180
FXUS63 KLSX 210737
AFDLSX

Area Forecast Discussion
National Weather Service Saint Louis MO
237 AM CDT Thu Aug 21 2025

.KEY MESSAGES...

- Near normal temperatures are expected the next few days with
  highs in the 80s. But a cold front this weekend brings an early
  taste of fall lasting into next week.

&&

.SHORT TERM...  (Through Late Friday Afternoon)
Issued at 236 AM CDT Thu Aug 21 2025

Cool surface high pressure continues to build slowly south through
the Great Lakes today leading to what will likely be our coolest day
of this air mass. Temperatures should top out a few degrees lower
than they were yesterday, in the low to mid 80s. Expect a fair
amount of afternoon cumulus again today, but a more pronounced cap
today will prevent any showers from forming. As the surface high
nestles into our region tomorrow night we`ll see the coolest night
of this air mass with many areas dropping into the low 60s. With the
modest lingering humidity and a favorable location relative to the
surface high center, there is a fairly good chance for some fog
formation mainly in sheltered valley locations provided the sky
remains otherwise clear.

As the high settles southward and weakens, upper ridging actually
slides down from the north and allows for a modest warm up on
Friday. We`re still looking at 80s, but a few degrees warmer than
Thursday.

Kimble

&&

.LONG TERM...  (Friday Night through Wednesday)
Issued at 236 AM CDT Thu Aug 21 2025

The big story in the long term is the next cold front arriving this
weekend which will bring the coolest air mass we`ve seen in months.
Ridging over the Four Corners region and a developing trough over
southern Canada and the Great Lakes will send this cool, dry air
mass southward. The cold front arrives during the day Saturday with
some uncertainty on temperatures here due to the timing of the front
and the corresponding cold advection. Northern areas stand a better
chance of seeing an early frontal passage and experience the cool
down quicker while southern areas warm up into the upper 80s again.
We`ll all feel that cool air by Sunday, though, and it`s going to
stick with us through the upcoming week.

Just how cool are we talking? Ensemble means have 850MB temperatures
dropping down to about +10C which is a far cry from the +23C or so
we saw during our recent heat wave. Assuming full mixing and bright
sunshine, that`s still only highs in the low to mid 70s, about 25
degrees cooler than our heat wave. There`s still some wiggle room in
the guidance on just how cool those 850MB temperatures get, with an
IQR spread in the 12Z ensembles of about 3C or about 5-6F. For
nighttime lows we take a look at the dewpoints in the incoming air
mass. The dewpoints taking aim at us are squarely in the 40s, and
deterministic NBM dewpoints continue to fall with each run, now
forecast to reach the 40s for most of the area by Tuesday. This kind
of dry air is hard to maintain during the growing season in the
absence of pronounced drought, as evapotranspiration typically buoys
those dewpoints into the 50s. But this dry air is a marker of just
how cool it can get at night if we get some clear, calm nights. The
center of the surface high looks to make its way into our area
sometime around Tuesday night/Wednesday morning so this would be our
best opportunity to see some lows touching the 40s if we can keep
the sky clear. That`s something our area hasn`t seen in three months
(May).

As far as rain chances, there`s not much to show for it at the
moment, as NBM keeps things largely dry. The earliest chance would
be for some light rain showers as the front moves through on
Saturday, but moisture is lacking and the best mid level forcing
stays either just to the west or far to the north. For the rest of
the weekend and into next week, the jet will mostly be off to our
southwest where multiple shortwave troughs track through a northwest
flow over the Southern Plains. While the focus for these is clearly
to our southwest, it remains possible that one or two of these waves
does inch close enough to our area to bring some light rain or more
likely some cloud cover. Cloud cover would have a tendency to keep
daytime highs even lower but also prevent strong nighttime cooling.
This type of scenario becomes a bit more likely toward the middle to
end of next week when NBM PoP does start to creep up above 20
percent.

Kimble

&&

.AVIATION...  (For the 06z TAFs through 06z Thursday Night)
Issued at 1033 PM CDT Wed Aug 20 2025

VFR cloud cover will exit the area overnight, allowing for at
least MVFR river valley fog to develop at KJEF and KSUS. Fog will
dissipate after sunrise. Diurnal cumulus is expected again today,
though more sparse than yesterday. Clouds will likely develop
between 2000 and 3000 ft and rise through the afternoon, but BKN
skies are not expected at this time.

Jaja

&&

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

$$

WFO LSX