868
FXUS63 KLOT 061930
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
130 PM CST Tue Jan 6 2026

.KEY MESSAGES...

- Patchy dense fog and drizzle are expected tonight/Wednesday
  morning.

- Periods of rain are expected on Thursday. Southwest winds may
  become gusty Thursday night

- A storm system may bring a mix of rain and snow this weekend.


&&

.DISCUSSION...
Issued at 130 PM CST Tue Jan 6 2026

Through Wednesday Night:

Stratus has finally filled in across the area and will persist
through tonight and possibly through tomorrow afternoon. A
subtle mid-level wave currently shifting across central Iowa
should result in a little deeper low-level saturation through
this evening. With just a hint of increased UVVs through the
low-level cloud deck, have added a mention of drizzle to the
gridded forecast late this afternoon and evening. With
widespread stratus in place and little in the way of cold
advection tonight, have nudged overnight lows up a bit and don`t
currently have significant icing concerns.

The subsidence inversion overhead will continually build down
tonight into Wednesday morning. This should result in gradually
lowering cloud bases and--to our west--the re-development of
fog. Any fog that does materialize near the Mississippi River
may shift across our region through Wednesday morning. Have
maintained patchy wording for the time being with some
uncertainty in coverage in our area related to 5-10 kts of flow
within the stratus layer. Should dense fog materialize,
conditions will only gradually improve through midday.

Most guidance suggests low stratus will eventually erode from
southwest to northeast through Wednesday afternoon as
southwesterly flow develops. With warm advection forecast to
intensify during this period, there is some potential that
stratus just continually re-develops across the area (NAM/SREF
indicate this scenario). Given the ongoing steady clearing
upstream, will continue to side with the multi-model consensus
here, but note that the cloud and temperature forecast has a
decent bust potential tomorrow afternoon. Quiet conditions are
expected Wednesday night.

Carlaw


Thursday through next Tuesday:

Toward the middle of the week, a Pacific trough will dig into
the far southwestern United States causing a cut-off low
pressure system (currently spinning off the southwestern coast
of California) to get shunted northeastward. There remains
strong agreement in ensemble guidance that the leading wave will
induce the development of a low pressure system along the
Oklahoma/Kansas border early Thursday and track northeastward
into northwestern Illinois Thursday evening. As a result,
confidence remains fairly high that there will be a period of
warm advection-driven rain across our area Thursday afternoon as
the low approaches (80 to 100% PoPs). At this point, the
ensemble signal for the steadiest rain (locally 1"/12 hours) is
just northwest of our area, to the northwest of the path of the
low. Regardless, Thursday will still feature rain in the
afternoon. Temperatures should rise into the upper 40s to
locally lower 50s by early afternoon.

Thursday evening, the low will make its closest approach while
lifting toward southern Lake Michigan. While increasing mid-
level warm air advection ahead of the Pacific trough in the
central Plains will act to weaken the lead trough as it moves
into the Great Lakes (stalling in surface pressure, if not
filling), a residually strong low-level wind field will
facilitate the efficient northward advection of the warm sector
across much if not all of our area. As a result, southwesterly
winds should increase after dark (gusts up to 30 mph, if not
higher per the GEFS) and help temperatures soar into the upper
50s to locally 60 after sunset Thursday. (So, the high
temperatures for both Thursday and Friday will probably occur at
midnight). Will have to watch the placement of the low-level
instability axis to support the development of a low-topped band
of showers (little to no lightning) along the approaching cold
front between sunset and midnight. With some 50 to 60kt of flow
from 900 to 800mb, any low-topped convection would be prone to
efficiently transporting strong to locally damaging winds to the
ground. With that said, would be more concerned if the low were
still deepening while moving into our area. We`ll keep an eye
on it.

The Pacific trough will eject eastward in the Friday through
Saturday timeframe presenting the next opportunity for
precipitation. The evolution of the second system remains vast
in ensemble guidance owing to differences in phasing
opportunities with a northern-stream shortwave, though am noting
a trend for a largely dry Friday and the development of rain
and snow across the Great Lakes on Saturday as a surface low
matures in Lower Michigan. In scenarios where the low is more
optimally phased, can easily envision fairly raw conditions
Saturday afternoon and evening with blustery northwest winds and
snow showers (12Z ECMWF as an example). We`re still well within
the range for big shifts in forecast guidance, so won`t fully
bite on any outcome for now and simply go with NMB-delivered
mid-range PoPs (40-60%) for a rain/snow mix centered on
Saturday.

Borchardt

&&

.AVIATION /18Z TAFS THROUGH 00Z THURSDAY/...
Issued at 1112 AM CST Tue Jan 6 2026

Key messages:

- MVFR and IFR cigs this afternoon and tonight. Some potential
  for LIFR conditions Wednesday morning, but uncertainty is too
  high for a mention in the current TAFs.

- Potential for -DZ this evening.

Low cigs will continue to expand across the terminals this
afternoon. While nearby observations indicate conditions may
predominantly be low-MVFR initially at the Chicago-area
terminals, have included a TEMPO for IFR conditions. Have
prevailed IFR at RFD based on ongoing observations there.

A disturbance currently across central Iowa will move across the
region this evening. Have introduced TEMPO groups for some
slightly lower vsbys, as well as -DZ given deepening low-level
saturation and some modest ascent modeled in the low-levels.
Thereafter, periods of -DZ, BR, etc. are possible overnight.
Towards daybreak on Wednesday, guidance suggests that stratus
may continue to build downwards, with a chance for LIFR
conditions. Uncertainty remains a bit too high right now to
introduce into the TAFs, but will continue to monitor guidance
and upstream trends.

Carlaw

&&

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

&&

$$

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