455
FXUS63 KLOT 202133
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
333 PM CST Thu Nov 20 2025

.KEY MESSAGES...

- Patches of drizzle and fog may continue to be observed
  through tonight beneath widespread stratus.

- A period of rain is possible on Friday in areas near and
  especially south of the Illinois and Kankakee rivers.

&&

.DISCUSSION...
Issued at 333 PM CST Thu Nov 20 2025

This afternoon, the region remains socked beneath a 4000-5000
ft deep stratus deck trapped beneath a temperature inversion at
the 850 mb level. Still seeing some occasional drizzle reports
in regional METARs, and with flow in the cloud layer expected to
increase modestly over the next few hours while an upper-level
shortwave trough passing to our north deals a glancing blow of
ascent, would not be surprised to see these drizzle observations
become more common going into this evening.

Low-level dry air advection behind a weak cold front dropping
in from the north late tonight should cause the stratus deck to
become patchier and eventually scatter out in our northern
counties sometime tomorrow, while the stratus status quo is
preserved through the daytime hours in our southern counties.
Areas that do manage to shed the stratus will still, however,
see increasingly thicker upper-level cloud cover move overhead
as a sheared-out upper-level shortwave pivots into the region
from the southwest. Thus, one way or another, the magnitude of
solar insolation that we`ll see will be blunted once again, and
tomorrow`s temperatures will likely end up being similar to
today`s as a result with highs in the mid 40s to low 50s.

The northeastward approach of a weak surface low associated
with the aforementioned shortwave clashing with the southward
push of cooler, drier air behind the aforementioned cold front
will yield a sharpening baroclinic zone and strengthening
frontogenesis across central Illinois/Indiana. An expansive
precipitation shield with a sharp cut-off on its northern
periphery (due to the impinging drier air from the north) will
be found in this region of stronger forcing for ascent Friday
morning through Friday evening. Some uncertainty still remains
regarding how far north the rain will get. The past several runs
of the deterministic GFS have actually been outputting QPF as
far north as the I-88 corridor. However, these runs appear to
be outliers relative to most other guidance, and more often
than not, the dry air coming in from the north tends to win out
in these kinds of setups. Therefore, suspect that the bulk of
the incoming precipitation should stay confined to the south of
the Illinois and Kankakee rivers, and possibly even south of our
forecast area altogether. Have kept PoPs capped in the chance
range in our southern counties to reflect this thinking.

After this late week weather system clears the area, dry
conditions with moderating temperatures are expected through the
weekend. An upper-level low ejecting northeastward out of the
Desert Southwest should then bring the possibility of
widespread rain back into the region in the Monday-Tuesday time
frame. A northern stream trough will follow closely on the heels
of this initial disturbance and should bring some colder air
into the region just in time for Thanksgiving. Depending on how
these two disturbances interact, it`s possible that some
precipitation could also be seen here on Wednesday and/or
Thursday. However, only a minority of global ensemble members
depict such a scenario at this time, and the latest NBM keeps
PoPs in our CWA largely below 15% as a result.

Ogorek

&&

.AVIATION /18Z TAFS THROUGH 00Z SATURDAY/...
Issued at 1200 PM CST Thu Nov 20 2025

- Periods of LIFR/IFR VSBYs CIGs and patchy -DZ BR possible at
  times through tonight.

Other than some minor visibility and ceiling improvements
little has changed across the area through midday with
continued LIFR to IFR CIGs under an expansive stratus deck and
IFR to MVFR VSBYs due to lingering light mist/BR (lowest outside
of Chicago). While guidance continues to suggest that there
could be further improvement through the afternoon, this could
be counteracted by potential redevelopment of drizzle (20%
chance). Thus, opted to not make big changes to the inherited
TAFs for now and continue to monitor trends. Additional periods
of -DZ may need to be added.

A weak cold front will push across the area late this evening
and overnight with ongoing light south to southwest winds
veering to the northwest in its wake. This feature could be
paired with another round of drizzle and associated VSBY/CIG
reductions (30% chance) from roughly 2-10Z, earliest northwest
and latest southeast.

CIG and VSBY trends are expected to improve through the morning
on Friday, returning back to VFR by the afternoon as the
stratus layer thins and erodes. Another system will graze parts
of the area on Friday, though any associated showers are
expected to remain south of the terminals toward central
Illinois.

Petr

&&

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

&&

$$

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