866
FXUS63 KLOT 302045
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
245 PM CST Sun Nov 30 2025

.KEY MESSAGES...

- Some slick travel across Porter County this evening with
  lingering lake effect. Threat for patchy freezing fog, mainly
  west of the Fox Valley, tonight.

- Accumulating snow with potential impacts to the Monday
  afternoon and evening commute and resulting hazardous travel
  conditions.

- Well below normal temperatures will persist through next
  weekend with wind chills -5 to -15 possible Thursday and
  Friday mornings.

&&

.DISCUSSION...
Issued at 245 PM CST Sun Nov 30 2025

Through Tonight:

Main focus for the rest of the afternoon and evening is the
potential for one final push of lake effect snow into parts of
Porter County, Indiana. Surface winds have turned solidly
northwesterly on this side of the lake, and the overall
orientation of the main trough axis suggests that convergence
will continue to focus primarily east of our area. However,
based on recent radar trends, it looks like at lease some
disorganized lake effect may continue backing into parts of
Porter County (maybe even parts of eastern Lake County, IN) late
this afternoon/early evening as a sharpening trough pushes
south out of western lower Michigan. The parameter space is
certainly supportive of heavier rates with equilibrium levels
pushing past 10 kft. However, with the main zone of convergence
focused farther to the east and the ongoing LES not looking
particularly well-organized, have just increased PoPs and snow
amounts a smidge across NE Porter county. Will need to keep a
close eye on radar trends this afternoon, however.

Otherwise, it looks like cloud cover could clear out across
parts of the area this evening. If this occurs, temperatures
will absolutely tank over the recently-minted snowpack, at least
prior to the arrival of increasing mid and high cloud cover
late. Have nudged lows down a bit from the Fox Valley and points
west with this in mind. Locally dense freezing fog could also
develop if this comes to fruition. At this time, still have
enough questions about what effects (if any) high cloud cover
will have, and the degree of clearing we`ll maintain in our
west, combined with recent trends with some guidance backing off
on this threat, that we`ve elected to hold off on a fog mention
in the grids right now.

Carlaw


Monday and Monday night:

A trough over the Great Basin this afternoon will track across
Oklahoma late Monday afternoon before phasing with a weak sub-
tropical wave across the Ohio River Valley Monday night. A
corresponding area of mid-level Pacific-based moisture will
provide sufficient saturation for a shield of snow to develop
over the area Monday afternoon and especially Monday evening.

As the phasing occurs, amplification of the main wave (around
400-600 hPa) and a proceeding lower-level wave (800-600 hPa)
will induce a broad area of modest ascent across the mid-
Mississippi River Valley. The exit region of an associated
strengthening mid-level jet streak will generate an embedded
corridor of stronger ascent across eastern Iowa that should
expand into at least northwest Illinois by mid-afternoon. As the
mid-level wave continues to strengthen and align with the
lower-level wave, a quickly intensifying 600 hPa frontogenesis
axis will develop across the southern forecast area and into
central Illinois and northern Indiana.

Expectations are that top-down saturation will result in a
period of light snow spreading west to east over the entire
area, reaching the I-39 corridor by mid-afternoon and the
Chicago metro and northwest Indiana by late afternoon. Most
guidance has been consistent in generating QPF generally in the
0.15-0.20 inch range, with higher amounts of at least 0.25 inch
in the broader ensemble envelope south of I-80. Meanwhile, a 3km
deep DGZ intersecting with most of the mid-level ascent would
support a higher ratio snowfall on the order of 15:1 to even
20:1. Putting this together, a widespread fluffy snow event of
1-3" north of I-80 and 2-4" south of I-80 appears likely. Have
some concerns that the 600 hPa f-gen noted above will focus a
narrow (county-wide) WSW to ENE oriented band of higher QPF and
higher SLR (>20:1) southeast of I-55 in the evening. In this
case, it is feasible that a narrow 4-6" band of snow will be
realized. Finally, while not expected (10% chance), synoptic
enhancement of a developing meso-low over southern Lake Michigan
tonight could back close to the Illinois shore as the low-level
synoptic flow turns SSE early Monday evening. Will therefore
need to monitor for locally higher snowfall totals along the
immediate shore from downtown Chicago to the IL/WI line.

Held off on issuing a Winter Weather Advisory at this time as
it is unclear where the potential heavier band settles, and
whether the longer duration of lower snowfall intensity and
amounts (half of which occur after the peak evening commute)
outside of the localized band would warrant an advisory.
However, it is likely an advisory will be needed for at least a
couple county-wide ribbon somewhere along or south of a Lee to
Cook County line.

Tuesday through Thursday: A deep trough crossing western Canada
will bring an arctic front across the area late Wednesday
afternoon into the evening. Ahead of the front and south of the
deeper synoptic support, modest WAA within a low-level saturated
layer may result in a wintry mix of poor-quality snow and
freezing drizzle during the afternoon. Very cold conditions
behind the front are expected Wednesday night through Thursday
night, with sub-zero lows possible and highs in the low to mid
teens. With continued stronger gradient flow behind the front
Wednesday night, wind chill values may fall close to criteria
(-20F) for a Cold Weather Advisory in parts of the area.

Friday through Sunday: The arctic airmass should push the
effective jet stream far enough south to keep conditions quiet
locally Friday into Friday night, but any northward shift of a
currently advertised storm system over the lower half of the
Mississippi River Valley will need to be monitored for the
potential of accumulating snow.

Kluber

&&

.AVIATION /18Z TAFS THROUGH 00Z TUESDAY/...
Issued at 1140 AM CST Sun Nov 30 2025

Forecast concerns for the terminals...

- Diminishing winds and cloud cover this afternoon

- 15-20% chance for patchy fog late tonight, mainly west of the
  Fox Valley

- Period of accumulating snow Monday afternoon and evening
  resulting in IFR ceilings and visibilities


The early snow showers have concluded across northeast IL and
much of northwest IN as of 1730z. High pressure will begin to
build into the area later this afternoon/evening which will
allow winds to gradually diminish through the afternoon. In fact
as the surface high moves overhead tonight, many sites may
actually go light and variable with winds but the Chicago
terminals look to maintain a north-northwest direction.

That said, if the low stratus currently over southern MN erodes
this afternoon then conditions may become favorable for patchy
fog to develop late tonight into Monday morning. While the
stratus has shown some signs of eroding on recent satellite
trends, the expansive nature and the increasing cirrus to the
west do not offer sufficient confidence (15-20% chance) to
warrant the inclusion of patchy fog in the TAFs at this time.
If fog does develop tonight the near single digit to lower teen
temperatures will likely generate instances of freezing fog
especially for areas west of the Fox Valley.

Heading into Monday, any fog should erode by mid-morning with
cloud cover increasing through the day as our next storm system
begins to move into the area. As the system approaches winds
will turn south-southeasterly Monday afternoon with speeds
generally in the 6-8 kt range. The system will also bring
another period of accumulating snowfall to northern IL and
northwest IN starting around 19-20z and lasting through at least
midnight Monday night. While accumulations with this system will
be notably lower than what occurred this weekend (around 2-4
inches), the snow will be fairly steady and result in IFR
ceilings and visibilities Monday evening.

Yack

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...Small Craft Advisory until 3 AM CST Monday for the IL
     nearshore waters.

     Small Craft Advisory until 7 AM CST Monday for the IN
     nearshore waters.

&&

$$

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