textproduct: Chicago

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 /00Z TAFS THROUGH 06Z TUESDAY/

Issued at 617 PM CST Sun Nov 30 2025

Key Messages:

- There is a potential for visibility reductions from fog at RFD and DPA tonight into tomorrow morning.

- A period of accumulating snow with IFR conditions is expected tomorrow afternoon and evening.

Winds will become lighter tonight as surface high pressure spreads into the region. If the primarily VFR stratus deck over interior northern Illinois manages to erode to a sufficient extent, then patchy fog may develop in some areas and affect RFD and/or DPA sometime overnight through mid-morning tomorrow. If this fog were to develop, then single digit to low teens temperatures would allow for the fog droplets to freeze upon contact with various surfaces. Model guidance doesn't have a good handle on the existing stratus deck, so confidence remains fairly low in the development and spatial extent of any potential fog, but even so, opted to hint at this fog potential in the RFD and DPA TAFs as it could potentially result in notable impacts if it were to develop.

Otherwise, the main forecast item of note for the current TAF cycle will be a period of accumulating snow tomorrow afternoon and evening. Total snow accumulations with this round of snow are expected to end up in the 1-4" range at our five TAF sites this time around, but steady snowfall will still likely knock visibilities down to IFR levels for several hours and bring ceilings down to MVFR or IFR levels as well. Winds are expected to be out of the south or southeast during the snowfall, but should remain at or under 10 kts for the most part.

Ogorek

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.