202
FXUS63 KLOT 150821
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
221 AM CST Fri Nov 15 2024

.KEY MESSAGES...

- Low chance (20%) of showers Saturday night into Sunday.

- Periods of rain expected Monday night.

- A cooler weather pattern, potentially accompanied by periodic
  showers, looks to materialize mid-late next week.


&&

.DISCUSSION...
Issued at 220 AM CST Fri Nov 15 2024

Through Saturday:

Broad upper ridging across the central CONUS will edge eastward
across the area through Saturday. The main forecast concern
during this period will revolve around cloud cover trends as an
expansive area of stratus over the western Great Lakes remains
trapped under a stout subsidence inversion ahead of the ridge.
Though cloud depths have not been all that impressive, weak low-
level isentropic ascent last evening resulted in patches of
light drizzle/mist. Recent radar trends and diminishing forcing
(per guidance) early this morning suggest remaining drizzle
should end prior to sunrise. Meanwhile, the western edge of the
stratus has been steadily drifting eastward through the night,
reaching just west of the Mississippi River as of 2am.
Expectations are that some clearing should reach areas west of
I-39 after sunrise, but increasing diurnal support from weak
mixing under the strong inversion will likely maintain a decent
stratus deck farther east across the CWA through much of the
day.

As the surface ridge cross the forecast area late this
afternoon and evening, shallow low-level isentropic ascent amid
veering winds will likely maintain the stratus deck while
ultimately expanding back to the northwest through Saturday.
Opted to lean toward colder guidance with daytime highs today
and Saturday, but given how guidance usually struggles with the
evolution of shallow stratus in the cool season, high
temperatures could easily overperform if clouds erode quicker
than expected. Additionally, any substantial clearing that
persists tonight may foster fog development within the passing
ridge.

Kluber


Saturday Night through Thursday:

Surface low pressure is forecast to lift northeast from the
Dakotas into Ontario Saturday night, in response to a fairly
strong short wave tracking north of the region. While the best
large-scale forcing remains north of the forecast area,
southwesterly lower and mid-level flow does look to provide
modest isentropic ascent and shallow moisture advection across
the forecast area later Saturday night into Sunday ahead of the
system`s trailing cold front. Guidance continues to depict some
spotty QPF particularly later Sunday night, and have maintained
slight chance (15-20%) pops especially along/northwest of the
I-55 corridor for some light rain. The front completes its
transit of the cwa Sunday, though with the bulk of the better
moisture advection shifting east of the area, fropa looks to be
largely dry. Weak surface high pressure develops across the area
behind the front Sunday night.

Attention then turns to another mid-level disturbance emerging
from northern Mexico Sunday night. Guidance is in good agreement
in lifting this negative-tilt short wave northeast into eastern
Kansas by late Monday, then becoming stacked above its
occluding surface low as it continues to lift across Iowa and
into the upper Mississippi Valley into Tuesday. For our forecast
area, shower chances develop again on Monday ahead of the
system`s approaching warm front, especially across our IL
counties. More widespread rain becomes likely Monday night
however, as the main mid-level vort and low-level occluded front
move into the area. Rain should taper off to showers or drizzle
Tuesday as the mid-level dry slot spreads in aloft. Daytime
temperatures are expected to be in the 50s to around 60 through
Tuesday.

Pattern evolution becomes more complex by the middle of next
week, as longer-range ensembles indicate another digging short
wave moves into the Plains, eventually phasing with the occluded
system over the upper Midwest. The general GEFS/EPS and CMC
ensemble consensus depicts a resulting closed upper low and deep
(sub-1000 mb) surface low moving through the region
Wednesday/Thursday. There is considerable spread amongst
individual ensemble members and deterministic runs however, and
given this is still out in the days 6-7 time frame it`s still
too early to get too specific with the details. NBM blended
guidance remains warm enough with low-level profiles to remain
all liquid precipitation, though a trend toward cooler
temperatures is expected in the wake of the system by the end of
next week.

Ratzer

&&

.AVIATION /06Z TAFS THROUGH 12Z SATURDAY/...
Issued at 1104 PM CST Thu Nov 14 2024

Forecast concerns for the terminals...

- Periods of drizzle possible through 08-09z

- MVFR ceilings to prevail through Friday night


A few bands of drizzle have developed within the last couple of
hours across southern WI and northeast IL in the wake of an
upper trough. While moisture depths continue to be marginal,
drizzle does look to linger through at 08z to 09z tonight at the
Chicago area terminals as some lake induced instability
persists. So far the drizzle has not generated any notable
visibility changes and that is expected to remain the case, but
isolated pockets of 3-5 SM may occur at times. Furthermore,
pockets of IFR ceilings have been observed in WI which are
expected to remain possible with the drizzle this evening.
Though confidence on longevity of IFR remains low so have
decided to handle with a SCT mention for now.

Otherwise, expect the MVFR ceilings to prevail through Friday
night as the low-level moisture becomes trapped beneath the
building surface high. However, some pockets of sunshine may
poke through the clouds Friday afternoon. Winds will remain
light through the period, but directions will become more east-
southeasterly Friday night under the high.

Yack

&&

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

&&

$$

Visit us at weather.gov/chicago