid/upper
level ridge will shift into the intermountain west by Tuesday, with
anomalous troughing digging into southeast Canada near Hudson Bay.
Clusters of the 500-hPa height pattern show some variance of course
in the upper air pattern, mainly with the strength of the
aforementioned troughing in southeast Canada. Regardless, the
pattern definitely appears favorable for a cold frontal passage
Monday night into Tuesday. Behind this feature, temperatures should
drop back to at least near normal levels along with lower humidity.
This pattern has occurred a few times this summer, and past
instances (i.e., early August) have trended cooler as we have gotten
closer. We will see if that happens once again for mid next week,
but at least a brief period of cooler than normal conditions would
not at all be surprising.


Gosselin

&&

.AVIATION...  (For the 06z TAFs through 06z Thursday Night)
Issued at 1036 PM CDT Wed Aug 13 2025

Fog is likely to develop between 07-09Z with with IFR/LIFR
visibilites possible at valley airports between 09-14Z including
JEF/SUS/CPS. Confidence has increased that some MVFR visibilities
in fog may occor at STL, so have also included a TEMPO group
between 09-13Z at that terminal. The fog is then expected to
dissipate after 14z leaving dry and VFR conditions with generally
light and variable winds.

Britt

&&

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

$$

WFO LSX