377
FXUS63 KILX 041708
AFDILX

Area Forecast Discussion...Updated Aviation
National Weather Service Lincoln IL
1108 AM CST Thu Dec 4 2025

.KEY MESSAGES...

- Bitterly cold weather is expected today and tonight. Wind chill
  values will range from 5 to 10 degrees below zero. Protect
  exposed skin and ensure proper heating for safety. Check on
  vulnerable populations and pets.

- Watch for slick spots and rapidly decreasing visibility this
  evening, especially east of the Illinois River Valley where
  freezing fog and/or hoar frost could develop in sheltered and
  low-lying areas.

- The threat for accumulating snow returns late Saturday into
  Sunday, which may cause travel disruption. While a dusting is
  most likely, be prepared for a reasonable (25%) chance of 1 inch
  or more in parts of Eastern Iowa and West- Central Illinois.
  Closely monitor forecast updates over the next 24 hours.

- A substantial warm-up is expected by the middle of next week,
  marking an end to this exceptionally cold spell, if only
  temporarily.

&&

.DISCUSSION...
Issued at 309 AM CST Thu Dec 4 2025

An arctic front delivered the season`s coldest air overnight.
Morning temperatures have dropped into the single digits north of
I-70, with wind chill values generally ranging from 5 to 10
degrees below zero. This exceptionally cold spell for early
December is approximately 25-30 degrees below average and could
potentially break a few records. Despite increasing sunshine,
afternoon highs will struggle to reach the 20s today.

The pattern is expected to continue tonight as surface high
pressure rapidly builds over the region. Notably, despite the
frigid temperatures, the near-surface airmass is not particularly
dry. Tonight`s modeled T-Td (temperature-dewpoint) depressions
are very small, in the 0-3F range, with relative humidity values
ranging 90-100%. Soundings from the GFS, NAM, RAP, and HRRR all
indicate strong, shallow temperature inversions this evening. This
is especially true east of the Illinois River Valley, where
surface winds will remain light and mid-level clouds will be
absent, suggesting a potential for freezing fog or even hoar frost
in sheltered and low-lying areas.

Friday is expected to bring marginally warmer and continued dry
conditions as a storm system passes to the south.

A signal for light accumulating snow returns this weekend,
attributed to a clipper-like system moving through the Corn Belt
embedded in choppy northwest flow. Current guidance from the NBM
and Grand Ensemble (GEFS, GEPS, and EPS) gives a 1 in 4 chance for
at least one inch of snow near and north of a Macomb to
Bloomington line. This probability may be an underestimate due to
existing model differences, and otherwise a classic case where the
mean guidance is camouflaging the underlying model variance, with
the ECMWF/EPS remaining the most bullish regarding QPF and
associated snowfall.

High-resolution guidance will begin to arrive this morning, which
should help to narrow the possibilities.

For now, the most likely scenario for late Saturday into Sunday
is a dusting of snow. A reasonable alternative is a 1-2 inch
snowfall across eastern Iowa into parts of west-central Illinois.
The outlier scenario, resembling the upper-tail of EPS guidance,
is around 4 inches.

A notable warm-up is anticipated into next week, driven by a
longwave ridge establishing itself over the western U.S. This
pattern will induce warm, downslope flow across the Plains, which
will then advect into the Midwest by midweek. Concurrently, a
parade of clipper systems are forecast to track across the Upper
Mississippi Valley and into the Great Lakes, potentially affecting
our area directly at times.

MJA

&&

.AVIATION...  (For the 18z TAFs through 18z Friday Afternoon)
Issued at 1105 AM CST Thu Dec 4 2025

Main concern this period is the potential for fog (or low stratus)
late this evening and overnight. Guidance suggests the greatest
potential for fog is across east-central IL (KCMI, KDEC), but it
could expand further west and impact all terminals. Have vis as
low as 1 mile at KCMI, KDEC, but confidence in the intensity of
the fog is low. With below freezing temperatures, if fog does form
and persist it could lead to slick surfaces. Fog development is
expected as early as 03-04z, with any fog eroding by 14-15z Fri.
Winds will be out of the north this afternoon, becoming variable
this evening/overnight, then south-southwesterly Friday.

Erwin

&&

.ILX WATCHES/WARNINGS/ADVISORIES...
None.
&&

$$