Area Forecast Discussion

574
FXUS63 KLSX 141933
AFDLSX

Area Forecast Discussion
National Weather Service Saint Louis MO
133 PM CST Sun Dec 14 2025

.KEY MESSAGES...

- Cold temperatures are expected tonight, but will rise 30 to 35
  degrees to near seasonal averages by mid afternoon.

- A round of showers is increasingly likely (40-70%) in many areas
  early Thursday, especially southeast MO southwest IL.

- A cold front later in the day Thursday will bring increasing
  west-northwest winds, followed by a brief cooldown overnight and
  Friday.

&&

.SHORT TERM...  (Through Late Monday Night)
Issued at 126 PM CST Sun Dec 14 2025

While there has been some modest improvement in wind chills this
morning, or at least just enough to allow the Cold Weather Advisory
to expire, conditions remain quite brisk Actual temperatures have
only risen into the teens to upper single digits, and when
combined with a 10-15 mph north wind, wind chills remain between
-5 and 10 degrees. While steadily decreasing winds will help with
the wind chill aspect over the next few hours, temperatures have
likely reached near their "warmest" values of the day, with
perhaps only a degree or two left to go. Overnight, surface high
pressure will slide into the region, allowing winds to go nearly
calm and temperatures to drop back to between zero and the low
teens. Fortunately these light winds will keep wind chills in
check through the night, although a couple of spots in south-
central Illinois may dip to near -10 by sunrise.

Early in the day tomorrow, the surface high will continue to slide
southeast, and a southwest / northeast surface pressure gradient
will develop locally. This will drive increasing southwesterly low
level flow, including breezy surface winds, along with substantial
warm air advection. In fact, temperatures remain likely to warm by
as much as 30 to 35 degrees from early morning to mid afternoon, and
high temperatures will likely reach into the 30s to even mid 40s in
the warmest locations.

Meanwhile, afternoon humidity is likely to be very low in some areas
thanks to a combination of lagging moisture return and the
aforementioned warmup, especially across the Ozarks where minimum
relative humidity may dip into the low to mid 20s. While ordinarily
this combination of breezy southwest winds and low humidity would be
cause for a fire weather concern, it is unlikely that most fuels
will be able to warm and dry fast enough to become receptive to
burning, considering how cold they will be to start the day. While
you may still want to use extra caution in exposed grassy areas that
can dry out more quickly, the overall threat appears to be marginal
tomorrow.

BRC

&&

.LONG TERM...  (Tuesday through Next Sunday)
Issued at 126 PM CST Sun Dec 14 2025

Between Tuesday and Wednesday, the primary story continues to be the
continuation of a rapid warmup, driven by persistent southwesterly
low level flow. While the pressure gradient and wind speeds are
likely to relax slightly Tuesday, warming will continue unabated,
and we can expect high temperatures to reach the 40s to low 50s
during the afternoon, and perhaps to near 60 in some areas by
Wednesday. The cooler values are likely to occur in areas where snow
lingers, but the effect of this will wane each day as more and more
of this snow melts. Our average high temperatures this time of year
are generally in the low 40s, so this forecast is roughly 5 to 10
degrees above seasonal averages.

By Wednesday night, attention turns to the arrival of a dynamic
trough moving quickly across the northern Plains and Upper
Mississippi Valley, which will drive a deep surface cyclone across
these same areas. This will trigger a significant mass response in
the form of substantial southerly low level flow and breezy surface
winds, reinforcing warm air advection and also more substantial Gulf
moisture return. Confidence continues to increment upward that this
will produce a round of showers across at least parts of our area,
most likely our eastern half, sometime Thursday. However, this
trough remains quite progressive, and there remains non-trivial
spread in the position and timing of the trough within individual
ensemble members, and also ensemble clusters. A faster trough would
likely shorten our window for precipitation by limiting our moisture
return before the warm sector is pushed to our east, while a
slower/deeper trough would lengthen our residence time and allow for
both a longer period of precipitation and greater coverage.

In spite of these differences, precipitation chances have risen
by roughly 5-10% areawide from the previous forecast, and to
between 50 and 70% across southeast Missouri and southwest
Illinois early in the day Thursday. Also, considering the strong
dynamics present with this trough, we will have to keep an eye on
the potential for thunderstorms, although the marginal moisture
quality continues to severely limit instability projections, and
this potential remains low (20% or less) overall.

What may be more impactful than the rain, though, is the cold front
that is likely to move through the area during the day Thursday.
While this front may not bring another round of significant cold
like the most recent iteration, it may bring some substantial west-
northwest winds. While the current NBM forecast calls for sustained
winds in the 15-25 mph range with gusts close to 35, a closer
examination at individual deterministic models reveals the potential
for more higher speeds, considering the anomalously strong 850 mb
winds and steep low level lapse rates behind the front. While it`s a
bit too early to nail down how this will translate to the surface
just yet, 75th to 90th percentile gust speeds reach between 35 and
45 mph already in both the LREF and NBM across northern/central
Missouri Thursday afternoon, and this appears plenty reasonable
considering the model sounding profiles present in deterministic
output.

Again, this front is not likely to bring another round of
significant cold like this most recent iteration, but it will
abruptly (albeit temporarily) halt this week`s warming trend.
Forecast temperatures for Friday have dropped slightly since
yesterday, and ensemble spread has also narrowed, increasing
confidence that a sharp drop in temperatures can be expected
Thursday night and Friday. NBM-forecasted morning low temperatures
Friday morning have dropped all the way into the teens and low 20s
after Thursday high temperatures well into the 50s to low 60s. To be
clear, this drop to occurs well after precipitation ends during the
day Thursday, so unless precipitation is significantly delayed in
subsequent forecasts, this is expected to fall entirely as rain.

Finally, as quickly as the cold air arrives, southerly flow will
return quickly over the weekend as another substantial trough moves
across the northern CONUS. This should lead to another sharp warmup
Saturday , followed perhaps by another cold front later in the
weekend, although confidence in the timing of these features and the
implications on our local weather remains low during this timeframe.

BRC

&&

.AVIATION...  (For the 18z TAFs through 18z Monday Afternoon)
Issued at 1015 AM CST Sun Dec 14 2025

VFR flight conditions are expected throughout the 18Z period.
Modest northerly winds will become nearly calm late this afternoon
through the evening, and gradually become southerly overnight.
Wind speeds will increase quickly tomorrow morning and veer to the
southwest, with moderately strong gusts. Temperatures will also
warm very quickly tomorrow, and most terminals will climb above
freezing by the end of the period after another frigid morning.

BRC

&&

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

$$

WFO LSX

NWS LSX Office Area Forecast Discussion