Area Forecast Discussion

751
FXUS63 KLSX 280902
AFDLSX

Area Forecast Discussion
National Weather Service Saint Louis MO
302 AM CST Sun Dec 28 2025

.KEY MESSAGES...

- There is a marginal risk for severe thunderstorms this afternoon
along a strong cold front. While confidence is lower in
thunderstorms forming, any thunderstorms that become severe will be
capable of damaging winds and brief tornadoes.

- Strong winds up to 40 mph and a significant drop in temperatures
  on Monday will result in near zero wind chills Monday morning
  and less than 20 degree wind chills Monday afternoon.


&&

.SHORT TERM...  (Through Late Monday Afternoon)
Issued at 302 AM CST Sun Dec 28 2025

Light rain with a few embedded thunderstorms have formed across
central and northeast Missouri and west-central Illinois from the
low-level jet overriding a warm front this morning. Thunderstorms
have been rather limited as the bulk of the available MUCAPE is to
the south of the mass of rain, though instability is expected to
surge northward over the next few hours and result in an uptick in
thunderstorms. 50-60kts of 0-6 km shear may be able to organize a
thunderstorm or two, mainly capable of small to marginally severe
hail due to the elevated nature of these storms. These storms will
shift north and east through the morning hours as the low-level jet
pushes northeast ahead of the approaching cold front.

After a lull mid-morning, the chance for showers and thunderstorms
returns along the much anticipated cold front as it drops southeast
through the forecast area. Guidance is in agreement that around 500
J/kg of SBCAPE will be available along the front where the cap will
also be minimized. Shear will continue to be robust with 50-70kts of
0-6 km shear and 40-50kts of 0-3 km shear along the front. While
instability was previously the conditional ingredient for severe
thunderstorm development this afternoon, uncertainty has redeveloped
in whether enough lift will be present along the front in our
forecast area. Over the last 24 hours the mid-level trough has begun
to lag behind the surface front, reducing the amount of expected
lift along the front. Reflecting this, most CAMs develop convection
to the east of our forecast area. If this limiting forcing is
realized, this would severely limit to eliminate the severe risk
across our forecast area.

Despite this recent change, it is still possible enough lift will
exist from low-level convergence and the upper level jet, or that
the trough is more closely tied to the front than guidance currently
suggests. If enough lift is present, all other ingredients are in
place to produce a rapidly moving linear system with the potential
for sections to become severe and produce damaging winds and a brief
tornado or two.

Behind the cold front, a tight surface pressure gradient will result
in winds gusting into the 30s this afternoon into tomorrow as the
surface low exits the area. The cold front will also usher in our
dramatic cool down with temperatures dropping into the teens to low
twenties tonight. Low rainfall amounts from this rapidly moving
system combined with increasing wind speeds increase confidence that
wet surfaces will dry before they freeze. The coldest portion of
this Arctic airmass won`t make it to the forecast area until Monday
afternoon, resulting in highs that struggle to make it above
freezing area wide. Wind chill values will plummet to single and
negative single digits overnight, reaching their afternoon maximum
Monday in the single digits to teens.

Delia

&&

.LONG TERM...  (Monday Night through Saturday)
Issued at 302 AM CST Sun Dec 28 2025

Temperatures will begin to rebound Tuesday as a low-level ridge
builds into the mid-Mississippi Valley and the Arctic airmass shifts
east. Low-level winds will become westerly and begin pulling warmer
air into the region, warming temperatures to near normal by
Wednesday.

Beyond the mid-week period, guidance diverges on how to handle an
approaching mid-level ridge and as well as the timing and strength
of a mid-level low over southeastern Canada. This low is expected to
bring a cold front into the region for the second half of the work
week. But as has been discussed in previous packages, there is a
great deal of uncertainty still in how far south this front
progresses as well as where the bulk of the associated Arctic air
goes. The result is a continued 15-20 degree interquartile spread
for high temperatures Thursday into the weekend. Confidence is
higher that a lack of moisture area wide will keep the region
largely dry into next weekend.

Delia

&&

.AVIATION...  (For the 06z TAFs through 06z Sunday Night)
Issued at 1102 PM CST Sat Dec 27 2025

A warm front has passed all terminals but KUIN, where IFR clouds and
fog will likely persist until sometime Sunday morning. Showers and a
few thunderstorms are expected to increase in coverage north of I-70
Sunday morning as well, potentially impacting KUIN. Elsewhere, MVFR
ceilings expand in coverage after sunrise, remaining overhead
through most of Sunday. A cold front will also pass through the area
midday Sunday into the afternoon, but confidence is decreasing that
it will be accompanied by showers and thunderstorms. The front will
at least veer winds from southerly to northwesterly with gusts
increase to 30 to 37 kt. These winds will continue through Sunday
night, but eventual lifting/scattering of MVFR ceilings is favored
sometime around or just beyond the valid TAF periods.

Pfahler

&&

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

$$

WFO LSX

NWS LSX Office Area Forecast Discussion