663
FXUS63 KLOT 210218
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
818 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.

&&

.UPDATE...
Issued at 818 PM CST Thu Nov 20 2025

Have issued a Dense Fog Advisory until 10 AM CST Friday for
southern parts of the forecast area.

Fog has developed this evening across parts of central IL and
IN, with visibilities dropping below a mile (and as low as
1/4SM) in spots. In the WFO LOT cwa, this is most largely across
the southern tier of counties, where both surface temps and dew
points are in the mid-upper 40s. Slightly weaker flow aloft and
a later arrival of a weak cold front into Friday morning
suggest these southern areas will remain at greatest risk for
dense fog development/expansion. Farther north, temp-dew point
spreads are slightly larger, with slightly more flow atop the
boundary layer supporting more of a persistent low-stratus
situation rather than widespread denser fog, though patchy fog
is certainly possible there as well. A weak surface cold front
currently stretching northeast to southwest across WI and
northeast IA will push into far northern IL overnight, and will
bring some weak dry advection which should further limit fog
impacts across the north.

Early evening AMDAR aircraft soundings from PIA, RFD and MDW
generally depict a shallower stratus layer than last night,
roughly 1500-2000` thick. While still maintaining the patchy
drizzle mention ahead of the front, suspect it will remain less
widespread than we saw 24 hours ago.

Other than expanding area and mentioning dense fog across the
southern cwa, no changes made to going forecast.

Ratzer

&&

.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 /00Z TAFS THROUGH 06Z SATURDAY/...
Issued at 533 PM CST Thu Nov 20 2025

Forecast concerns include...

Mvfr/ifr cigs through Friday morning.
Patchy drizzle tonight.
Wind shift to northwest late this evening and northerly Friday.

Prevailing low mvfr cigs will continue through at least late
Friday morning and then may begin to scatter out from north to
south. A weak frontal boundary will move across the area later
this evening into the early overnight hours. As it moves across
the area, a period of ifr cigs and patchy drizzle is expected.
Ifr cigs may develop earlier this evening and persist into
Friday morning with overall low confidence. Visibilities may
also dip into the mvfr range for much of the area, along with
ifr vis with any periods of heavier drizzle.

Light southwest winds currently will become more westerly
through the evening then shift to the northwest with the frontal
boundary noted above. Directions will turn more northerly Friday
morning and may become north/northeast Friday afternoon for the
Chicago terminals. Speeds are expected to be less than 10 kt
through the period.

There will be a chance of showers south of the terminals Friday
afternoon into Friday evening and mvfr cigs may persist across
these areas, central IL into central IN, through late Friday
afternoon. cms

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Dense Fog Advisory until 9 AM CST Friday for ILZ032-ILZ033-
     ILZ039.

IN...Dense Fog Advisory until 9 AM CST /10 AM EST/ Friday for
     INZ010-INZ011-INZ019.

LM...None.

&&

$$

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