Tue Feb 13 Meteorology for Chicago

[Brett Crapser]

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1131 AM CST Tue Feb 13 2024

KEY MESSAGES...

- Chance (30-45 percent) for snow south of I-80 midnight to 6
am tonight with the potential for some slippery travel.

- Rain moves in Wednesday night with an expected transition
over to snow Thursday morning for areas north of I-80. Slushy
accumulations possible near the WI/IL state line.

- Another system leading into the weekend with the potential
for more precip.

&&

DISCUSSION...
Issued at 327 AM CST Tue Feb 13 2024

Despite winds being less than 10 mph, there seems to be just
enough mixing taking place to allow for (relatively) warmer air
temperatures (compared to yesterday morning) in the low 30s
closer to the lake and upper 20s elsewhere. High temperatures
are expected in the upper 30s to low 40s this afternoon. Other
than increasing cloud cover, quiet conditions are expected
through the day before the next chance for snow tonight.

As a subtle mid level short wave passes over the area tonight,
RAP guidance has amplified the signal for a shallow f-gen
circulation south of I-80. Low level dry air could inhibit
snowfall production, but with sufficient warm air advection and
strong ascent aloft, it is plausible that the snowfall will be
efficient enough to overcome the dry layer via melting and
sublimation and survive down to the surface.

As hi-res models have come into better agreement, PoPs were
increased for areas south of I-80. It should be noted, however,
that models tend to struggle with narrow systems similar to
this with strong f-gen circulations and can waffle the location
in each model run. The current thinking has the highest PoPs
along an Ottawa to Rensselaer line, but were buffered with
slight chances (15 to 25 percent) to the north and south to
account for wobbles of the storm track. If dry air erodes
sufficiently, this region will likely see a light coating of
snow, with localized areas that fall under the strongest parts
of the circulation seeing over an inch. Lastly, with warmer
temperatures expected in Central Illinois, there is a slight
chance for some rain to mix; however, confidence remains that it
will be primarily snow.

The next upper level disturbance with a paired surface low is
expected to transit over Northern Illinois on Wednesday night
into Thursday morning. Models are trending the system a little
farther south which has allowed for slightly colder thermal
profiles to sag down to around I-80. The current forecast
maintains that the system will still arrive with rain as the
dominant precip type at the start Wednesday night, and then
slowly allowing snow to mix in Thursday morning. With models
trending south with the colder air, there is a potential for it
to transition to all snow, but the better confidence in that
would be along the Wisconsin state line. Both the Canadian and
Euro models have ramped up the potential for snow accums,
focusing most of the snowfall in Wisconsin. The snow forecast
was increased slightly, but did not buy into going higher as
there remains uncertainty in the exact track (of note: 24 hours
ago, models had a more northern track which influenced the
forecast to be all rain).

Given the active weather pattern that is developing, it is no
surprise that ensemble members continue to show another wave
passing over the region late in the week. However, it seems like
each model run shifts the phasing with the northern stream jet
slightly to which precludes us from being able to hone in on
specifics. However, this next system could bring the next chance
for snow and maintained PoPs from 30 to 40 percent for the time
being.

DK

&&

AVIATION /18Z TAFS THROUGH 00Z THURSDAY/...
Issued at 1131 AM CST Tue Feb 13 2024

Forecast concerns for the terminals...

- VFR ceilings arrive this afternoon

- Winds become northeasterly this evening

Another quiet day weather wise is expected across the terminals
with generally light northwest winds around 5 to 10 kts. While
skies right now are mostly clear, an area of SCT to BKN VFR
clouds continues to drift southward out of Wisconsin. These
clouds are expected to arrive at the terminals over the next
hour or so and persist through the evening before skies scatter
out overnight. Winds will become northeasterly this evening as
speeds ease before eventually settling into a southeasterly
direction Wednesday morning. Though, there is the chance that
winds could briefly (about an hour or two) turn west of south
late Wednesday morning into early afternoon but given the short
duration and light speeds (around 5 kts) have decided to forego
a formal TAF mention.

Lastly, there is a chance (30 to 40 percent) for a narrow band
of accumulating snow to develop after midnight tonight as a
subtle disturbance passes through. While guidance continues to
vary on the amount of low-level moisture, there is good
agreement that this band of snow will remain south of the TAF
sites with areas along and south of a VYS to IKK line the most
favored. Regardless, any accumulation that does occur with this
band looks to be between a dusting to around 1 inch and snow is
expected to end around daybreak Wednesday morning.

Yack

&&

LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...Small Craft Advisory until 3 PM CST this afternoon for Gary to
Burns Harbor IN-Burns Harbor to Michigan City IN.

&&

$$