textproduct: Chicago

KEY MESSAGES

- A brief period of freezing rain may result in a glaze of ice on untreated surfaces this evening, impacting part of the PM commute (~5-8 PM CST), mainly northwest of a Dixon to McHenry, IL line (30% chance).

- Strong southwest winds are expected late this evening before becoming northwest on Wednesday (gusts to 35-40+ mph).

- The potential for hazardous travel conditions is increasing for the Wednesday AM commute due to robust snow showers or squalls.

- More fast-moving clipper systems could result in some additional periods of accumulating snow in or near our area late this week into the weekend.

- Temperatures will turn colder for the latter half of the week and especially this coming weekend, when dangerously cold wind chills may also be observed at times.

DISCUSSION

Issued at 405 AM CST Tue Dec 9 2025

Through Wednesday night:

A complex forecast is in store through the next 24-48 hours with multiple periods where there may be travel impacts.

The overnight snow showers and flurries are on their way out of the area but likely resulted in a quick dusting in some spots. In their wake, southerly warm and moist advection is already well underway with winds steadily increasing out of the south here locally. In fact, above freezing temperatures have already lifted out of Missouri into southwest Illinois. The freezing line will continue to lift northeast across Illinois and into Indiana through the day today.

The potential for freezing drizzle continues to wane early this morning with the lingering stratus deck expected to thin through the morning. This should inhibit additional drizzle development. However, there are weak echoes on radar across parts of Wisconsin but so far have not seen any observations of this reaching the ground. Still something to keep an eye on for counties along the WI/IL stateline. As temperatures warm over the snowpack, any lingering stratus is expected to continue thinning which will limit any potential drizzle developing later this morning, but overall, not expecting any hazardous weather through the daylight hours.

The big forecast question after sunset early this evening is whether or not there is a brief window for freezing rain across northwest Illinois and along the WI/IL state line (generally north of a Dixon to McHenry, IL line (including the Rockford metro). A strengthening surface low is forecast to track across southern Wisconsin overnight. Out ahead of it, light precipitation will move into the area after sunset. Forecast vertical profiles suggest that the primary precipitation type is expected to be liquid/rain (perhaps mixed with some snow and/or sleet initially). If ground temperatures are even just an hour or two slower at warming above freezing than forecast, this could lead to a quick glaze of ice on untreated surfaces coinciding with part of the PM commute. Confidence in this occurring here locally remains quite low, however, and have noted a trend toward a slower onset of precipitation with some of the latest guidance which further narrows this window (closer to 5-8 PM CST). Have opted to message this with an SPS for Ogle, Winnebago, Boone, and McHenry counties for now outlining the threat of slick roadways. If confidence increases in slower warming and a larger window for icing, a short-fused Winter Weather Advisory may be needed given it would coincide with the latter half of the PM commute. Stay tuned!

Once temperatures warm solidly above freezing across the area late Tuesday, cold rain is expected to expand across the area through the evening and continue overnight (total QPF around 0.1-0.2"). Winds will also become strong out of the southwest through the evening, gusting up to 35 to 40+ mph.

There may be a bit of a break in the precipitation late tonight into early Wednesday morning before a formidable cold front races across the area. This will likely be paired with gusty snow showers to potentially robust snow squalls. Winds along the front will likely approach or reach wind advisory criteria for a 1-3 hour window for any given location. Opted to hold off on a wind advisory issuance with this update in case the day shift determines that the snow squall potential itself warrants a Winter Weather Advisory (as some offices have done upstream). The trend toward higher Snow Squall Parameter values is also telling (incorporates low-level instability, moisture, and winds). This could result in a quick coating of snow up to an inch within a short window of time paired with very low visibility and strong winds. Accordingly, impacts to the Wednesday morning commute are becoming increasingly likely across the area between roughly 5-10 AM CST from northwest to southeast.

A few show showers may linger into the afternoon with temperatures in the upper 20s to around 30 degrees. Temperatures then steadily fall into Wednesday night with lows back into the teens (around 20 near the immediate lakeshore). There is a signal for a robust lake effect band to drive down the lake in the wake of the front Wednesday afternoon into the evening. This could bring a quick 1-2" of snow to portions of northwest Indiana before ending overnight.

Petr

Thursday through Monday:

The stubborn northwest flow pattern will persist over much the CONUS through the latter half of the week and into the upcoming weekend. With the energetic jet stream and associated baroclinic zone lining up directly over Illinois and Indiana, the barrage of clipper-type systems getting sent into the Midwest/Great Lakes will continue through at least this weekend, and each of these systems will likely produce additional snow accumulations somewhere in or very near our forecast area.

While there are still some differences across the broader suite of medium range deterministic and ensemble guidance that are leading to persisting spatiotemporal uncertainties with our precipitation forecast, there appear to be three primary disturbances of note that will affect the region between Thursday and Sunday. The first of these disturbances will be a somewhat sheared-out shortwave trough that looks to arrive at our longitude sometime late Thursday/Thursday night. Strengthening mid-level frontogenesis along the attendant baroclinic zone during this time should help induce an elongated band of snow somewhere over or just southwest of our forecast area as this wave passes by.

