423
FXUS63 KILX 030042
AFDILX

Area Forecast Discussion...Updated
National Weather Service Lincoln IL
642 PM CST Thu Jan 2 2025

.KEY MESSAGES...

- A brief period of snow this evening will bring between a dusting
  and 1.5 inches of accumulation in areas north of a Macomb to
  Lincoln to Paris line, with the higher amounts favoring areas
  north of Interstate 74.

- A winter storm system will impact central and southeast Illinois
  from overnight Saturday night into Monday morning. There is
  between a 60-80% confidence in moderate winter weather impacts
  along and south of a Macomb to Lincoln to Hoopeston line.

- Cold temperatures will grip the region from Friday onward, with
  even colder temperatures arriving by the middle of next week.
  Confidence is high (60-80% chance or greater) that wind chill
  values will fall below zero across central and southeast
  Illinois from Tuesday night on.

&&

.UPDATE...
Issued at 640 PM CST Thu Jan 2 2025

Band of light snow stretches from near the Illinois River to the I-
57 corridor at 630pm CST. This will continue to track east-
southeast across the area producing a swath of around 1-2 inches
of accumulating snow mainly near and north of a Havana to Danville
line. Have shifted the accumulating snow line further south
slightly from previous forecasts and bumped up totals...partially
due to higher than expected snow-to-liquid ratios and a narrow
east-west oriented banding apparent on radar. In addition,
pavement temps have fallen since sunset and area webcams are
showing snow sticking in spots on untreated roads.

Deubelbeiss

&&

.SHORT TERM...  (Through Late Friday Night)
Issued at 230 PM CST Thu Jan 2 2025

A tongue of low-level moisture will continue to advect northward
ahead of a subtle mid-level shortwave trough currently digging
across SE Iowa. The net effect will be a broad area of light precip
spreading eastward across central Illinois this evening. Thermal
profiles support a change-over to snow north of a Macomb to
Lincoln to Paris line where the sfc wet-bulb zero line is
anticipated to evolve.

A quick glance at our mesoanalysis fields show strong upward
omega through a saturated DGZ across SE Iowa. This has resulted
in 0.5-1"/hr snowfall rates, but lasting only 1-2 hours before
moving east. As this disturbance continues to push ESE into
central IL this evening, we anticipate a similar scenario across
I-74 and north.

Despite the quick-hitting snowfall, we don`t anticipate widespread
travel impact. Sfc temps warmed into the lower 30s this afternoon,
and with several hours of partial sunshine, pavement temps have
warmed well into the 40s. It is certainly plausible that snowfall
rates this evening become robust enough to overcome the
relatively- warm pavements, but given the short duration of the
event, thinking roads will become more slushy than icy along and
north of I-74. Then, as temperatures steadily fall into the upper
teens tonight, any residual wetness on untreated bridge decks
could become icy. We have no plans at this point to issue a winter
weather advisory, but will continue to monitor trends upstream.

Temperatures will fall sharply behind this evening`s little
clipper system. Drier and blustery conditions will help force
overnight lows into the upper teens. When coupled with a north
wind gusting 20-25 mph, the wind chill values will fall into the
single-digits to begin Friday morning. These cold and dry
conditions then stretch through most of Saturday as an area of
elongated surface high pressure lingers ahead of an impending
winter storm.

MJA

.LONG TERM...

This forecast period will be dominated by a major winter storm
late this weekend, followed by a prolonged period of very cold
temperatures.

Regarding the impending winter storm, the general narrative more
or less remains the same: a deepening upper- level disturbance
currently positioned over the western Pacific will serve as a
catalyst for mid- latitude cyclogenesis by this weekend. The storm
system will combine with a seasonably cold airmass to bring
moderate- to- major winter impacts to portions of the Midwest, and
likely directly to central and southeast Illinois between Sunday
morning and midday Monday.

---------------------------------------------------------------------

Here are the two forecast outcomes we are currently analyzing,
both of which bring moderate-to-major winter impacts:

(A) The Most Likely Outcome:
Global ensemble guidance from the main players (GEFS, ENS, and
GEPS) seems to slightly favor the odds of a faster/opened mid-
level trough translating across the Mid-Mississippi Valley by
early Monday morning. This would result in a slightly less-deep
surface low pushing through our area, and in turn, provide a lower
probability of mesoscale banding (e.g. mid-level frontogenesis &
TROWAL enhancements). If this outcome is achieved, it will result
in less snow. That is not to say we won`t still achieve a
significant snowfall under this pattern. It might just mean the
snowfall range is more like 5-10" across the Winter Storm Watch
area rather than 8-15".

Additionally under this outcome, the thermal profile across our
I-70 corridor may favor a less-deep warm layer, resulting in more
sleet than freezing rain.


(B) A Reasonable Alternative:
This scenario would favor the GFS/GEFS solution where the upper-
level trough remains slower/closed as it moves into the Ohio River
Valley by Monday morning. The net effect would be a slightly
deeper surface low, resulting in stronger mid-level frontogenesis
and a strengthening TROWAL, both of which will prolong the
duration of the moderate-to-heavy snow event. If this scenario
unfolds, QPF will be higher. Snowfall will be higher, perhaps in
the 8-15" range across the Winter Storm Watch area. And, freezing
rain across the I-70 corridor will be higher, with a much deeper
warm layer. If this scenario plays out, one-tenth to one-quarter
inch of flat ice is certainly plausible along and south of I-70,
though the higher QPF rates could actually limit the icing
potential due to excess runoff.

Another factor to consider if this scenario becomes favored, is a
northward expansion of the Winter Storm Watch as the deeper synoptic
lift (more favorable jet coupling signature) results in a broader
QPF/snow shield.

-----------------------------------------------------------------------

Either way, we are facing a high probability (60-80% chance) for
at least moderate winter weather impacts. Either of the above
outcomes meet our Winter Storm Watch criteria for snow, and with
blustery north winds (20-30 mph) whipping around, we have
concerns for blowing/drifting snow as well as sporadic power
outages.

Not to be outdone, but in the wake of our winter storm, a
prolonged stretch of very cold temperatures will bring its own set
of impacts to the region. Confidence in well-below normal
temperatures remains high as cross-polar flow disperses some of
the coldest weather of the season. Some question remains as to how
prolonged the cold snap will be, but confidence is high (at least
60% chance) that overnight wind chill values will fall below zero
Tuesday night - Thursday night.

&&

.AVIATION...  (For the 00z TAFs through 00z Friday Evening)
Issued at 525 PM CST Thu Jan 2 2025

A band of light precip and accompanying MVFR ceilings is moving
across central Illinois this evening associated with a passing
cold front. Primarily snow is favored further north at SPI/BMI,
while rain or a rain/snow mix is possible to the south. Precip
will taper off by late this evening and expect winds to increase
behind the front with gusts picking up to around 20kt.

Deubelbeiss

&&

.ILX WATCHES/WARNINGS/ADVISORIES...
Winter Storm Watch from late Saturday night through Monday
morning for ILZ040>057-061>063-066>068-071>073.

&&

$$