348
FXUS63 KLSX 221134
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
634 AM CDT Fri Aug 22 2025

.KEY MESSAGES...

- Dry weather will continue through the period with fall like
  conditions starting Sunday and continuing through the end of
  next week.


&&

.SHORT TERM...  (Through Late Saturday Afternoon)
Issued at 251 AM CDT Fri Aug 22 2025

A few isolated sites across the region have already developed fog
this morning. More fog is expected to develop within the next few
hours, mainly in valleys and hallows across the area, including
river valleys. Fog may be intermittently dense, but is not expected
to pose a widespread hazard and will begin to dissipate with sunrise.

The surface high stretching from the Great Lakes region into the mid-
Mississippi Valley will persist through the day, supporting another
dry day punctuated by afternoon cumulus. Mixing will be a little
deeper today, up to around 850 mb, and combined with the increased
afternoon sunshine will result in high temperatures a few degrees
warmer than yesterday. With the loss of afternoon cumulus tonight,
combined with the light and variable winds from the surface high,
fog is expected again tonight. River valleys and hallows where cool
air will pool are most likely to see patchy fog develop.

The strong mid-level trough currently moving east across south-
central Canada will continue its eastward push Saturday. In response
a surface low will slide into the Great Lakes region, dragging a
cold front through the forecast area during the day. This front will
go largely unnoticed as moisture convergence along the front is very
weak. Only roughly 20% of ensemble guidance indicate measurable
rain, with a continued downward trend. Given the dry air this
front will be moving into, moisture pooling along the front is
unlikely and I`ve continued with a dry forecast through Saturday.

While the surface front will move through on Saturday, the elevated
front won`t move through the region until Sunday, delaying the onset
of cooler, drier conditions. 850 mb temperatures will pool slightly
ahead of the advancing 850 mb front on Saturday, and result in high
temperatures on Saturday a few degrees warmer than today.

Delia

&&

.LONG TERM...  (Saturday Night through Thursday)
Issued at 251 AM CDT Fri Aug 22 2025

The long term forecast will be dominated by two key features. The
first is the mid-level trough discussed above. Over the end of the
weekend and first half of the week this feature will track eastward
and linger over the eastern CONUS. While the initial center will
weaken and lift to the northeast, additional shortwaves within the
flow will keep the trough largely in place. This will keep our
region within northwesterly flow, pulling dry continental air into
the forecast area aloft. The second feature will be a persistent
surface high spread from south central Canada into the mid-
Mississippi Valley that will aid in keeping cool, dry Canadian air
advecting into the region at the surface.

The result will be dry conditions cooling below normal starting
Sunday when the elevated front makes it through the area and ushers
the new airmass into the region. By Monday 850 mb temperatures will
be between 8-12C, which translates to high temperatures in the 70s,
a few degrees higher if clouds don`t develop. These temperatures
will stick around through the mid week period, and confidence in the
high temperature forecast is high with only a 3-4 degree spread in
the high temperature interquartile range. The continental air moving
into the region will bring dewpoints into the mid 40s to low 50s
during the week, and will allow overnight temperatures to cool
nearly as low. Some mornings next week may be downright chilly,
particularly after the heat last weekend. The widespread subsidence
and lack of moisture will keep the region largely dry through the
period.

The pattern begins to shift towards the end of the work week as the
surface high shifts eastward and both heat and moisture begin to
rebound. Forecast pops increase starting Thursday as guidance hints
at the potential for disturbances coming off the Rockies to take
advantage of the moisture return. However, confidence in
measurable rainfall is low at this point.

Delia

&&

.AVIATION...  (For the 12z TAFs through 12z Saturday Morning)
Issued at 632 AM CDT Fri Aug 22 2025

VFR conditions are expected today with light and variable winds.
There is the potential for fog tonight into Saturday morning, most
likely at valley locations like SUS.

Kimble

&&

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

$$

WFO LSX