533
FXUS63 KLOT 050936
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
336 AM CST Wed Feb 5 2025

.KEY MESSAGES...

- Hazardous travel expected after sunset this evening into
  tonight as freezing rain and freezing drizzle causes untreated
  roadways to become icy.

- West to northwest winds gusting 25 to locally 40 mph are
  expected Thursday and evening, with the highest readings near
  the Wisconsin state line.

- There will be another threat for a wintry mix Friday night
  through Saturday.

&&

.DISCUSSION...
Issued at 336 AM CST Wed Feb 5 2025

Through Tonight:

Main forecast concern continues to center around the period of
icy weather expected tonight. Some notable changes were made to
the going forecast for today and tonight, these include:
- Significantly slowing the onset timing of the wintry
  precipitation. Aside from some low (20%) chances for a band of
  light snow across northern IL this morning, the precipitation is
  now expected to hold off for much of the day, likely not
  onsetting in most areas until afternoon sunset early this
  evening.
- Pushed off the start time of the winter weather advisory due to
  the later anticipated onset time. We now have areas generally
  south of I-80 beginning at 4 pm this afternoon, while areas
  farther to the north now start at 6 pm this evening. Did not
  make any changes to the current 6 am end time.
- Modified precipitation types to push the forecast more towards
  freezing drizzle/freezing rain, in favor of a mix of snow sleet
  and freezing rain.

As mentioned in the previous discussions, there remains some
low potential (~20%) for a band of light snow to materialize
this morning, mainly across far northern IL. While there are
some decent mid-level lapse rates and forced ascent aloft to
support this potential, suspect the very dry lower level layer
noted in the 00Z DVN RAOB will win out and result in sublimation
of the snow before making it to the surface.

Otherwise, our focus today will be on the freezing
precipitation expected to develop northward across downstate IL
through the day in advance of an approaching mid-level impulse.
As noted above, we have slowed down the onset of this
precipitation by several hours, with it now likely not onsetting
in our area until after sunset this evening. The peak timing is
looking to be this evening from around 6 pm through around 2am
tonight. During this timeframe, it appears that the primarily
precipitation type will be freezing rain and/or freezing
drizzle, owing to the presence of a dry mid- level airmass
moving overhead through the evening. This looks to result in
cloud temperatures remaining too warm (likely warmer than -5C)
to support heterogeneous ice nucleation needed for snow and
sleet production. Accordingly, we have steered the forecast more
towards a freezing rain/freezing drizzle outcome.

In spite of the expectation for the precipitation to fall as
freezing rain and/or freezing drizzle, this is not going to be a
significant ice storm for the area. We thus are not expected
significant power outages and downed tree limbs. However, the
combination of light precipitation rates (possible even just
drizzle at times), breezy southeasterly winds (10 to 15 mph) and
subfreezing temperatures (and wetbulb temperatures in the mid
to upper 20) will make for efficient ice accretion. Accordingly,
hazardous travel conditions are expected tonight, especially
after 6 PM this evening as untreated streets and sidewalks
quickly become glazed in ice. Conditions will begin to slowly
improve towards daybreak Thursday as temperatures gradually warm
above freezing and the precipitation comes to an end.

KJB


Thursday through Tuesday:

The forecast thinking during this period has not changed.
Reference the previous discussion below for more details.

In the immediate wake of the upper-level wave responsible for a
wintry mix of precipitation Wednesday night, a secondary upper-
level wave and associated surface low will race across the
Upper Great Lakes on Thursday. At this point, ensemble model
guidance favors additional precipitation chances staying north
of our area across Wisconsin and Michigan. With that said, the
tightening pressure gradient between the 990mb low to our
northeast and a 1020mb surface high building into the Plains
(forcing 6hr pressure rises on the order of 6-9mb locally) and
tumbling temperatures (850mb values dropping some 7-9C/6 hours)
will lead to a pop in west to northwesterly winds particularly
during the afternoon and evening. Forecast thermal profiles from
the GFS in particular are bullish showing some 45 to 55kt of
flow available to mix downward to the ground, though feel a more
conservative approach is prudent at this range. So, will
advertise gusts of 25 to 40 mph Thursday afternoon and evening,
with the highest gusts near the Wisconsin state line (tighter
pressure gradient).

