521
FXUS63 KLSX 290820
AFDLSX

Area Forecast Discussion
National Weather Service Saint Louis MO
320 AM CDT Fri Aug 29 2025

.KEY MESSAGES...

- Outside of an isolated shower or two along and east of the
  Mississippi River this afternoon, largely-dry conditions will
  continue through the holiday weekend. Seasonable temperatures
  will gradually cool through early next week as well.

&&

.SHORT TERM...  (Through Late Saturday Afternoon)
Issued at 319 AM CDT Fri Aug 29 2025

An analysis of regional surface observations show a backdoor cold
front advancing west through central Illinois at this hour. While
the air behind it is not significantly cooler, drier dewpoints will
accompany this frontal passage. Aloft, dry northwest flow resulting
from a very amplified ridge-trough pattern spanning the CONUS
continues to promote largely-tranquil conditions in the Mississippi
Valley. The only sensible weather of "concern" in the next 36 hours
will be the threat for isolated showers along the cold front this
afternoon. High resolution guidance continues to hint that
sufficient low-level forcing may exist to warrant a slight increase
in PoPs (now roughly 15%). The front appears to stall near the
Mississippi River (where PoPs are highest) before becoming very
diffuse in the evening. From tonight through Saturday, weak easterly
winds will promote dry, comfortable conditions across the region.
Some deterministic guidance attempts to bring a mid-level
shortwave down through the northwest flow aloft to spark
additional showers on Saturday in northeast Missouri and west-
central Illinois, but most solutions keep this feature north of
the forecast area.

MRB

&&

.LONG TERM...  (Saturday Night through Thursday)
Issued at 319 AM CDT Fri Aug 29 2025

The trough currently situated over the Great Lakes region begins to
advance downstream by late Saturday, leading to a dampening in the
amplified upper-level flow pattern. Meanwhile, more solutions are
now pointing to a shortwave slowly drifting southeast amidst the
weaker steering flow into the Mid-Mississippi Valley. While the
depictions of the wave vary drastically in amplitude and speed,
nearly all of them show the feature weakening drastically by the
time it reaches north-central Missouri. While some low (15-35%)
chances for rain do exist in this part of the forecast area Sunday
into early next week as the wave gradually advances south and
east, don`t expect much of a relief from the worsening drought
conditions in the region. Even if this feature can manage to force
precipitation, PWATs are fairly unimpressive which won`t lead to
much total rainfall (NBM probabilities of 24-hr rainfall over 0.25
inches never exceed 25%).

Following this shortwave, confidence is very high (80-90%) that we
will see another late summer cooldown ushered in by a strong cold
front. A few showers and thunderstorms along the front are certainly
possible, but the big story will be temperatures that fail to warm
into the 80s on Thursday. A very strong mid-level trough, near the
1st climatological percentile, dives south into the region by then
along with 850mb temperatures near the 10th climatological
percentile. These anomalous features vary in strength among the
available ensemble guidance, but even the warmest side of the
guidance still sends temperatures slightly below-normal. Our current
forecast blankets the region with highs in the 70s, which is 10-15F
below normal depending on the location. While this abrupt cooldown
appears short-lived when looking beyond the 7-day forecast, with the
NBM temperature envelope warming generally going into next weekend,
it will be refreshing nonetheless.

MRB

&&

.AVIATION...  (For the 06z TAFs through 06z Friday Night)
Issued at 1008 PM CDT Thu Aug 28 2025

VFR conditions are expected throughout the 06Z TAF period. Light
and variable winds will become primarily east-northeasterly
tomorrow afternoon and continue through the end of the period, but
will remain light. A low chance of patchy steam fog development
exists along main stem rivers overnight and early tomorrow
morning, but this is not likely to impact nearby terminals
(SUS/JEF/CPS).

BRC

&&

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

$$

WFO LSX