Wed Feb 07 Meteorology for Chicago

[Brett Crapser]

Area Forecast Discussion...CORRECTED
National Weather Service Chicago/Romeoville, IL
1135 AM CST Wed Feb 7 2024

KEY MESSAGES...

- Dense fog and freezing fog persists this morning, mainly
across interior northern IL.

- Strong south winds develop Thursday afternoon and evening
(gusts up to 40 mph)

- Two rounds of showers Thursday afternoon and evening (40-60%)
a few storms possible (20%)

- Unseasonably warm temperatures expected Thursday and Friday

- Pattern change brings cooler temperatures and chances for
winter precipitation next week (20%)

&&

DISCUSSION...
Issued at 332 AM CST Wed Feb 7 2024

Discussion:

Areas of dense fog/freezing fog continue early this morning,
mainly along/west of the Fox River and I-55 where a Dense Fog
Advisory remains in effect through 11 am. Patchy fog will remain
possible farther east and into northwest Indiana. As we head
into the late morning afternoon hours warmer air begins to lift
into the region with southerly flow returning as the surface
high shifts east of the area. There are still some questions as
to the impact the AM fog/stratus will have on high temperatures,
though think that mid 40s to near 50 are within reach.

Attention then turns to an expected deepening northern Plains
surface low and trailing cold front progged to sweep across the
region late Thursday night. Well ahead of the front a narrow
band of pre-frontal showers are expected to move through as
early as noon on Thursday (NW IL) and sweep across the area
during the afternoon. The fast movement and isolated to
scattered coverage along the line would suggest not much more
than 0.05-0.15" of QPF with this initial round, even with high
PWATs for this time of year, upwards of 0.7- 0.9". In the wake
of the afternoon pre-frontal showers, a narrow plume of
steep mid-level lapse rates and marginal elevated instability
(250-500 J/kg of MUCAPE) lifts into the region. This combined
with strong shear would be supportive of a few storms. There
remain questions on the impact of mid-level drying behind the
first wave of showers which could reduce the overall coverage,
but if the dry slot pivots into the area even just a few hours
later a stronger storm or two can`t be ruled out during the 4 to
9 pm timeframe, highest chances across interior northern
Illinois. There are a few drier solutions (namely RAP/NAM) where
showers struggle to even develop this far south, keeping precip
mainly north into Wisconsin. For now consider this an outlier
but will monitor if other guidance trends in this direction with
later updates.

In addition to the shower potential, strong southerly winds
develop in response to a tightening pressure gradient as the
northern Plains low deepens. Wind gusts up to 40 mph could mix
down to the surface Thursday afternoon and evening in the warm
advective region ahead of the approaching cold front. High
temperatures will respond to this strong warm advection and
mixing with April-like temperatures currently forecast with in
the 50s to near 60 possible Thursday and Friday and Thursday
night lows still in the 40s.

Looking beyond the workweek, a pattern change results in cooler
temperatures (albeit still near to slightly above normal for
mid- February), with highs in the upper 30s to lower 40s and
lows in the 20s to lower 30s. As far as precipitation chances
go, a subsequent weakening shortwave ejects across the Rockies
toward the area Saturday night into Sunday which could bring a
chance for rain, currently favoring areas along/south of I-55 as
an associated a weak surface reflection/low moves across the
area. Beyond the weekend the details become a bit more muddled
as there remains differences in the handling of the high
amplitude upper trough moving across the central CONUS and
subsequent trailing shortwave diving out of Canada across the
Great Lakes. This wave could bring chances for some winter
precipitation to the region during the Monday-Tuesday timeframe
(20%).

Petr

&&

AVIATION /18Z TAFS THROUGH 00Z FRIDAY/...
Issued at 1116 AM CST Wed Feb 7 2024

Aviation Forecast Concerns:

* Period of LLWS conditions with 45 kt low level jet late
tonight/early Thursday morning.

* Strong south winds Thursday from mid-morning on, with gusts
around 30 kts.

* Period of MVFR ceilings (2000-2500 ft) from midday/early
afternoon on, along with some scattered showers.

* Low chance (<25%) of isolated thunderstorms across northwest IL
Thursday afternoon.

Morning fog/low stratus continues to burn off west of Chicago,
with some MVFR/VFR haze lingering at KRFD and KDPA. This should
continue to improve early this afternoon.

Otherwise, south winds near 10 kts expected, becoming more
south-southeast by this evening with a high-level VFR ceiling
(above 150). Surface low pressure developing/deepening across
the Plains will lead to increasing low level southerly winds
tonight into Thursday, with surface winds gradually increasing
to 10-15 kts late tonight. Just above the boundary layer,
at/just above about 1200 ft AGL, a 45 kt southwest low level jet
will develop after midnight which will present LLWS conditions
conditions into Thursday morning.

Surface winds will strengthen and become gusty from the south to
south-southwest by mid-morning Thursday, with gusts approaching
30 kt by midday and likely 30+ kts by mid-late afternoon. An
MVFR cloud deck is expected to develop by afternoon (along with
a few scattered showers) which may help limit wind gusts to
less than 35 kts. If coverage of low clouds ends up less than
currently expected, this could support winds/gusts higher than
in current TAFs. Also, there is a low-chance potential for
isolated thunderstorms across northwest IL (mainly for KRFD area
or even northwest of there) which could also produce locally-
enhanced wind gusts Thursday afternoon. Low confidence that
sufficient low- level moisture will available for thunder, but a
non-zero potential does exist.

Ratzer

&&

CLIMATE...
Unseasonably warm temperatures Thursday into Friday may
approach or reach records for both Rockford and Chicago. The
following are the record highs and record high minimum
temperatures for these days.

Thursday, February 8th:

Chicago: Record High: 62 in 1900, 1925.
Record High Min: 54 in 1925.

Rockford: Record High: 58 in 1990.
Record High Min: 40 in 1925.

Friday, February 9th:

Chicago: Record High: 56 in 1886.
Record High Min: 47 in 1966.

Rockford: Record High: 54 in 1925.
Record High Min: 43 in 1966

Petr

&&

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

&&

$$