Area Forecast Discussion

379
FXUS63 KLSX 160804
AFDLSX

Area Forecast Discussion
National Weather Service Saint Louis MO
204 AM CST Tue Dec 16 2025

.KEY MESSAGES...

- Mild weather returns today and lasts for most of the week.

- The best chance of rain comes Thursday morning as a cold front
  moves through the region.

&&

.SHORT TERM...  (Through Late Wednesday Afternoon)
Issued at 203 AM CST Tue Dec 16 2025

Our warm up continues today with southwesterly winds in the low
levels and ridging expanding eastward aloft. This type of regime is
typical for dry and breezy conditions. Thankfully, though, there is
some moisture return upstream which will allow the dewpoint to rise
through the day and keep humidity above critical levels. The
temperature warms into the 50s downstream of the Ozarks across the
Missouri River Valley toward St Louis. Downsloping southwesterly
flow off the Ozarks will enable deeper mixing and may allow for even
warmer readings in those downslope areas.

A trough moving east through southern Canada today will drag a cold
front southeast behind it, entering our area from the north
overnight. It loses its momentum as it arrives, though, with the
front largely washing out by Wednesday morning. The bulk of the cold
air behind this front heads east through the Great Lakes, so we
won`t see much effect locally. Winds go light and turn southerly on
Wednesday and for some areas it`s even a bit warmer than Tuesday.
Still largely in the 50s, though.

Kimble

&&

.LONG TERM...  (Wednesday Night through Monday)
Issued at 203 AM CST Tue Dec 16 2025

On Thursday a deeper trough develops across the Upper Midwest and
sends a stronger cold front southeast toward our area. Ahead of the
front, Gulf moisture gets pulled northward, meeting with the front
in the vicinity of our area early Thursday morning. This moist
advection as well as convergence along the front will result in
showers developing ahead of and just behind the front. There`s also
the potential for a little bit of instability to tap into as well.
Of the ECMWF ensemble members, 30 percent or more produce at least
100 MUCAPE from St Louis southward indicating that there may be some
rumbles of thunder. While this does represent our best chance of
rain in this forecast, not everyone is going to get it. The Gulf
moisture is arriving and meeting up with the front over our area, so
how much of our area gets rain depends on how early that meeting
happens. Either way, the better chances of rain are in the east
where confidence is greater that there will be enough moisture ahead
of the front before it moves through. Chances are lower in the west
for the same reason in reverse. Overall amounts are light, with less
than 50 percent of long range ensemble members producing 0.25 inch
or more, with those chances highest in southern Illinois.

The air behind this cold front will be quite cold. The temperature
will likely be falling through the day on Thursday, especially in
northern areas. Winds will be strong as well, with cold advection
aiding in the efficient mixing of stronger winds aloft down to the
surface. The NBM forecast winds and gusts are likely too low in this
scenario, and we collaborated with neighboring offices to increase
these during the day on Thursday. Forecast soundings show close to
45KT at the top of the mixed layer, and NBM probabilities of 40KT
(45 mph) gusts at the surface are up to 20 percent in northern
Missouri on Thursday.

This cold air mass doesn`t last long. Ridging quickly spills
eastward behind the departing trough by Friday morning. So by Friday
we`ll already be beginning the warm up, though for most areas it
will still be colder Friday than it was early Thursday morning
before the front. That warm up accelerates on Saturday as much of
the area reaches the 50s. In fact, the probabilistic NBM has greater
than 20 percent chance of reaching 60 south of the Missouri River.

There`s more uncertainty on the temperature forecast Sunday. Another
trough is expected to push through southern Canada and the Great
Lakes Saturday into Sunday with a cold front pushing southward
behind it. While this front is likely to reach our area before
washing out, it is becoming clear that the greater thrust of cold
air is to the east rather than to the south. So while cooler
temperatures are expected, just how much cooler is less certain.
Northern areas have the greater chance of being affected briefly by
this more intense cold on Sunday, while southern areas may not cool
much at all. NBM interquartile range remains high (about 12 degrees)
for high temperatures Sunday, but this has come down a bit from
prior forecasts indicating improving confidence even if there
remains a good deal of uncertainty. Ridging aloft, though, will
ensure that any colder air doesn`t last long and we return quickly
to the more mild temperatures.

Kimble

&&

.AVIATION...  (For the 06z TAFs through 06z Tuesday Night)
Issued at 1103 PM CST Mon Dec 15 2025

An area of MVFR stratus over eastern Missouri and southwest
Illinois is expected to clear slowly from west to east overnight.
VFR flight conditions are expected to prevail thereafter. Light
southwest flow will increase to 10-15kts Tuesday morning. Gusts to
around 20kts are likely across central Missouri.

Carney

&&

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

$$

WFO LSX

NWS LSX Office Area Forecast Discussion