815
FXUS63 KLSX 311940
AFDLSX

Area Forecast Discussion
National Weather Service Saint Louis MO
240 PM CDT Thu Jul 31 2025

.KEY MESSAGES...

- Dry and cooler than normal conditions along with low humidity
  are forecast through this weekend before temperatures moderate
  back closer to normal next week.


&&

.SHORT TERM...  (Through Late Friday Night)
Issued at 233 PM CDT Thu Jul 31 2025

A weak surface convergence zone/inverted trough has served as the
focus for drizzle/very light rain showers earlier this morning
into the afternoon. This trough axis will continue to gradually
shift southward, with dry weather forecast behind its passage.
Looking upstream, there is some smoke/haze across much of the
Upper Midwest originating from wildfires in southern Canada. This
should advect south/southwest into the area through tonight. I
don`t expect much in the way of visibility restrictions (maybe
down to 4-5 miles), but you may notice a bit of a more milky/hazy
sky tonight into Friday.

Well-below normal temperatures are forecast both tonight and
during the day on Friday. Lows tonight are expected to drop into
the mid 50s (northeast Missouri) to the low 60s (St. Louis
metro/southeast Missouri/southwest Illinois). These temperatures
would be about 5-8 degrees below normal for the date. Highs on
Friday will be a bit warmer in most locations than today, mainly
due to more sunshine expected. Highs in the upper 70s to low 80s
are forecast. What will be more noticeable will be the lower
humidity. Dewpoints in the upper 50s to low 60s are forecast
tomorrow afternoon, almost 10 degrees in spots lower than what is
currently being observed. To put these dewpoints in some more
perspective, climatologically speaking the values forecast are
expected to be between the 10th and 25th percentile for early
August.


Gosselin

&&

.LONG TERM...  (Saturday through Next Thursday)
Issued at 233 PM CDT Thu Jul 31 2025

(Friday Night - Sunday)

The cool and dry weather pattern will continue through the weekend
along with seasonably low humidity levels. This is all thanks to
persistent northeasterly flow around a sprawling area of high
pressure quasi-stationary over the Great Lakes. Highs in the upper
70s to low 80s with lows in the mid 50s to low 60s are forecast
through the period.


(Sunday Night - Next Thursday)

Moderating temperatures (and humidity levels) are expected as we
head into next week as ensemble guidance is in good agreement that
mid/upper level ridging will move eastward into the Plains.
Meanwhile, at the surface, the surface high over the Great Lakes
will continue to weaken and finally shift eastward into the
northeast. This will allow low-level flow to shift more out of the
southeast, helping to advect the warmer and more humid air back into
the mid-Mississippi Valley.

Overall, ensemble guidance is actually in very good agreement with
the overall pattern change, with very subtle differences with the
location/strength of the mid/upper level ridging out through midweek
next week. Our region is on the eastern/northeastern periphery of
the ridge, which puts us at the risk for "ridge-runners" and chances
of showers and thunderstorms. This puts high temperatures more in
doubt for next week, with spread on the inter-quartile range of the
NBM increasing into the 6-10 degree range for next
Wednesday/Thursday. That is very high for anytime in summer, even
for a day 6/7 forecast. This spread is the difference between highs
in the mid 80s (slightly below normal) to mid 90s (5-7F above
normal). Overnight low temperatures have less spread, not
surprisingly, with readings in the mid 60s to low 70s. Even the 25th
percentile is at least a couple of degrees below normal. Therefore,
while there is a lot of uncertainty on how warm it will get relative
to normal during the day next week, mild low temperatures are a near
certainty to return.

As for precipitation chances, it is hard to pinpoint when/where
higher PoPs should be focused this far out, but the pattern does
suggest at least the possibility of 1 or more MCSs next week. At
this early stage, signs favor the the central Plains to be the
focus, which is why PoPs for now remain mainly in the 20-30% range
in our area.


Gosselin

&&

.AVIATION...  (For the 18z TAFs through 18z Friday Afternoon)
Issued at 1204 PM CDT Thu Jul 31 2025

Low stratus will gradually lift and may eventually scatter out but
late afternoon. Some drizzle and/or very light rain showers may
continue to impact central Missouri and metro St. Louis terminals
early this afternoon before shifting south of the area. Another
concern is impacts from wildfire smoke. Upstream visibilities are
mainly in the 4-6SM range. Added some 6SM HZ at the terminals as
it moves in from the north/northeast. KUIN would have the best
chance of seeing MVFR visibilities, but wasn`t confident enough to
go that low during the afternoon hours as mixing increases.

Gosselin

&&

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

$$

WFO LSX