637 
FXUS63 KLSX 052324
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
624 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 western 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 00z TAFs through 00z Monday Evening)
Issued at 621 PM CDT Sun Oct 5 2025

Gusty winds diminish quickly this evening and don't return
tomorrow. Will see some increasing mid level clouds overnight
which will help to prevent fog and also keep winds reduced
tomorrow. There's some potential for showers or thunderstorms to
develop in various places on Monday, though right now confidence
in these occurring at any terminal in particular is too low to
include in any TAF. Storms over southeast MO and southern IL are
likely to remain south of the St Louis metro while any storms
associated with an approaching cold front from the north Monday
evening are likely to remain north of Quincy as well. There is the
potential for lower ceilings to develop Monday night, just outside
of the current TAF period, as the cold front oozes southward.

Kimble

&&

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

$$

WFO LSX