Fri Feb 16 Meteorology for Chicago

[Brett Crapser]

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1204 PM CST Fri Feb 16 2024

KEY MESSAGES...

- A period of snow is expected from mid-morning to mid-
afternoon mainly along and south of Interstate 80. The
northern edge to the snow will be sharp.

- Total snow accumulations of 1 to 3 inches are expected south
of Interstate 80 through this afternoon with locally 3 to 4
inches south of a line from Sibley, IL to Fowler, IN.

- A short-lived cold snap will move through the area with
below-zero wind chills tonight and in the teens throughout the
day on Saturday.

- Above-average temperatures will return Sunday and last through
next week.

&&

UPDATE...
Issued at 1002 AM CST Fri Feb 16 2024

The going forecast for today looks to be in good shape. For this
reason, there is little change in going details of the forecast
mentioned in the overnight discussion below. While a brief couple
hour period of flurries and/or very light snow will be possible
north of I-80 this morning, we expect the accumulating snow, and
impactful travel to largely remain south of I-80 into this
afternoon. The highest snowfall amounts and most persistent
higher rates (0.5 to 1"+ per hour rates) into this afternoon will
remain even farther south (generally south of U.S. 24) into
central parts of IL and IN. In this region, total snow amounts
could push up around 4 inches before the snow tapers off through
the mid to late afternoon hours.

KJB

&&

DISCUSSION...
Issued at 223 AM CST Fri Feb 16 2024

Through Thursday:

Regional radar imagery shows an elongated west-to-east oriented band
of snow from eastern Nebraska to west-central Illinois, though a
pocket of dry air from the surface to around 700mb has thus far
chewed up all precipitation from reaching the surface east of the
Mississippi River. Toward or just after daybreak, an approaching
upper-level trough and increasing upper-level diffluence in the left
exit region of an upper-level 250mb jet streak will lead to the
development of a robust low-level frontogenetical circulation.
As a result, the aforementioned elongated band of snow is
expected to "shift" and expand across northern Missouri and
central Illinois this morning and chew up the low-level pocket
of dry air, allowing for snow to readily reach and accumulate on
the ground. With the transitory nature of the upper-level wave
and associated jet streak, the duration of snow may be limited
to 6 or 8 hours at most, with peak snow rates from mid-morning
to early afternoon. As is often the case with frontogenetically-
forced bands of snow in resident dry airmasses, the northern
edge of snow should be very sharp, in this case near or just
south of I-80.

With a deep layer of forcing through the heart of the DGZ, snow
ratios should average between 12:1 and 18:1 from mid-morning to
early afternoon. When combined with anywhere from 0.05 to 0.2" of
QPF, total snow should range from 1 to 3 inches from I-80 south
(again, we`re expecting a sharp edge from no snow to accumulating
snow). Interestingly, forecast vertical cross sections of -ePV and
theta-e lapse rates through the frontogenetical circulation show
signs of ephemeral CSI, suggesting channels of enhanced snow rates
may flare from time to time within a broader band of snow. It is for
this reason that deterministic guidance depicts streaks of 0.25 to
0.33" of QPF primarily across central Illinois but possible as far
north as southern Ford, Iroquois, and Benton counties. As a result,
opted to increase snow totals south of a line from Sibley, IL to
Fowler, IN to the 3 to 4 inch range. With relatively light
winds below 15 mph and the snowfall falling outside the
traditional "peak" in travel times, felt we weren`t quite there
for a Winter Weather Advisory (especially since the 3 to 4 inch
snow totals would probably fall within the southern-most 10
miles or so of our southern tier of counties). For now, will
issue a Special Weather Statement for areas south of I-80 and
let the day shift monitor trends.

Snow should taper quickly from west to east by sunset as the
eastern periphery of a surface ridge in the Plains expands eastward
into the Lower Great Lakes. Tonight, a secondary upper-level trough
anchored by 850mb temperatures of -15 to -16C will usher in a brief
period of robust CAA leading to tumbling temperatures. In fact,
overnight lows may fall as low as the upper single digits to lower
teens, which with northwest winds of 10 to 20 mph, will drop wind
chills below zero. Even as temperatures rebound toward the freezing
mark on Saturday, blustery west winds will still make it feel some
10 to 25 degrees colder.

Thankfully, the cold shot looks short-lived as a warm-advection
regime resumes Sunday onward. Temperatures are expected to rebound
into the 40s on Sunday and the low to mid 50s by the middle of the
week. Ensemble support for precipitation looks weak until Thursday
or Friday of next week (and even then, there are no signs of a heavy-
precipitation producer).

Borchardt

&&

AVIATION /18Z TAFS THROUGH 00Z SUNDAY/...
Issued at 1145 AM CST Fri Feb 16 2024

Main Concerns:

- -SN mainly GYY and south shifting southeast this afternoon

- Temporary MVFR CIGs and flurries late this afternoon through
the evening

- Gusty winds, northwest tonight, becoming west-southwest
Saturday afternoon

The northern edge of light snow and reduced VSBY has primarily
made it to I-80 and south including GYY and will sink south this
afternoon. MDW will observe flurries through 19z or 20z. A
reinforcing push of colder air into this evening will likely
bring at least temporary MVFR CIGs along with intermittent
flurries. This will be followed by northwest winds increasing
and gusting to 20-25 kt through early Saturday. Winds will
ease and become westerly during Saturday morning and then
steadily increase mid-late Saturday afternoon from the west-
southwest with gusts in the 25-30 kt range 20z onward.

Castro

&&

MARINE...
Issued at 223 AM CST Fri Feb 16 2024

Saturday into Sunday, a low pressure system is expected to move
across southern Ontario while a surface high pressure system settles
into the southern United States Saturday into Sunday. As a result, a
tight pressure gradient will develop across the Great Lakes and
support breezy west to southwest winds. With relatively warm water
temperatures and near-surface temperatures remaining seasonably cool
on the backside of a departing cold air mass, the marine airmass may
become neutral to weakly unstable and allow for southwesterly to
westerly gale force winds to develop particularly Saturday night.
For this reason, a Gale Watch is now in effect from Saturday evening
through Sunday morning.

Borchardt

&&

LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...Gale Watch from Saturday evening through late Saturday night
for the IL and IN nearshore waters.

Small Craft Advisory from 9 PM this evening to noon CST
Saturday for the IN nearshore waters.

&&

$$