Cold in Illinois, December 24 to 25, 2022

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
548 PM CST Sat Dec 24 2022

.SHORT TERM...
Issued at 232 PM CST Sat Dec 24 2022

Through Sunday...

The Great Lakes reside on the western side of broad upper-level
cyclonic flow across eastern North America anchored by a powerhouse
966mb low pressure system centered in west-central Quebec. With a
1035mb high pressure system across the central US, there is a nearly
70 mb pressure gradient across the Great Lakes! It is thus no
surprise winds continue to gust 35-40 mph this afternoon which
continues to lead to pockets of blowing snow particularly in open
and rural areas. Thanks to the position of the surface low, the
predominant wind direction has become west to even west
southwesterly over the past few hours, confining lake effect snow
showers to LaPorte and Berrien counties and points northward. Mostly
sunny skies are treating our area, which is no doubt making it feel
nicer than the sub-zero wind chills that continue at this hour.

Tonight, the sprawling storm system in Canada is expected to
continue lifting further away from our area. Even as winds ease this
evening, mostly clear skies allowing temperatures to fall toward
0F will support a continuation of nippy wind chills down to
around - 20F. For this reason, we will hoist a Wind Chill Advisory
for all but central Cook county where the urban heat island
should hold values closer to -15F (criteria is -20F). Meanwhile, a
wiggle in the departing upper-level cyclonic flow will allow for
winds to modestly veer northwesterly bring a return of lake
effect snow showers in far northeastern Porter county. For now,
will let the Winter Storm Warning go though a few spots (really
within a mile or two into the northeastern part of the county) may
still see low visibility from time to time tonight.

Compared to the past few days, Sunday looks wonderful with mostly
sunny skies and temperatures in the lower teens. West winds of 10-15
mph will continue to support sub-zero wind chills, so still cover up
from head to toe to be safe.

Borchardt

&&

.LONG TERM...
Issued at 216 PM CST Sat Dec 24 2022

Monday through Saturday...

The long term forecast begins with a chance for some snow Christmas
evening and night. Through the day on Sunday, an Alberta clipper
will quickly take shape and drop through the northern Plains and
eventually on into the western Midwest. Despite the rather compact
size of the system and the low amplitude of the associated trough,
some impressive forcing is anticipated to set up around the base of
the trough, especially in the low levels. However, it appears that
the better forcing will miss the area to the west and to the south.
Additionally, the strong positive tilt of the trough will result in
a quasi-isothermal profile through from the surface through roughly
700mb. Whereas prior model runs were suggesting this isothermal
layer would sit near -9 or -10C, the profile has been on a cooling
trend, especially in the RAP and HRRR which have since pulled the
layer down into the DGZ with temps near -11 and -12C resulting in a
DGZ potentially 10,000-12,000 ft deep. Despite a nearly saturated,
deep DGZ, the lack of forcing aloft will keep the snow production
and resultant snowfall rates somewhat humble. Snow totals are
looking like around an inch across much of the area with locally
higher totals possible in the western and southwestern CWA and
lesser totals likely across northwest IN. Mid-evening Sunday through
the earlier part of Monday morning appears to be the most likely
timeframe for snow.

Monday and Tuesday will see highs in the lower and middle 20s before
a robust low level jet moves over the Midwest on Wednesday and reels
in a much warmer airmass from the southwest with the aid of the SW
return flow from the backside of the prominent Bermuda high. Lower
40s are looking likely on Wednesday followed by 50s potentially on
Thursday as the efficient WAA continues. As a trough digs in the
western US Wednesday into Thursday, a surface Low is expected to
spin up in the Plains and push into the upper Midwest. As it does,
the system's cold front looks to slowly drag across the area and
provide us with some rain showers through much of Thursday and
Friday. As the trough continues to dig, guidance shows a second low
pressure center forming along the front dropping widespread showers
along its warm front as it pushes through the CWA on Saturday. With
the center of each system missing the area just to the west, the WAA
will continue through the end of the week likely keeping the upper
40s/lower 50s hanging on into the weekend.

