Area Forecast Discussion

311
FXUS63 KLSX 261143 CCA
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
536 AM CST Fri Dec 26 2025

.KEY MESSAGES...

- Temperatures nearly 30 degrees above normal will stick around
through Sunday.

- The best chance for rain over the next 7 days will move through
Sunday, followed by a 30-40 degree temperature drop or Monday`s
highs.


&&

.SHORT TERM... (Through Saturday Evening)
Issued at 337 AM CST Fri Dec 26 2025

Fog has been more inconsistent across the area overnight compared to
previous nights, and the fog that developed early this evening has
lifted northward out of our northern CWA with the warm front. A weak
cold front is approaching the forecast area from the west, and will
bring an initial period of dense fog along the front, which will
transition to low stratus deeper behind the front. So while there is
unrestricted visibility across northeast Missouri and west-central
Illinois as of 3 AM, conditions are expected to degrade in the few
hours surrounding sunrise as the cold front approaches. Therefore
I`ve made no change to the Dense Fog Advisory, which continues until
mid-morning. The weak cold front will push through the forecast area
this morning, keeping winds from the west today. This will result in
shallow mixing as the strong warm air advection will create a robust
warm nose. Mixing will extend up to roughly 925 mb, resulting in
temperatures quite similar to Christmas Day.

Despite continued low-level warm air advection, southeasterly to
southerly flow will maintain at least some of an elevated warm nose,
resulting in shallow mixing again on Saturday. High temperatures
will peak similar to previous days in the 60s to low 70s. Meanwhile,
there is an low chance (15-20%) for afternoon/evening showers across
most of the CWA, driven by warm air advection and an increasing low-
level jet. Confidence is currently highest in showers during the
evening hours as the low-level jet picks up in intensity, so have
increasing pops during this period to 15-20%.


Delia

&&

.LONG TERM... (Sunday through Thursday)
Issued at 337 AM CST Fri Dec 26 2025

Through the end of the work week and into the weekend a mid-level
trough will come onshore across British Columbia and swing into the
Rockies; this feature will bring our anticipated pattern change and
significant cool down. By Sunday the deepening trough will approach
the forecast area, and an anomalously strong surface low (10th
percentile) will develop across Nebraska before moving northeast.
The attendant cold front will move into the forecast area Sunday,
bringing showers and a thunderstorms or two to the area largely
along and behind the front as a majority of the upper-level support
lags behind the surface front. Strong warm air advection and deeper
mixing than previous days ahead of the front, combined with an
increasing low-level jet may be enough to produce isolated to
scattered showers and a few thunderstorms ahead of the front as
well.

As has been noted in previous discussions, the two airmasses at play
in this interaction are particularly strong, with 70 degree air
temperatures and 60 degree dewpoints to the south of the boundary and
temperatures and dewpoints in the 40s immediately north of the
boundary. This increases the potential for severe thunderstorms to
develop with this system, given the strong dynamics at play. While
machine learning guidance and CSU severe guidance are highlighting
the mid-Mississippi Valley for 15-30%, there are a few caveats that
could keep us from seeing severe weather.

1. Uncertainty exists in how much surface based instability will be
available ahead of and along the front. NBM 25th-75th percentiles
indicates anywhere from 0-300 J/kg, which is fairly representative
of other guidance: some sources have 0 J/kg available while others
have up to 500 J/kg in some locations. While we won`t need much, the
low end amounts (less than 100 J/kg) would preclude severe weather.

2. Lift ahead of the front, where the instability resides, is
relatively weak and confined to an increasing low-level jet and warm
air advection. The best lift associated with the mid-level trough
and the upper level jet remain displaced from the front. This means
that if instability does develop, there is less forcing available to
generate thunderstorms.

This doesn`t mean that we`re out of the woods however, and we`ll
continue to monitor the potential for severe weather Sunday over the
next few days.

Most locations are expected to receive at least 0.10" of rain (>70%
chance), though probabilities of at least 0.50" of of rain drop off
to 20-50% with the highest probabilities across Illinois, closes to
the low center. So while this system is expected to bring some much
needed rain to the area, confidence is low that it will be enough to
quell our dry conditions. Cold air will rush into the region behind
the front and light precipitation could linger into the cold air,
producing a brief period of light snow before the front and
associated precipitation exit the area. Timing of the surge of cold
air will be a big factor, yet any snowfall is expected to be
superficial given the light amounts and warm ground temperatures.

As the mid-level trough exits the forecast area Monday, and the
brunt of the cold air will impact the area, pulling 850 mb
temperatures into the negative teens. This will result in high
temperatures on Monday struggling to make it above freezing, a 30-40
degree temperature swing from Sunday. By Tuesday the mass of cold
air will shift eastward as a low-level high move east into the
region and southwesterly flow kicks up. Subsequent warm air
advection will warm temperatures back to near normal ahead of
another approaching cold front. A mid-level trough will move through
the Great Lakes region during the second half of next week, bringing
a backdoor/hybrid cold front into the region. There is a lot of
uncertainty in how far south this front advances, which is reflective
in the 20 degree high temperature spread Thursday and beyond.


Delia

&&

.AVIATION...  (For the 12z TAFs through 12z Saturday Morning)
Issued at 528 AM CST Fri Dec 26 2025

Fog has dissipated across the terminals this morning, leaving MVFR
stratus building into KUIN, KCOU, and KJEF. Conditions are
expected to degrade to IFR at KUIN, closest to the bulk of the low
stratus. The stratus is expected to continue to build south and
eastward over the next few hours, impacting the St. Louis metro
terminals, including KSTL, mid-morning. Through the late morning
daytime mixing is expected to scatter and lift the deck, returning
the terminals to VFR flight conditions. When exactly this
happens remains uncertain, but the best timing is outlined in the
TAFs.

Low stratus is expected to develop again tonight, though
uncertainty remains in how far south the stratus will extend. The
most likely location is across northeast Missouri and west-central
Illinois, including KUIN. How low stratus gets is also uncertain,
so I have left the mention at KUIN to MVFR for now.

Westerly to northwesterly winds will become light and variable
this evening and overnight as a surface high pushes across the
region. Winds will pick back up from the east Saturday morning.

Delia

&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...Dense Fog Advisory until 9 AM CST this morning for Knox MO-Lewis
     MO-Marion MO-Monroe MO-Pike MO-Ralls MO-Shelby MO.

IL...Dense Fog Advisory until 9 AM CST this morning for Adams IL-
     Brown IL-Pike IL.

&&

$$

WFO LSX

NWS LSX Office Area Forecast Discussion