On Friday, the aforementioned surface high will slide through
the Great Lakes as a muted upper-level ridge builds into the
Great Lakes. Upper-level clouds will mark an otherwise
uneventful day characterized by light winds and highs near the
freezing mark.

Ensemble model guidance is in agreement that the next upper-
level wave will traverse the country in the Friday night to
Saturday timeframe. There is a fair amount of agreement that the
the associated surface low will track directly over or nearby
our area, leading to another opportunity for a messy mix of
wintry precipitation types. For these reasons, will carry
relatively high PoPs (>75%) and continue a wintry mix as the
precipitation type into our forecast.

Looking toward next week and the middle of February, ensemble
model guidance supports the development of broad upper-level
troughing across the central United States. Such a regime favors
a period of below average temperatures and perhaps episodic
opportunities for snowfall in the general region.

Borchardt

&&

.AVIATION /06Z TAFS THROUGH 12Z THURSDAY/...
Issued at 1049 PM CST Tue Feb 4 2025

Forecast concerns include...

Mvfr cigs through Wednesday morning.
Wintry mix changing to light freezing rain Wednesday evening.
Ifr cigs, possible lifr cigs early Thursday morning.

Mvfr cigs are expected to continue overnight, scattering out
from southeast to northwest around and just after daybreak.
There may still be a few flurries at times, but confidence is
low and have removed mention from this forecast.

A wintry mix is expected to develop early Wednesday evening and
transition to light freezing rain during the evening. There is
still uncertainty for start and transition timing, and changes
are possible with later forecasts. The freezing rain is expected
to end during the early overnight hours Thursday morning. Cigs
will lower into mvfr and then slowly lower into ifr early
Thursday morning with lifr cigs possible. There may also be some
fog early Thursday morning, but confidence is low.

Northeast winds around 10kt will turn easterly overnight and
southeasterly Wednesday morning when gusts into the 15-20kt
range are expected. These gusts will end Wednesday evening with
wind directions turning south/southwest early Thursday morning.
cms

&&

.MARINE...
Issued at 336 PM CST Wed Feb 5 2025

As a low pressure system moves through the northern Great Lakes
and a surface high pressure system builds into the Mississippi
River Valley, a period of westerly to northwesterly gale force
winds in excess of 35 kt are expected, mainly north of Gary,
Indiana. With confidence increasing, the Gale Watch has been
upgraded to a Gale Warning, which is in effect along the
Illinois nearshore waters of Lake Michigan from 12 PM Thursday
to 12 AM Friday morning.

Borchardt/KJB

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Winter Weather Advisory from 6 PM this evening to 6 AM CST
     Thursday for ILZ003-ILZ004-ILZ005-ILZ006-ILZ008-ILZ010-
     ILZ011-ILZ012-ILZ013-ILZ020-ILZ103-ILZ104-ILZ105-ILZ106-
     ILZ107-ILZ108.

     Winter Weather Advisory from 4 PM this afternoon to 6 AM CST
     Thursday for ILZ019-ILZ021-ILZ023-ILZ032-ILZ033-ILZ039.

IN...Winter Weather Advisory from 6 PM this evening to 6 AM CST
     Thursday for INZ001-INZ002.

     Winter Weather Advisory from 4 PM CST /5 PM EST/ this
     afternoon to 6 AM CST /7 AM EST/ Thursday for INZ010-
     INZ011-INZ019.

LM...Small Craft Advisory from 6 PM this evening to 3 AM CST
     Thursday for Winthrop Harbor to Wilmette Harbor IL.

     Gale Warning from noon Thursday to midnight CST Thursday night
     for the IL nearshore waters.

&&

$$

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