Sun Feb 11 Meteorology for Chicago

[Brett Crapser]

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1149 AM CST Sun Feb 11 2024

KEY MESSAGES...

- Chilly morning temperatures with above normal afternoon temps
are expected through the middle of the week

- Drier conditions are expected despite a stronger system
passing to the south Mon/Tues

- A more favorable pattern for active weather develops mid week
with colder temperatures by the weekend

&&

DISCUSSION...
Issued at 305 AM CST Sun Feb 11 2024

Clear skies and light winds have allowed temperatures to fall
into the mid to upper 20s this morning. A region of low stratus
continues to slowly approach from the north. Most guidance is
expecting this cloud cover to fill in through the morning, but
only briefly. Winds will remain light and temperatures are
expected to rise back into the upper 30s and 40s in the
afternoon. There are better chances for clearer skies Sunday
night, which will make for another chilly morning on Monday
with temperatures once again in the mid 20s, with upper 20s
expected closer to the city of Chicago.

A deep upper low, currently in eastern New Mexico, will
gradually start moving northeastward toward the Ohio River
Valley by late Monday. An upper level trough will start to dig
down over Canada late Monday and help keep the trajectory of the
low south of the forecast area. However, models have trended
slightly more northerly with the low`s track.

While the the center of low pressure is not expected to pass
over the area, a sharp baroclinic region will set-up just to the
south. Strong frontogenetic circulations are expected to
develop, with a very moist layer at mid levels and steep lapse
rates, along with a strongly divergent jet core at 300 mb that
will provide additional synoptic lift. However, just north of
the front stronger subsidence and drier air mixing down is
expected which will develop a sharp gradient of who can and
cannot get precipitation. For now this sharp gradient looks to
remain south of Ford, Iroquois and Benton counties, but if it
drifts a little farther north, there could be some snow
developing south of I-80 late Monday into early Tuesday morning.
While the official forecast remains dry, PoPs were increased to
be just below mentionable (less than 15 percent), but may need
to be increased depending on trends.

As the upper wave passes to the east, winds will transition to
the north to northwest. Model soundings from the NAM are
suggesting a potential for lake effect snow on Tuesday for
northwest Indiana, although this seems to be a relative outlier
compared to other guidance. Lake effect parameters are generally
marginal, with lake induced CAPE 100 J/kg or less along with
under 5 kft. It was decided to leave PoPs out of the forecast
for now due to the model disagreement, but yet something else to
monitor as the pattern transitions to a more active period.

By Wednesday, the surface low (the one currently in New Mexico)
will push through New England and stall as a ridge grows to the
west; thereby setting up northwesterly flow and a more active
weather pattern over the Midwest. Models have been consistently
showing a wave passing through late on Wednesday and into
Thursday morning.

Temperatures look to be warm enough to start as rain, but then
potentially have the chance for snow to mix in as the system
passes overnight. Despite the consistency in bringing a system
over the area, there are still discrepancies in the details. The
GFS is showing a broader wave with warmer temperatures, while
the Euro has a more well defined system that passes directly
over Chicago. So, while details can still be ironed out,
confidence is growing in the chance for the next precipitation
coming mid week.

After that system passes, northwest flow remains in place which
will to allow for additional precip chances in the long range.
Ensemble guidance is suggesting a colder air mass could be
arriving by the weekend as well. With additional disturbances,
the potential for snow may increase during this time frame,
although confidence in specifics at this time remains low.

DK

&&

AVIATION /18Z TAFS THROUGH 00Z TUESDAY/...
Issued at 1149 AM CST Sun Feb 11 2024

Forecast concerns for the terminals...

* Lingering SCT to BKN MVFR stratus this afternoon

* Small (<20%) chance for patchy ground fog late tonight into
Monday morning

Overall a generally quiet weather day is upon us with dry
conditions and light northwest winds. However, an area of 2000
to 2500 ft stratus continues to gradually expand across
northern Illinois generating MVFR ceilings at some area
airports. While recent trends do show the ceilings gradually
expanding east, the thin nature of the stratus and recent signs
of erosion within the deck makes confidence low as to eastward
extent and duration. Given this lower confidence and the
anticipated limited impact to operations I have decided to
maintain the SCT025 mention at the Chicago terminals.
Regardless, any MVFR ceilings that do develop should erode by
early evening (02z to 03z) leaving us with VFR conditions for
the remainder of the period.

As clouds clear out this evening there is the chance, albeit
small, for some patchy ground fog late tonight (after 08z) into
Monday morning. While there continues to be a decent signal
within some guidance, the marginal moisture profiles lead me to
believe that any fog tonight will be very isolated and shallow
(visibilities generally between 5 to 7 SM). Due to lower
confidence in fog coverage have decided to hold off on a formal
mention for now.

Otherwise, expect winds to remain light (speeds between 5 to 10
kts) through the period. Though wind directions will become
west-southwest tonight before returning northwesterly Monday
morning.

Yack

&&

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

&&

$$