Area Forecast Discussion

513
FXUS63 KLSX 251754
AFDLSX

Area Forecast Discussion...Updated
National Weather Service Saint Louis MO
1154 AM CST Thu Dec 25 2025

.UPDATE...
Issued at 1151 AM CST Thu Dec 25 2025

Although visibility is finally improving, it remains low in spots.
We expect renewed low visibility in fog shortly after sunset, so
in collaboration with neighboring offices we have extended the
Dense Fog Advisory for parts of the area through tomorrow morning.
A front pushing in from the west on Friday morning will finally
bring some drier air and make fog less likely


Issued at 1018AM CST Thu Dec 25 2025

Latest satellite and observations show cloud cover thinning across
the southern half of the area, while dense fog has held tough
mainly north of I-70. The Dense Fog Advisory was extended through
noon, and by then visibility should improve at least for a few
hours. Northern areas are unlikely to see the sun today, and in
fact, renewed fog tonight is looking likely. Further south, the
thinner clouds will disperse sooner and sun will allow quick
heating through the 60s into the low to mid 70s.

Kimble

&&

.KEY MESSAGES...

- A quasi-stationary boundary meanders through the region through
  Saturday with well above normal temperatures favored. Where
  partial clearing occurs, temperatures will be near-record
  territory.

- Showers and thunderstorms are becoming more likely Sunday along
  a strong cold front. Much colder temperatures return next week.


&&

.SHORT TERM...  (Through Late Friday Afternoon)
Issued at 256 AM CST Thu Dec 25 2025

Broad midlevel ridging remains in place across the mid-Mississippi
Valley through the period, resulting in weak height tendencies and
little synoptic-scale forcing. Water vapor imagery continues to
depict subsident flow aloft, supporting the maintenance of a strong
low-level inversion. Beneath the inversion, a quasi-stationary
frontal boundary remains positioned near the I-70 corridor, with
shallow cool air north of the boundary and warmer, more humid air to
the south.

Surface observations as of 08z this morning show widespread fog
north and west of the St. Louis metro, with visibility reductions
around 1/4 mile and low in some cases. Fog is slowly expanding
southward along the boundary, but has made slower progress in the
last hour. That builds confidence that the Dense Fog Advisory is in
good standing. Continued low-level moisture advection atop the
shallow cool layer, combined with weak mixing, will support low
stratus and fog through at least mid-morning, especially along and
north of I-70. The primary challenge otherwise, will be the impact
of persistent fog and low clouds on temperature trends. Recent
trends favor slower erosion than guidance suggests, curbing
potential highs at affected locations.

Despite anomalously warm temperatures (15-18C at 850MB) aloft, high
temperatures today will be highly conditional on cloud evolution.
Where fog and stratus persist into the late morning or even early
afternoon, particularly across northern Missouri and west-central
Illinois, highs will remain confined to the 50s. Manual adjustments
were made to lower temperatures a few degrees north of the front,
where fog and stratus is favored to persist later into the morning.
Farther south/southwest, including areas south of the Missouri River
and the St. Louis Metro, partial clearing, improved mixing and
downslope southwest flow off the Ozarks will allow temperatures to
rise well into the 60s. Faster-than-expected clearing would result
in warmer outcomes with 70s favored in similar areas as Wednesday.
To the contrary, slower clearing would result in high temperatures
cooler-than-forecast. HREF show mean cloud cover of 90% from KCOU to
KSTL through southwest Illinois. This coincides with an area where
30-50% of the HREF ensembles push high temperatures above 70
degrees. While this may curb confidence in warmth of this magnitude,
yesterday`s temperature behavior serves as the best reference for
today`s expectations.

By Friday morning, the front begins to shift back to the north.
Another round of fog is expected over northern sections of the
forecast area, with locally dense fog possible. The northward
adjustment in the boundary draws warmth and moisture northward,
resulting in partial clearing over much of the area. This will warm
high temperatures slightly over areas that have already been in the
warm sector, while temperatures north of I-70 climb roughly 4-7
degrees higher than Thursday.


Maples

&&

.LONG TERM...  (Friday Night through Wednesday)
Issued at 256 AM CST Thu Dec 25 2025

Friday night marks the transition to a more progressive pattern that
continues into Saturday as a deepening western CONUS trough advances
eastward. Height falls and strengthening southwesterly flow aloft
will allow the boundary to lift even further northward, supporting
renewed moisture advection and continuation of above-normal
temperatures.

A strong cold front moves through the region very late Saturday
night into Sunday with latest trends showing a much later arrival
that just 24 hours ago. This keep warmer air over at least the
southeast half of the CWA through Sunday. Additionally, it boosts
convective potential over the region Sunday afternoon. Increasing
large-scale ascent associated with positive vorticity advection and
front convergence will support widespread showers and thunderstorms
along and ahead of the front. The magnitude of instability is
uncertain at this range with the latest LREF data showing mean
SBCAPE of 200-300 J/kg from Springfield, MO through St. Louis
through Northern Indiana. CAPE values might sound low, but winter
systems tend to perform better with much colder air aloft.
Additionally, mean shear values approach 60 knots with a slowing
trend in boundary arrival. This maintains the moisture advection
Sunday and opens the window for better thunderstorm potential, some
of which could produce strong wind gusts Sunday afternoon.
Considering the uncertainty in timing, this will be something to
continue to monitor as we head into the weekend.

Timing differences between guidance is also impacting precipitation
types among some of the ensemble data, showing a very brief and low
chance (

NWS LSX Office Area Forecast Discussion