921
FXUS63 KLOT 081646
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1046 AM CST Wed Jan 8 2025

.KEY MESSAGES...

- Accumulating snow likely for much of the Chicago metro and
  areas around and east of I-55 late Thursday night into Friday
  afternoon, with slippery conditions possible during the
  Friday morning commute.

- 30-50% chance of accumulating snow on Sunday ahead of a period
  of arctic air crossing the area early next week.

&&

.DISCUSSION...
Issued at 253 AM CST Wed Jan 8 2025

Through Thursday:

It will be another chilly start to the day with temperatures in
the single digits to teens. Continued cold air advection into
the region should limit our warming yet again with forecast
highs in the low to mid 20s (~5-10 degrees below normal for this
time of year).

A mid-level wave, currently over the UP of Michigan and Lake
Superior, is expected to dive across Lake Michigan and Lower
Michigan through the day today. Model guidance has trended a
bit further east with this feature and the associated low-
stratus deck. However, given the proximity to the wave, still
think at least some stratus will try to graze far northeast
Illinois into northwest Indiana near the lake this afternoon.
Occasional flurries will be possible where there is stratus, so
opted to maintain a slight chance of flurries in those areas. In
tandem, the location of the main lake effect snow band has
trended farther east across western lower Michigan and
northwest/north central Indiana, which keeps the higher snowfall
rates and potential for accumulations mainly east of the
forecast area. Forecast soundings suggest cloud tops just barely
reach the lower end of the DGZ as well, which is more
indicative of poorer quality snowflakes/lower SLRs and should
limit accumulations to not much more than a dusting (if that).
Given expected northwesterly wind directions, do think that at
least light snow showers/flurries could occur at times through
this evening in northern Porter County and felt comfortable
maintaining slight chances for snow (20%).

Clearing skies are expected this evening and overnight as the
surface ridge axis pivots overhead which should allow
temperatures to once again drop into the single digits to low
teens prior high clouds streaming into the region early Thursday
morning ahead of our next system. Forecast highs on Thursday
are slightly warmer than today, but given that southwest winds
will be moving across a deep snowpack across central/southern
IL, suspect highs will remain in the 20s, coldest south of
I-80.

Petr


Thursday Night through Tuesday:

A pair of mid-level waves over southern Canada on Thursday will
phase into an amplifying wave over the Upper Mississippi River
Valley. Meanwhile, a large upper-level low over northern Mexico
will induce a Gulf low over the Texas coast. Between these two
areas, a potent 150 knot anticyclonically-curved upper-level jet
streak will cross the Ohio River Valley Thursday night in
conjunction with broad mid-level diffluence. This will foster
strong forcing that will spread across the area through Friday
morning. A modestly dry airmass ahead of the northern wave
(PWATs around 0.3") will limit overall precip intensity, but the
magnitude of forcing should produce an area of light snow over
at least the southeast 2/3 of the forecast area, including much
of the Chicago metro. Some mid-level Pacific moisture advecting
into the area Friday morning should enhance the precip shield
with eastward extent before further amplifying east of the CWA
late Friday afternoon and night. Latest indications are for
snowfall amounts ranging from a light dusting across the
northwest CWA to up to 2"+ in the far southeast CWA. This would
put much of the Chicago metro at around 1", with some impacts
possible with the snow falling right around the morning rush.

The phased trough will carve out a longwave trough across much
of North America this weekend, gradually bringing an expansive
area of arctic air over the Great Lakes and Northeast Sunday
into the middle of next week. As this transition occurs Sunday
into Sunday night, a clipper system will bring a swath of
accumulating snow over the Upper Mississippi River Valley and
western Great Lakes. Compared to 24 hours ago, several more
ensemble members are supporting a solution favoring the bulk of
the associated snow shield crossing Wisconsin and into northern
Illinois. Details on the exact timing and location still need
to be ironed out, but the overall message is for increasing
chances of snow Sunday morning (primarily WAA-driven) and Sunday
afternoon/evening (trough and arctic front forcing).

A series of weaker waves embedded within the larger longwave
trough will rotate across the western Great Lakes early next
week as much colder conditions settle across the area. On and
off snow showers can be expected amid blustery NW winds and sub-
zero wind chills for much of the period.

Kluber

&&

.AVIATION /18Z TAFS THROUGH 00Z FRIDAY/...
Issued at 1046 AM CST Wed Jan 8 2025

There are no aviation weather concerns anticipated through the
period. Overall, expect VFR conditions to prevail, with
northwesterly winds persisting through the afternoon. Winds will
then turn westerly and become light (<5kt) tonight, before
settling into a south-southwest direction around 10 to 12 kt
during the day on Thursday.

KJB

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...None.

&&

$$

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