Mon Feb 12 Meteorology for Chicago

[Brett Crapser]

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1128 AM CST Mon Feb 12 2024

KEY MESSAGES...

- Seasonably mild temperatures through Wednesday

- Rain moves in late Wednesday with the potential for some snow
to mix in by Thursday morning

- Another system moves in end of week with colder temperatures
leading into the weekend.

&&

DISCUSSION...
Issued at 310 AM CST Mon Feb 12 2024

Nighttime satellite imagery has very little to view over Northern
Illinois due to the lack of cloud cover. This along with light
winds will drive chilly temperatures in the 20s, with coldest
temperatures by day break west of the Fox Valley. Winds are
expected to remain light through most of the day. Afternoon
temperatures will warm back up above normal into the 40s, with
slightly cooler conditions in northwestern Indiana due to the
potential for a brief, weak lake breeze developing late in the
day before sunset.

An upper level low over western Oklahoma will continue its track
northeastward. Model consensus has continued to shift its track
more to the south as it moves toward Kentucky. There is still a
brief non- zero chance that the edge of the system graze the
southern part of the forecast area with maybe a flurry or two,
but it was kept out of the forecast. A short wave trough aloft
will move down out of Manitoba (which will help force the
stronger low toward south of the Ohio River), with only a weak
cold front reflected at the surface. The main impacts will be
winds turning to the northwest with the potential for gusts up
to 20 mph tonight into Tuesday.

Northwest winds will persist on Tuesday providing sufficient
fetch over the lake into northwest Indiana. The risk for lake
effect snow is fairly low, so PoPs were kept below 15 percent.
While models are depicting marginal lake effect snow parameters
(such as 850 mb temps around -10 and lake-induced CAPE as high
as 100 J/kg), the convective boundary layer currently looks too
dry. As a result, the potential for lake effect precip is very
conditional, and if it did develop, it would mainly just scrape
eastern Porter County with the main area being east of there.

Beyond Tuesday, the synoptic weather pattern develops steady
northwest flow over the CONUS, due to a long wave trough to the
east and a ridge to the west. This will allow several waves to
pass over the Chicago area providing an uptick in activity by
the weekend. The first wave that is starting to get a better
signal is late Wednesday into Thursday morning. With sufficient
warm air advection ahead of the wave and steeper lapse rates
aloft in association with a robust f-gen circulation, the
probability for the next chance for precipitation is looking
more likely. The main question with this system will be
temperatures as well as the speed at which the system passes
through to the east. Current projections keep most of the cold
air up in Wisconsin to start, suggesting that precip will begin
as rain and then may mix with, or briefly change to snow from
the north into Thursday morning.

While global models suggests another wave moves over the area at
the end of the week, they are struggling with the exact phasing
as it moves east. However, there is some agreement that colder
wet bulb temperatures are expected, which would translate in
better chances for snow. PoPs were kept at chance wording
(between 25 and 35 percent) given the uncertainty. Beyond that
wave, a colder airmass looks to drop southward, increasing the
potential for air temperatures to once again return to below
normal.

DK

&&

AVIATION /18Z TAFS THROUGH 00Z WEDNESDAY/...
Issued at 1128 AM CST Mon Feb 12 2024

Forecast concerns for the terminals...

- Light lake breeze may cause brief wind shift at MDW and GYY

- Increasing VFR cloud cover Tuesday

Another mostly quiet afternoon weather wise is expected at the
terminals with mostly sunny skies and light west-northwest winds
around 5 kts. However, there continues to be a signal in
guidance for a light lake breeze to develop late this afternoon.
Given that water temperatures off of Chicago are in the upper
30s and the forecast highs today in the low to mid-40s, I
suspect that the lake breeze will struggle to make much inland
progress. Therefore, the expectation is for only sites nearest
the lake (GYY and MDW) to see the resultant easterly wind shift.
Though the aforementioned weak temperature gradient does make
confidence low as to whether or not the lake breeze will
actually reach MDW. Regardless, wind speeds with the lake breeze
are expected to remain light (around 6 kts) so impacts to
operations should be minimal.

A more notable wind shift is expected this evening with a dry
cold front which will turn winds northwesterly for the remainder
of the forecast period. Additionally, wind speeds will also
increase behind the front with occasional gusts in the teens on
Tuesday. Otherwise, expect increasing VFR cloud cover through
the day on Tuesday.

Yack

&&

LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...None.

&&

$$