Doom

&&

.AVIATION...
For the 00Z TAFs...

Aviation forecast concerns:
 * Gusty westerly winds persist through tonight
 * Low chance for brief MVFR cigs - more likely remain VFR
 * Chance for snow late Sunday evening/night

Winds remain gusty out of the west early this evening to around
30kt. This will very gradually ease with time overnight and into
tomorrow with gusts still in the low 20kt range through the daytime
hours. There is a signal for low clouds (1500-2000ft range) to
develop overnight into Sunday morning, though a similar signal
existed for today and it remained cloud free across the area, so
think the more likely scenario is that we remain VFR.

Toward the end of the 30-hr TAFs light snow may begin moving
through the region with a quick moving weather system late Sunday
evening and overnight. Have left this out of the TAF for now and
will be addressed with later updates as confidence increases.

Petr

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Wind Chill Advisory...ILZ003-ILZ004-ILZ005-ILZ006-ILZ008-ILZ010-
     ILZ011-ILZ012-ILZ013-ILZ019-ILZ020-ILZ021-ILZ023-ILZ032-
     ILZ033-ILZ039-ILZ103-ILZ105-ILZ106-ILZ107-ILZ108...9 PM
     Saturday to 10 AM Sunday.

IN...Wind Chill Advisory...INZ001-INZ002-INZ010-INZ011-INZ019...9 PM
     Saturday to 10 AM Sunday.

LM...Gale Warning...nearshore waters until 10 PM Saturday.

     Heavy Freezing Spray Warning...nearshore waters until 10 PM
     Saturday.

&&

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Hazardous Weather Outlook...UPDATED
National Weather Service Chicago/Romeoville IL
1009 PM CST Sat Dec 24 2022

ILZ003>006-008-010>013-019>021-023-032-033-039-103>108-INZ001-002-
010-011-019-260415-
Winnebago-Boone-McHenry-Lake IL-Ogle-Lee-De Kalb-Kane-DuPage-
La Salle-Kendall-Grundy-Kankakee-Livingston-Iroquois-Ford-
Northern Cook-Central Cook-Southern Cook-Northern Will-
Southern Will-Eastern Will-Lake IN-Porter-Newton-Jasper-Benton-
1009 PM CST Sat Dec 24 2022 /1109 PM EST Sat Dec 24 2022/

This Hazardous Weather Outlook is for portions of North Central
Illinois...Northeast Illinois and Northwest Indiana.

.DAY ONE...Tonight.

Weather hazards expected...

  Elevated Excessive Cold Risk.

DISCUSSION...

Wind chills will drop to -20 to -25 degrees tonight away from the
urban center of Chicago.

.DAYS TWO THROUGH SEVEN...Sunday through Friday.

Sunday...
  Elevated Excessive Cold Risk in the morning.
  Limited Snow Risk late in the evening.
Monday...
  Limited Snow Risk in the morning.

DISCUSSION:

Snow is expected Sunday evening into Monday morning with minor
accumulations up to 1 inch possible.

.SPOTTER INFORMATION STATEMENT...

  Spotter activation will not be necessary through tonight.

LMZ740>745-260415-
Winthrop Harbor to Wilmette Harbor IL-
Wilmette Harbor to Northerly Island IL-
Northerly Island to Calumet Harbor IL-
Calumet Harbor IL to Gary IN-Gary to Burns Harbor IN-
Burns Harbor to Michigan City IN-
1009 PM CST Sat Dec 24 2022

This Hazardous Weather Outlook is for portions of the Illinois
nearshore waters of Lake Michigan and the Indiana nearshore
waters of Lake Michigan.

.DAY ONE...Tonight.

Weather hazards expected...

  Hazardous conditions for small craft.
  Limited Freezing Spray Risk.

.DAYS TWO THROUGH SEVEN...Sunday through Friday.

Sunday...
  Hazardous conditions for small craft are likely.
  Limited Freezing Spray Risk in the morning.
Tuesday and Wednesday...
  Hazardous conditions for small craft are likely.

Petr

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