505
FXUS63 KLSX 272300
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
500 PM CST Thu Nov 27 2025

.KEY MESSAGES...

-A winter storm watch is in effect for portions of northeast and
east-central Missouri and west-central Illinois from midnight
Saturday morning to midnight Sunday morning.

-The highest confidence and most significant impacts exist across
northeast Missouri and west-central Illinois.

-Confidence is lowest in the snow-rain transition and subsequent
 amounts in the St. Louis metro.


&&

.SHORT TERM...  (Through Late Friday Night)
Issued at 149 PM CST Thu Nov 27 2025

A surface high and deep northwesterly flow has kept the region dry
today with below normal temperatures. Temperatures will cool tonight
into teens to 20s tonight under the influence of the surface high,
light winds and a mostly clear sky. Friday will dawn cold, and
temperatures will struggle to make it out of the 30s north of I-70,
and peak in the low to mid 40s south of I-70. High clouds will start
to move into the area midday ahead of the approaching winter system,
contributing to the cooler highs.

Delia

&&

.LONG TERM...  (Saturday through Next Thursday)
Issued at 149 PM CST Thu Nov 27 2025

There have been few changes in the forecast over the last 12 hours.
Precipitation will start falling in the forecast area after midnight
Saturday, starting as snow as the lower atmosphere wet bulbs to
dewpoints which are expected to be in the mid to upper 20s. The
timing of the change over from snow to rain continues to be a large
point of uncertainty as the low-level jet strengthens with
precipitation onset and increases both QPF and warm air advection.
How long dynamical cooling keeps snow falling at the surface wins
out over the warm air advection remains uncertain.

What is certain is that across central and southeast Missouri and
southwest Illinois snow will transition to rain quickest and
snowfall accumulations are expected to be light to non-existent. The
nose of the 55kt low-level jet will stick right into these areas and
warm air will eventually be able to overwhelm the snow. In these
areas I can`t rule out a quick 1-2 inches of snow accumulation
before the rain sets in. Across northeast Missouri and west-central
Illinois confidence is high that a majority, if not all the
precipitation, will fall as snow. The close proximity to the bulk of
the cold air and the distance away from the warm air advection will
allow for significant snowfall accumulations. LREF ensembles
indicate at a 50-60% chance of exceeding 7" of snow, and a 30-40%
chance of exceeding 9" of snow in this region with a 10:1 ratio.
This is slightly higher than our current snow-to-liquid ratio
forecast, and while the LREF probabilities isn`t a direct
correlation with our forecast, it does give some context. If snow
does transition over to rain, it would be late in the day Saturday
once the snow has already done its damage. Where uncertainty is the
highest is across the St. Louis metro area. This area will have
access to some of the cold low-level air up across Iowa/northern
Illinois, but will also have access to the low-level jet and warm
air advection. We expect that snow will hang on here a few hours
longer than points to the south and west, but the exact timing of
the change over, and therefore amounts, remains uncertain.

HREF 3 hourly PMM guidance is indicating 0.25-0.50" of QPF
widespread across the forecast area. This can give us a proxy of the
highest snowfall rates, which can be up to 1.0 inch across the St.
Louis metro and areas south and west. Across our north where colder
air and higher SLR will be, we could see 1.0-1.5 inch rates. The
heaviest precipitation is still expected Saturday morning, and
stronger rates will be able to overcome increasing warm air in the
low-levels to keep snow falling longer and produce snowfall amounts
on the higher end of our forecast range.

The system will exit the area Saturday evening with colder air
quickly moving in behind. A quick shot of light snow is possible
along the back edge of this system as the 300mb jet exit region
slides across the forecast area. This will have little impact on the
region, particularly in comparison to the snow falling earlier in
the day.

A surface high will push into the region Sunday behind Saturday`s
system while cold Arctic air surges into the region along
northwesterly low-level flow. This will drop 850mb temperatures into
the negative single digits, near the 10th percentile for late
November/early December. This cold air will linger for the first few
days of the work week, pulling high temperatures nearly 20-25
degrees below normal. This Arctic airmass has the potential to set
record low maximum temperatures Monday, but will depend on fine
details like snow cover, additional snowfall, and cloud cover. At
the same time mid-level disturbances ahead of an approaching mid-
level trough and low-level frontogenesis will provide forcing Monday
into Tuesday for snow showers. QPF is expected to be very light
given a lack of significant moisture return and the cold
temperatures, resulting in likewise light snow accumulations. While
amounts, timing, and location are uncertain right now, confidence is
high that wherever snow falls, it will stick, including on untreated
roadways.

Low-level ridging and subsequent warm air advection kicks off
Wednesday, starting a warming trend that will pull high temperatures
back into the 30s to 40s through the end of the forecast period.

Delia

&&

.AVIATION...  (For the 00z TAFs through 00z Friday Evening)
Issued at 454 PM CST Thu Nov 27 2025

Dry and VFR conditions will prevail through the TAF period. Winds
will remain light through the majority of the period with mid-
level to high clouds entering from the northwest throughout the
day. The chance of snow will begin just outside of the 00z TAFs.
Will include mention during the next package.

Jaja

&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...Winter Storm Watch from late Friday night through Saturday
     evening for Knox MO-Lewis MO-Lincoln MO-Marion MO-Monroe MO-
     Pike MO-Ralls MO-Saint Charles MO-Saint Louis City MO-Saint
     Louis MO-Shelby MO.

IL...Winter Storm Watch from late Friday night through Saturday
     evening for Adams IL-Bond IL-Brown IL-Calhoun IL-Fayette IL-
     Greene IL-Jersey IL-Macoupin IL-Madison IL-Montgomery IL-
     Pike IL.

&&

$$

WFO LSX