textproduct: Chicago

KEY MESSAGES

- Accumulating snow this evening. Most accumulations (coating to an inch) focused roughly northeast of a Rockford to Rensselaer line.

- Another period of accumulating snow again Wednesday afternoon.

DISCUSSION

Issued at 233 PM CST Tue Dec 30 2025

An initial batch of warm advection-driven light snow continues to press east across the region early this afternoon. Overall weak ascent continues to support mainly small flakes. This first round of light snowfall will wind down through mid-late afternoon, with a brief gap in activity before snowfall re- develops this evening.

There hasn't been much change in the guidance depiction of the main wave for later this evening with water vapor loops suggesting this feature is just now pressing south of the Minnesota Arrowhead Region. Light snow looks like it'll fill in from northwest to southeast after about 6 PM or so, with the highest coverage expected north of a roughly Rockford, IL to Rensselaer, IN line. Have commensurately increased PoPs this evening across the northeast half of the forecast area, and also smeared lower values farther southwest given the latest model guidance. Forcing overall is pretty lackluster, but there will be a brief 2-4 hour window this evening where at least modest mid-level fgen looks to provide a bit of a boost to UVVs. No major changes to the going snowfall forecast, with a general coating to 1 inch looking reasonable, with the highest values along and NE of I-90/290 and into parts of Lake and Porter Counties. Air temperatures in the 20s will result in slippery travel conditions even within the limited snowfall amounts.

The bulk of the accumulating snowfall will be ending through 10 pm to midnight with a rapid loss of deeper moisture as the main pocket of ascent swings east of the region. Have noted some periodic freezing drizzle reports across parts of the Twin Cities metro area today. Forecast soundings in our region overall look a bit less supportive of a transition to FZDZ with a very quick loss of moisture above 800 mb and transition to general downward motions within the main stratus layer. Right now, this looks more supportive of things ending as a little snizzle as opposed to all FZDZ which tends to be less impactful, but will obviously keep a close eye on trends this evening.

The second clipper arrives on Wednesday, with light snow expected to develop towards late morning/midday. Frontogenesis looks like it'll be a bit more of a player with this second system with an increase in upper jet support and slightly more robust thermal gradients. As a result, the higher precipitation amounts might end up a bit streakier in nature, and it's a bit difficult at this range to say where the highest amounts will fall as a result. With this current wave sort of setting up the main baroclinic zone, it seems reasonable to expect the main swath of snowfall (amounts near or locally a bit over an inch) near or perhaps a bit SW of a Rockford to Rensselaer line. In this vicinity, will need to watch for a potential for a little given the stronger fgen signal, but currently not seeing anything too concerning in the guidance. Temperatures also look to be warmer with this system, and there's some potential these push into the upper 30s south of the Kankakee River during the afternoon which could end up cutting into ratios and amounts leading to a snowfall gradient somewhere in the vicinity.

Quick shot of seasonably cold air is expected Wednesday night into Thursday. As winds veer to more northerly direction and prior to column really drying out, there could be a short window of some lake effect snow into northeast Porter County Wed evening, but latest forecast soundings don't look too impressive, with a rapidly building subsidence inversion overnight.

There are indications that we could temporarily transition to more of a zonal flow pattern over the weekend. This would allow for some moderation in temps and with dry conditions.

Carlaw

AVIATION /18Z TAFS THROUGH 00Z THURSDAY/

Issued at 1203 PM CST Tue Dec 30 2025

A period of SN across southern Wisconsin and northwest Illinois will spread across all sites this afternoon, with MVFR visibility possible at the Chicago sites and IFR visibility possible at RFD. This area of SN will briefly diminish toward sunset before a more potent area of SN spreads across the area this evening. IFR visibility is expected to prevail in the 00-04Z window for ORD/MDW, with the expected drier and smaller snowflakes lowering visibility into LIFR levels at times roughly in the 01-03Z window. SN intensity will quickly wane late this evening, with a low (<20 percent) chance that FZDZ mixes with the SN for an hour or two before ending. MVFR ceilings with perhaps some scattering are then expected overnight through Wednesday morning.

Another period of SN is forecast Wednesday afternoon into early evening as a strong jet streak shifts southeastward across the area. While SN is expected to prevail for much of the period, it will likely exhibit a streaky presentation on radar and result in varying visibility from IFR to VFR levels.

Otherwise, W/WSW winds around 10 knots will back SW and gust to around 20 knots during the SN this evening. Winds will then settle WNW around 10 knots overnight into the morning before veering NW and gusting over 20 knots during the afternoon.

Kluber

LOT WATCHES/WARNINGS/ADVISORIES

IL...None. IN...None. LM...Small Craft Advisory from 9 AM Wednesday to 3 AM CST Thursday for the IL nearshore waters.

Small Craft Advisory until 3 PM CST this afternoon for Calumet Harbor IL to Burns Harbor IN.

Small Craft Advisory from 9 AM Wednesday to 10 AM CST Thursday for Calumet Harbor IL to Burns Harbor IN.

Small Craft Advisory until 3 PM CST Thursday for Burns Harbor to Michigan City IN.