722
FXUS63 KLOT 170506
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1106 PM CST Thu Jan 16 2025

.KEY MESSAGES...

- Friday will be blustery with well above normal temps, then the
  coldest air mass of the season thus far will arrive Sunday and
  linger through next Tuesday night with bitterly cold temps
  and dangerously low wind chills (<=-20F) probable at times.

- Rain will develop Friday evening primarily for areas near and
  southeast of I-55, and then change to wet snow before ending,
  though accumulations if any will likely be a slushy coating on
  colder surfaces.

- Most of the precip during the cold snap will be lake effect
  snow east of our northwest Indiana counties, though flurries
  are possible on Monday.

- Temperatures forecast to moderate mid to late next week.

&&

.DISCUSSION...
Issued at 233 PM CST Thu Jan 16 2025

Through Friday:

A transient surface high pressure ridge axis will slide eastward
across the area this evening before moving off the the Atlantic
coast later in the day on Friday. Meanwhile, a potent and fast
moving surface low (~986 mb) will race eastward across southern
Manitoba into southern parts of Ontario tonight through Friday.
While no precipitation is anticipated through the daylight hours
of Friday as a result of this system, gusty south winds and
even warmer temperatures are expected as a result for Friday.

Winds will diminish this evening as the surface ridge axis
shifts overhead, then gradually back to a southerly direction
overnight following its eastward departure. Thereafter, a rapid
strengthening of the surface pressure gradient is expected to
set up Friday within the warm sector of the Canadian low. This
is expected to foster strong and gusty southerly winds across
our area through the day. These winds are expected to quickly
ramp- up shortly after daybreak Friday morning, with gusts
likely reaching 30-35 mph prior to midday as we begin to mix
into higher momentum aloft. Frequent gusts around 35 mph
(possibly briefly as high as 40 mph) are then expected through
the afternoon hours, before readings ease into the evening.

While it will be windy on Friday, temperatures in this strong
warm air advection regime will turn warmer. Accordingly, we have
bumped temperatures for Friday up a few degrees, with readings
expected to top out near/around 40F in the snow covered areas
in the south, and more into the low to middle 40s in the snow
free areas farther north and northwest.

KJB


Friday Night through Thursday:

The cold front trailing from the deep 986 mb low pressure over
central/southern Ontario will move across the area Friday
night, bringing an abrupt end to the short-lived (but welcome
for many) milder stretch. Following temps reaching the upper 30s
to mid 40s on Friday and still being in the upper 30s to around
40F past sunset, temperatures will steadily fall behind the
frontal passage, down to the upper teens to mid 20s by early
Saturday. Blustery northwest winds gusting up to 30-35 mph will
also develop in the wake of the fro-pa, knocking wind chills
down to the single digits and teens toward daybreak Saturday.

There are no significant changes in the thinking for generally
light precipitation (max liquid equivalents <0.20") ahead of and
just behind the cold front Friday night. Banded precip will
blossom in the frontal trough Friday evening as cold air advection
ensues aloft, primarily near and southeast of I-55. Expect a
sharp gradient on the northwest edge of the precip shield, with
little to no precip expected near and north of a Mendota to
Evanston line. Have PoPs peaking in the categorical range
southeast of I-55 in the late evening and overnight. PoPs will
continue to be further refined in subsequent updates.

An initially above freezing profile will result in the precip
starting as rain, though for areas that do receive precip, ptype
should mix with and then change to wet snow before ending in the
pre-dawn hours of Saturday. A slushy coating of snow is possible
on colder surfaces near and southeast of a Michigan City to Paxton
line by early Saturday, however mostly above freezing air temps
will likely limit travel impacts. Already minor snow amounts were
nudged further downward with this issuance.

Temperatures will primarily flat-line Saturday amidst blustery
north-northwest winds gusting up to 30 mph. PM wind chills will
remain in the single digits and lower teens. Lake effect parameters
will be somewhat marginal on Saturday, though sufficient to hold
onto slight chance (~20%) PoPs into northeast Porter County during
the morning hours.

As cross-Polar flow takes hold Saturday night and onward, a
truly Arctic air mass will arrive with 850 mb temps bottoming
out as low as -25 to -30C in the Sunday night-Tuesday morning
timeframe. In fact, recent deterministic guidance has indicated
850 mb temps as low as -31 to -32C into far northern Illinois
Monday night. The lack of snow cover in parts of our area (which
will likely see some further erosion on Friday) and upstream makes
it a bit uncertain exactly how cold temps and wind chills may get
in this stretch. Sunday morning wind chills bottoming out in the
~10 to 20 below range (coldest northwest CWA) unfortunately will
be the appetizer for the "main course" of cold to follow.

In the official forecast, the overall lowest temps will occur
Sunday night-Monday AM and Monday night-Tuesday AM when sub-
zero lows are forecast area wide along with minimum wind chill
values ranging from -15 to -25F (locally as cold as -25 to -30F
possible Sunday night-early Monday AM). Ensemble probabilities
of lows of -10F or colder anywhere in the CWA are highest on
Monday night. Daytime high temps will likely be held into the
positive single digits for much of if not the entire area Sunday
through Tuesday. Some moderation back to merely below normal
will occur by next Wednesday and Thursday.

The northwest to westerly flow pattern during the cold snap will
likely relegate precip chances to persistent lake effect snow
for the favored snow belts, possibly clipping northeast Porter
County at times. A possible minor exception to this is on Monday
and Monday evening as the 500 mb trough axis brings another
reinforcing surge of CAA. Model forecast soundings during this
time suggest sufficient saturation depths for ice nucleation and
tiny "Arctic dust" snowflakes. It wouldn`t be surprising if low
ratio snow in this regime could yield localized coatings of snow
and hazardous travel (due to the bitterly cold temps rendering
road treatments ineffective). For now, introduced a chance of
flurries to the official forecast. The overarching message during
this period is to prepare for the at times dangerously cold
conditions. Chances for appreciable accumulating snow through next
week are low, with less than 20% of guidance members supporting
recent deterministic GFS solutions next Thursday.

Castro

&&

.AVIATION /06Z TAFS THROUGH 12Z SATURDAY/...
Issued at 1106 PM CST Thu Jan 16 2025

The main aviation weather concerns for the 06Z TAF period
include:

* Low level wind shear during the day on Friday

WSW winds below 10 kt will back to SSW by early Friday.
Direction will remain SSW (190-210) into Friday afternoon as
gusts build into the 25 to 30 kt range. In addition to the gusty
winds, some low level wind shear is anticipated during the day
on Friday with as many as 50-55 kt expected at 2 kft AGL. The
gusts and wind shear will both ease for the evening as direction
veers to westerly. Gusts are expected to return to the 20 to 25
kt range overnight and remain breezy into Saturday.

Some light rain may move over the Chicagoland sites Friday
evening, although the better chances exists farther to the
south and southeast. With the exception of GYY, seemingly low
chances kept a mention out of the TAF for the time being. VFR is
likely throughout the period, although low-end VFR cigs moving
overhead Friday evening and night could go MVFR.

Doom

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...Small Craft Advisory from 8 AM to 9 PM CST Friday for the IL
     and IN nearshore waters.

&&

$$

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