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FXUS63 KLSX 111027
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
427 AM CST Thu Dec 11 2025

.KEY MESSAGES...

- Dangerous cold is expected this weekend. The most brutal wind chills
  will be north of I-70 Sunday morning.

- Light snow will glance portions of northeast Missouri and central/west-
  central Illinois this afternoon into tonight. The chance of
  significant impacts is low, but a small shift southwest could
  result in accumulations of 1"-2".

- Measurable snow is expected (80 - 100%) north of I-70 on Saturday.
  Any snow that falls will stick and cause travel impacts.


&&

.SHORT TERM...  (Through Late Friday Afternoon)
Issued at 314 AM CST Thu Dec 11 2025

The focus for today - Friday is the potential for light snow in
northeast Missouri southeastward through south-central Illinois in a
corridor roughly extending from Quincy, IL to Salem, IL and
northeast. The clipper responsible for the snow has been trending
southwest over the past day or so, placing the stripe of higher snow
accumulations on our northeastern doorstep. The latest 00z 12/11
HREF notably does not continue this southwestward trend, and the 00z
LREF actively pushes it back north. The majority of CAMs reflect
this as well, having the snow gradient just along our border with
higher reflectivity glancing in every now and again. This gradient
is expected to be tight, so where it places does matter. The latest
HREF LPMM gives us an idea of an "extreme" scenario. Currently this
extreme is completely northeast of our CWA, which reflects the
decreasing probabilities of 24 hour snowfall >1" (20 - 30% from 30 -
50%). However, the footprint is not gospel, and may (although not
climatologically favored) still shift southwest. It would only need
to shift a couple dozen miles to dramatically increase the chance of
accumulating snow. In this case, 1-2" will be possible in the
aforementioned corridor. Very brief periods of sleet are also
possible with any snow that falls, but are not expected to cause
impacts.

Jaja

&&

.LONG TERM...  (Friday Night through Wednesday)
Issued at 314 AM CST Thu Dec 11 2025

A brutal, anomalously cold airmass will follow the Thursday night
clipper. Temperatures will begin to fall as early as Friday
night, but the brunt of the cold will be felt Saturday night
through Sunday night with an emphasis on Saturday night. The once
warming NBM has consistently cooled over the past 2 days, with
KUIN dropping from a Sunday forecast high of 15-20 degrees to a
new high of 9. Given the strength of the Arctic air (850 mb temps
within the 10th percentile of climatology) and the strength of the
surface high pressure (97.5th percentile of climatology), and the
increasing likelihood of a snowpack, I think the NBM is on the
right track, even if its Saturday and Sunday night lows are still
above the 50th percentile. Despite potentially having more room to
drop, Saturday`s forecast overnight lows and wind chills will be
bitterly cold. LREF probabilities of 6am Sunday wind chills <-10F
have increased to 80 - 100% for nearly everywhere north of
I-70/I-64. The chance of wind chills reaching -20F has increased
as well, now 40 - 70% extending from about Kirksville, MO to
Centralia, IL and northeast. There are also indications among
guidance that Sunday night will be colder than currently forecast,
but the chance of reaching -10F again is much lower. Daytime
Sunday will not provide much relief to the cold. Highs at KUIN and
KSTL are approaching record cold high territory, with KUIN
forecast to reach 9 degrees (Record: 7 - 1989) and KSTL to reach
15 (Record: 13 - 1903). Wind chills around KUIN will remain below
0 for most if not all of the day, potentially creating an extended
period of negative wind chills lasting from Saturday afternoon
into Monday morning.

Ahead of the cold is another chance of snow during the day Saturday
with no potential for mixed precipitation. The track of this storm
has shifted south and broadened in the past 24 hours. The ECMWF,
GEPS, and Canadian all bring the system well into the northern half
of the CWA with a 100% chance of measurable snow. Soundings show
strong lift within a deep DGZ, and an isothermal layer just colder
than the most efficient temperature range for maximum aggregation.
This supports high SLRs and efficient accumulating where snow does
fall. The LREF now shows a 70 - 90% chance of 1" of snow over 24
hours in portions of northeast Missouri and west-central Illinois,
dropping to 30% when upped to 2". Given the LREF calculates these
probabilities using SLRs = 10:1 and SLRs are trending closer to
15:1, I think the chances of reaching 1-2" of accumulation are
higher. However, I don`t have much certainty in exactly how much
higher. Regardless, any snow that falls will stick and will not melt
until early next week.

The region will warm up quickly next week as mid-level ridging and
low-level warm air advection take hold. Lows Monday night are
forecast to be warmer than Sunday`s highs. There is reasonably high
confidence in at least some of the CWA reaching the 50s again on
Wednesday, with the NBM 25th percentile high at St. Louis that day
at 49 degrees.

Jaja

&&

.AVIATION...  (For the 12z TAFs through 12z Friday Morning)
Issued at 422 AM CST Thu Dec 11 2025

An MVFR cloud deck over Illinois will continue to push east,
leaving VFR conditions for most through the end of the TAF period.
For those that remain VFR, winds will pick up from the southeast
today before quickly shifting to northwesterly by tonight.

Portions of west-central and south-central Illinois may see MVFR
clouds come in again with a bout of snow to their northeast
tonight. MVFR conditions will be closely tied with the snow, so
if these locations remain clear, VFR conditions will likely
prevail. However, MVFR visibilities and ceilings will take hold if
snow trends further southwest, which upstream radar indicates it
may do. A trace to 0.5" of snow is the most likely scenario, but
up to 1-2" may fall in these areas depending where the snow lines
up.

Jaja

&&

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

$$

WFO LSX