153
FXUS63 KLOT 202325
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
525 PM CST Mon Jan 20 2025

.KEY MESSAGES...

- Scattered flurries expected this afternoon and evening. A few
  snow showers are also possible for areas south of the
  Kankakee River where some snow accumulations may occur.

- Another bitterly cold night ahead with wind chills in the -20F
  to -30F range. Cold Weather Advisory remains in effect until
  noon Tuesday.

- 30-40 percent chance of accumulating snow on Wednesday which
  could lead to slippery travel.

&&

.DISCUSSION...
Issued at 300 PM CST Mon Jan 20 2025

Through Tuesday Night:

Cloud cover has been increasing across much of the Midwest
(including northern IL) this afternoon as an upper-level
disturbance pivots through the region. Given the cold air mass
in place, cloud top temperatures are sitting below -20C which
has yielded some flurries and even a few bonafide snow showers
where low-level instability has maximized. These flurries and
snow showers are expected to drift into northern IL and
northwest IN over the next 1 to 3 hours and persist through at
least midnight before tapering. While accumulations with any
flurries are not expected, the fine snow grains and blustery
winds may produce local visibility reductions in the 3 to 5 mile
range with lower values possible with any true showers.
However, some snow accumulations (generally under a quarter
inch) may occur with the snow showers; especially for areas
south of the Kankakee River where better shower coverage is
forecast. That said, those planning to be out and about tonight
should use caution in case of slippery roads and visibility
changes.

The bigger weather story of the night will be the return of
bitterly cold temperatures and wind chills as the snow departs.
While some guidance is showing cloud cover persisting overnight,
the thin nature of any clouds shouldn`t inhibit radiational
cooling much. Therefore, expect overnight lows to dip below zero
with wind chills expected to bottom out in the -20F to -30F
range by daybreak. Since the -30F wind chills will be more
isolated, have decided to maintain the Cold Weather Advisory for
now but if more widespread coverage of these wind chills occur
an upgrade to a Extreme Cold Warning may be considered.

Temperatures will rebound a bit on Tuesday, but despite the
sunshine, highs will only top out in the low single digits.
Couple this with the 15 to 20 mph winds, wind chills will remain
in the negative teens through the day on Tuesday. However, as
winds turn more south-southwesterly Tuesday night temperatures
should gradually increase heading into Wednesday morning. Though,
lows Tuesday night will still be around zero to negative single
digits before this occurs.

Yack


Wednesday through Monday:

Southwesterly warm advection will be well underway at the
beginning of the extended forecast period with the region
situated between a 1030+mb high over the Mid-Atlantic and low
pressure developing across northern Minnesota/southwest Ontario.
While temperatures will accordingly be increasing steadily
throughout the day, sub-zero wind chills are expected to persist
through Wednesday morning due to strong winds.

In addition to the warming temperatures and gusty winds to 25-30
mph, there remains the potential for accumulating snow Wednesday
morning through the evening. A pair of shortwaves are forecast to
move through the broader region during this time, one paired with
the aforementioned surface low across the northwest Great Lakes
and a separate sheared out wave pivoting out of the central
Plains. While these features don`t appear to phase fully (at least
at this range), ascent associated with them should be sufficient
for snow.

Modeled vertical profiles show a stout but dry layer in the
lowest levels, at least initially Wednesday morning. However, the
mid-levels are expected to saturate, with the saturation becoming
quite deep through the DGZ (2-3+ km at times) which would support
large enough dendrites to punch through this dry layer. While it
will be notably warmer than the first part of the week, air
temperatures still in the teens and lower 20s would allow snow to
readily accumulate on untreated surfaces. Models continue to
support a mostly lighter QPF scenario with this system, with
snowfall amounts generally in the 1" or less range, just enough to
make untreated roads slippery. For now have maintained mainly
30-40 percent chances for snow with this system. Would like to see
how some of the hi-res guidance handles things before introducing
"likely" or higher PoPs (55%+), but suspect that is where things
are headed.

The cold front eventually moves through late Wednesday night
into Thursday morning, temporarily stalling our warming trend
with highs in mid-upper 20s. Lake effect snow may linger in
northeast Porter County on Thursday, otherwise dry conditions are
forecast through the end of the week with temperatures gradually
moderating each day back above freezing on Saturday. While a
quick hit of light snow can`t be ruled out at times, ensemble
trends favor generally drier conditions continuing.

Petr

&&

.AVIATION /00Z TAFS THROUGH 06Z WEDNESDAY/...
Issued at 525 PM CST Mon Jan 20 2025

The main aviation weather concerns for the 00Z TAF period
include:

* Flurries and light snow showers this evening into tonight with
  associated MVFR possible

A system of light snow is pushing across the Mississippi as of
2330Z. This snow will progress eastward and approach RFD near
00Z and closer to 02Z at the Chicago sites. The snow should be
mostly non-impactful with largely VFR conditions, although brief
periods of MVFR cigs and, more likely, vsbys will be possible,
especially through this evening. Light snow is expected in the
area into the overnight before moving away prior to daybreak.
Light dustings of accumulation will also be possible.

Meanwhile, WNW winds will continue gusting into the 20 to 25 kt
range through the evening and into tonight before subsiding
closer to 15 to 20 kt after about 08Z. Expect westerlies gusting
into the low 20 kt range during the daytime tomorrow, subsiding
for the evening.

Doom

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Cold Weather Advisory from 9 PM this evening to noon CST
     Tuesday for ILZ003-ILZ004-ILZ005-ILZ006-ILZ008-ILZ010-
     ILZ011-ILZ012-ILZ013-ILZ019-ILZ020-ILZ021-ILZ023-ILZ032-
     ILZ033-ILZ039-ILZ103-ILZ104-ILZ105-ILZ106-ILZ107-ILZ108.

IN...Cold Weather Advisory from 9 PM CST /10 PM EST/ this evening
     to noon CST /1 PM EST/ Tuesday for INZ001-INZ002-INZ010-
     INZ011-INZ019.

LM...Small Craft Advisory until 3 AM CST Tuesday for the IL and IN
     nearshore waters.

&&

$$

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