870
FXUS63 KLOT 191109
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
509 AM CST Sun Jan 19 2025

.KEY MESSAGES...

- Dangerously cold wind chills (frostbite potential in ~30
  minutes) expected this morning across interior far northern
  Illinois, areawide Sunday night-Monday morning, and again
  Monday night - early Tuesday.

- Slight chance for lake effect snow in Porter County Indiana
  today. Flurries are possible areawide Mon PM and evening.

&&

.DISCUSSION...
Issued at 223 AM CST Sun Jan 19 2025

Through Monday:

Winds remain out of the northwest providing a decent fetch over
the lake into northwest Indiana. There is also a strong
temperature difference between the 850 mb layer and the surface,
which will result in around 100-300 J/kg of lake induced CAPE.
Slight chance PoPs were maintained for lake effect snow through
this evening, mainly in Porter County Indiana. As winds turn to
the west overnight into Monday, precip chances diminish as the
lake effect focus turns primarily to southwest Michigan.

As mentioned, winds are currently out of the northwest with
gusts around 20 knots. Temperatures continue to tumble this
morning, with the coldest temperatures in the far northwestern
portions of the forecast area. Guidance stayed in line with this
thinking for wind chills around -20F, so no changes were made
to the Cold Weather Advisory in effect for that area through
this morning.

Temperatures will gradually "warm" into the single digits with
wind chills between -5F to -15F this afternoon. However,
temperatures will drop once again Sunday night into Monday
morning. Wind chills are expected to be at or below -20F across
the forecast area. Given that wind chills are expected to warm
above -15F during the day on Monday, no changes were made to the
end time of the current Cold Weather Advisory, which runs
through noon. See the discussion below for more on the continued
cold weather Monday night and beyond.

Lastly, other than the lake effect over Porter County
previously mentioned, most of the forecast area should see dry
conditions through Monday afternoon as a 1030+ high pressure
system gradually moves east through the Ohio River Valley.
However, there is a fairly stout vort max that will come down
out of the northern Plains Monday afternoon. With temperatures
as cold as they are and high-res models putting some ascent
through the cloud layer, it is not out of the question for a few
flakes to materialize. Therefore the mention for flurries
areawide was maintained in the forecast for Monday afternoon,
though no accumulations are expected.

DK

Monday Night through Saturday:

The main push of truly arctic air will arrive Monday night into
Tuesday morning. During this time, guidance remains in good
agreement depicting 850 mb temperatures plunging below -30 C
across far northern Illinois. While persistent sprockets of fairly
intense DCVA will meander overhead (leading to a potential for
lingering flurries in the evening), it does look like we`ll
steadily lose low-level moisture through the night resulting in
clearing skies into Tuesday morning. This will set the stage
for air temperatures to fall below zero. Given the lack of any
snow pack of note in the vicinity, it`s unclear just how low
temperatures will go, however. The ECMWF continues to run
colder than the raw GFS/NAM by several degrees, and its MOS-
based output (ECS) indicates lows solidly in the -10 to -15
range across northern Illinois. Given the lack of snow cover,
felt the NBM-delivered lows in the negative single digits
looked reasonable, but will note there`s uncertainty here.
Regardless, with the arctic high immediately to our southwest, a
light westerly breeze will help send wind chills into the -15
to -25 F range into Tuesday morning as the cold episode hits its
peak.

Given the fairly notable moderation in air temperatures and wind
chills Monday afternoon, elected not to extend the current Cold
Weather Advisories (which go through midday Monday) through
Monday night. Instead, thinking is to handle the Monday night -
Tuesday period with a separate advisory, and since this is
beyond 36 hours, will defer to the next shift for this.

As high pressure shifts to our southeast Tuesday night, winds will
swing around out of the south. Wind chills may hover near the -20
F mark through Tuesday night, so any additional headlines will
probably need to run through early Wednesday morning.
Temperatures will rebound fairly significantly on Wednesday as
blustery southwest winds develop ahead of a shortwave which will
be dropping through the Dakotas. Model guidance depicts 925 mb
winds increasing through 40 knots in the warm advection wing
which could necessitate bumping wind gusts up a bit.
Additionally, there`s a little better signal tonight that the
column will saturate down to around or below 850 mb Wednesday
afternoon and evening, and some guidance (GFS, ECMWF in
particular) are producing some swaths of light precip across
northern Illinois during this period. Elected to add some slight
chances for snow given this continued trend towards deeper
saturation with this next wave.

For Thursday and Friday, extended guidance suggests, to varying
degrees, a potential for some additional precipitation chances as
a broad trough swings across the central CONUS. Significant spread
in the guidance at this point precludes making any changes to the
dry NBM-delivered forecast, however.

Carlaw

&&

.AVIATION /12Z TAFS THROUGH 18Z MONDAY/...
Issued at 508 AM CST Sun Jan 19 2025

There are no significant aviation weather concerns. Aside from
perhaps some brief MVFR cigs near GYY and a few flurries, VFR
will prevail today with northwest winds gusting to around 20
knots. Winds should ease and back towards the west tonight
before increasing again out of the west on Monday morning.

Carlaw

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Cold Weather Advisory until noon CST Monday for ILZ003-ILZ004-
     ILZ005-ILZ008-ILZ010-ILZ011-ILZ012.

     Cold Weather Advisory from 6 PM this evening to noon CST
     Monday for ILZ006-ILZ013-ILZ019-ILZ020-ILZ021-ILZ023-
     ILZ032-ILZ033-ILZ039-ILZ103-ILZ104-ILZ105-ILZ106-ILZ107-
     ILZ108.

IN...Cold Weather Advisory from 6 PM CST /7 PM EST/ this evening to
     noon CST /1 PM EST/ Monday for INZ001-INZ002-INZ010-
     INZ011-INZ019.

LM...Small Craft Advisory until 4 AM CST Monday for Winthrop Harbor
     IL to Gary IN.

     Small Craft Advisory until 10 PM CST Monday for Gary to Burns
     Harbor IN-Burns Harbor to Michigan City IN.

     Heavy Freezing Spray Warning from noon today to 4 AM CST
     Monday for Gary to Burns Harbor IN-Burns Harbor to
     Michigan City IN.

&&

$$

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