A second shortwave trough then looks to follow closely on the heels of this first shortwave, arriving in the region on Friday. Depending on when this wave arrives and how far south it dips, it could help prolong the Thursday night snowfall into Friday, or it could induce its own separate area of snow that may largely remain to our north. Finally, a third shortwave trough is then favored to zip into the region on Saturday, with an overwhelming majority of 00Z EPS members and a smaller majority of 00Z GEFS members outputting QPF in our forecast area as this disturbance passes overhead. It's still too early to have much confidence in snow amounts with each of these snowfall opportunities. However, it can be said that the cold and dry air infiltrating the region should support generally lower QPF amounts, but higher snow-to-liquid ratios that correlate to fluffier snow characters (and possibly even dust-like snow, particularly in the case of this weekend).

Of potentially greater note for this coming weekend will be the bitterly cold temperatures. Successive shots of cold air brought in by the aforementioned disturbances will likely culminate in single digit and sub-zero lows and possibly even single digit highs in many areas over the weekend. Confidence in our overnight/morning wind magnitudes isn't particularly high at this point in time, but until a Canadian surface high settles over the region sometime Sunday into Monday, suspect that there's a good chance that northwesterly winds will remain high enough for minimum wind chills to reach (or at least get close to) our -20F threshold for Cold Weather advisories in some locations on at least one morning this coming weekend.

Ogorek

AVIATION /12Z TAFS THROUGH 18Z WEDNESDAY/

Issued at 640 AM CST Tue Dec 9 2025

Key Messages:

- Strong winds are expected tonight and into tomorrow, initially from a southwesterly to westerly direction before turning northwesterly behind a cold front tomorrow morning.

- A period of rain is expected this evening, possibly preceded by a brief wintry mix.

- Gusty snow showers/squalls are likely tomorrow morning with a brief period of sharply reduced visibility.

- MVFR/IFR ceilings are expected through most of the TAF period.

A busy TAF period is on hand as a deepening low pressure system passes to the north of the area.

First, the southern edge of an MVFR stratocumulus deck is bisecting the Chicago metro this morning. The expectation is that this cloud deck will gradually ooze southward over the course of the morning, but early morning satellite trends suggest that there will be some VFR time at our Chicago metro TAF sites as well until the ragged edge of the cloud deck has made a more solid push southward. A lowering/strengthening temperature inversion should then cause ceilings to lower later today, possibly to IFR levels. The greatest probability of IFR ceilings occurring is at RFD.

Early this evening, precipitation is expected to develop and/or spread over the terminals. The precipitation is predominantly expected to fall as rain, but at the onset of it, a brief wintry mix could be seen. While the chances for that at the Chicago metro terminals may have decreased slightly since the 06Z TAF issuance, elected to largely maintain the inherited PROB30 groups for -PLSN for now.

It appears that there will likely be a relative lull or break in the precipitation overnight before snow showers/squalls closer to an incoming cold front move into the area. While these wind-whipped snow showers will not last long, they will result in sharp visibility reductions, briefly lowered ceilings, and possibly some quick coatings of snow on the ground as well. Prior to the onset of the snow showers, the lower clouds from earlier may scatter out or lift to VFR levels, but a return to MVFR ceilings is favored behind the cold front.

Lastly, winds will ramp up tonight as the low pressure system gets closer by. 30+ kt gusts may begin as early as this evening while winds are still from a southwesterly direction. Winds will then gradually turn westerly tonight before shifting northwesterly behind the aforementioned cold front, with 30+ kt gusts remaining likely through this whole time. Some 40+ kt gusts could be seen as well late tonight or tomorrow morning. It's also possible that the gustiness may be slow to get going this evening. If that happens to be the case, then a 50+ kt low- level jet could result in a period of low-level wind shear. Didn't have enough confidence in that happening to include a formal LLWS mention in the TAFs, but that may need to be reconsidered in forthcoming TAF packages.

Ogorek

MARINE

Issued at 405 AM CST Tue Dec 9 2025

- Gale Warning in effect late Tuesday evening through Wednesday afternoon.

The Gale Watch has been upgraded to a Gale Warning for strong southwest winds quickly increasing through this evening ahead of an approaching weather system to 35-40 kt gales. Wind directions then turn northwest behind a cold front Wednesday morning paired with gusty snow showers/squalls. A lake effect snow band diving down the lake Wednesday afternoon may lead to another brief period of higher gusts to 40 kt Wednesday afternoon before winds gradually ease late afternoon into the evening.

Petr

LOT WATCHES/WARNINGS/ADVISORIES

IL...None. IN...None. LM...Small Craft Advisory until noon CST today for Winthrop Harbor IL to Gary IN.

Gale Warning from 9 PM this evening to 3 PM CST Wednesday for Winthrop Harbor IL to Gary IN.

Small Craft Advisory until 3 PM CST this afternoon for Gary to Burns Harbor IN-Burns Harbor to Michigan City IN.

Gale Warning from 9 PM this evening to 6 PM CST Wednesday for Gary to Burns Harbor IN-Burns Harbor to Michigan City IN.