764
FXUS63 KLSX 051917
AFDLSX

Area Forecast Discussion
National Weather Service Saint Louis MO
217 PM CDT Sun Oct 5 2025

.KEY MESSAGES...

- Much cooler temperatures are forecast starting Tuesday and lasting
  through Thursday behind a cold front.

- There is a 30-60% chance of showers and a few thunderstorms
  Monday afternoon through Tuesday morning. Unfortunately, no
  widespread beneficial rainfall is anticipated.


&&

.SHORT TERM...  (Through Late Monday Afternoon)
Issued at 217 PM CDT Sun Oct 5 2025

Dry and quiet weather conditions are expected tonight with light
southerly winds. A mostly clear night will give way to increasing
midlevel clouds moving northward late tonight. These clouds should
help limit the temperature fall, with lows mainly in the low to
mid 60s across the region.

A weak midlevel shortwave trough is forecast to move toward the
confluence of the Mississippi and Ohio Rivers by late Monday
afternoon. Increasing ascent is expected ahead of this feature, with
at least scattered showers moving into southeast Missouri and
southern Illinois during the afternoon. There remains some
uncertainty on the exact track of the aforementioned shortwave. A
slight shift south for instance could leave even our southeastern
counties mostly high and dry. The best chances for rainfall remain a
bit to our south across western Kentucky/bootheel of Missouri
region. This is where probabilities for measurable rainfall climb
above 80% from the HREF. Conversely, a slight shift northwest could
expand the higher rainfall probabilities further to the northwest,
and bring the chances for more significant rainfall into our far
south/southeastern counties. Currently, the 12Z HREF shows chances
for at least 0.25" of rain of only about 10% in these locations but
climb rapidly to around 50% just to our southeast. The disturbance
moving across the lower Ohio Valley definitely has some tropical
origins however. Precipitable water values over 1.75" are likely in
far southeast Missouri/southern Illinois. Those values are very
abnormal for early October and near the 99th percentile of
climatology. This means that showers, and especially any convective
elements, should be quite efficient. There may be a few lucky
locations that see several showers/weak thunderstorms on/off during
the afternoon hours in our just southeast of our area. If this were
to occur, there may be some isolated areas that see 1-2" of rain
like the LPMM of the 12Z HREF suggests.

Further to the northwest, there also should be some more pure
diurnal showers and a few thunderstorms where there is an axis of
modest instability. This activity should focus a bit closer to the I-
44/I-70 corridors in Missouri/Illinois respectively. For northeast
Missouri and west-central Illinois, the synoptic cold front begins
to move into the area by late afternoon. However, kept PoPs in the
slight chance category as most, if not all, of the precipitation
looks to be post frontal where stronger mid/upper level forcing for
ascent is located along with increasing low/mid level frontogenesis.

High temperatures will be a bit tricky due to increasing cloud cover
over much of the area. There is higher confidence in parts of
southeast Missouri and southwest Illinois staying mainly in the
upper 70s to low 80s due to thicker cloud cover earlier in the
morning. Highs in the mid 80s are expected further northwest, and
there may be some spotty upper 80s in a southwest-northeast axis
where the thicker midlevel clouds do not reach and where diurnal
cumulus develops later in the afternoon.


Gosselin

&&

.LONG TERM...  (Monday Night through Next Sunday)
Issued at 217 PM CDT Sun Oct 5 2025

(Monday Night - Tuesday)

The cold front is expected to gradually move through the region
overnight Monday night through Tuesday morning. Mainly scattered
showers and possibly a few rumbles of thunder should focus along the
trailing 850-hPa frontal zone. Both the low-level moisture
convergence as well as the low/mid level frontogenesis gradually
weaken however late Monday night into Tuesday morning, so the
coverage of showers may tend to decrease with time and southeastward
extent. Deterministic model guidance also show a subtle midlevel
shortwave quickly moving across the Mississippi Valley during the
day on Tuesday, which could prolong the shower chances through the
afternoon. The bad news is it continues to look like there will not
be widespread beneficial rainfall associated with this cold frontal
passage. The chances for at least a quarter inch of rain are below
20% across the entire area.

The main change behind the front will be the much cooler air
temperatures. For  most locations, highs on Tuesday afternoon will
be some 10-15+ degrees cooler. Chances of showers, increasing low-
level cold air advection, and a strong signal for low stratus (925
hPa mean RH from the LREF of 80-95%) all suggest a much more typical
fall-like day. Highs are expected to range from the mid 60s to upper
70s from northwest to southeast. These values aren`t exactly "cool"
for early October, but after day after day with highs well into the
80s, this cooldown is notable nonetheless.


(Tuesday Night - Thursday Night)

There is high confidence in dry weather with near-normal
temperatures (lows 40s/50s and highs in 70s) for the midweek period.
This will be mainly due to the presence of a seasonably strong
(1030+ hPa; >95th percentile of climatology) surface high moving
across the Great Lakes during this time period.


(Friday - Next Sunday)

Forecast uncertainty begins to increase on Friday heading into the
following weekend. Ensemble guidance shows quite a bit of spread
with the location of a mid/upper level ridge and how far/fast it
moves eastward. Deterministic guidance also shows some subtle, weak
midlevel shortwaves that may transverse the eastern periphery of
this ridge axis. Some moderation in temperatures is expected, but
how warm it will get is definitely a question mark. The spread in
the inter-quartile range of the NBM for highs and lows is generally
on the order of 6-10 degrees. However, even the 25th percentile of
the NBM climbs to above normal values for early/mid October by
Friday. Therefore, at least slightly above normal temperatures are
very likely. Mostly dry weather is also expected, though low chances
of showers return Thursday night and Friday as there may be a
midlevel shortwave moving across the region. There remains little/no
signal for anything widespread nor significant however in the
rainfall department for the foreseeable future.


Gosselin

&&

.AVIATION...  (For the 18z TAFs through 18z Monday Afternoon)
Issued at 1216 PM CDT Sun Oct 5 2025

Dry/VFR conditions with light southerly winds are forecast through
Monday morning. There will be some increasing midlevel clouds from
south to north late tonight into Monday, with a mostly clear sky
beforehand. This increase in midlevel cloudiness and a light
southerly breeze should prevent river valley fog from threatening
KSUS/KCPS/KJEF.

Gosselin

&&

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

$$

WFO LSX