textproduct: Chicago

KEY MESSAGES

- Mild temperatures through Thursday should melt much of the remaining snow cover.

- Rain showers and drizzle will develop late tonight with a period of steady rain ahead with a strong cold front on Thursday.

- Blustery and briefly colder conditions Thursday night into Friday will be followed by variable but generally above normal temperatures into next week.

DISCUSSION

Issued at 321 AM CST Wed Dec 17 2025

Through Thursday night:

A shortwave trough is in the process of swinging through the region early this morning. We managed to wring out some spotty sprinkles/light showers at the office a bit earlier from a 15 kft cloud deck. Given modeled ascent increasing through daybreak, have added some sprinkle wording to the grids roughly south of I-80. Thankfully, temperatures remain much warmer than previously thought, and are currently solidly above freezing. Seeing a few road temperature sensors still hovering around the 32 F mark, but given the light nature of things and warmer ambient conditions, don't have significant icing concerns before the column rapidly dries out through 5-6 AM.

Upstream satellite trends reveal post-frontal stratus is thinning across central Wisconsin. As a result, it looks like we'll probably clear out here this morning towards midday. Forecast soundings suggests we'll probably attempt to mix into at least some spotty lower stratus/stratocu this afternoon, but it remains a bit unclear just how widespread this will become. Continue to advertise general partly cloudy/partly sunny wording this afternoon as a result.

Dry conditions will continue through this evening, but cloud cover will thicken up ahead of our next robust storm system. Model guidance continues to indicate rapid saturation taking place under 700 mb during the 3 to 6 AM timeframe. This will likely manifest as a quick blossoming of showers and drizzle across much of the area, and PoPs commensurately increase during this time frame. Wetbulb temperatures will be solidly above freezing, and even pushing above 40 degrees, as precipitation develops, so no concerns with slick spots at this point. Widespread rain/showers will continue to expand across the region Thursday morning. Given the dynamics at play, wouldn't be totally surprised to see a lightning flash approach our southern CWA mid-morning towards midday. Any low-level instability of note appears like it'll remain decidedly south of the area.

An initial front/pre-frontal trough will push through the area Thursday afternoon sending temperatures falling quickly into the mid to upper 30s. Have worked to sharpen up this non-diurnal trend a bit with the NBM-delivered grids not quite capturing this sharp gradient. While we can't totally rule out some snow mixing in, the sharp nature of this boundary suggests that precipitation will come to an end prior to the column cooling enough to support a widespread changeover to snow.

The true push of deep cold advection will lag this initial boundary a bit, but will arrive decidedly early Thursday evening and overnight with temperatures plunging through the teens. As the main blast of cold air arrives, saturation through a thickening DGZ is forecast to occur. This setup looks favorable for continued intermittent gusty snow showers and widespread flurries through Thursday night. Continuing to keep an eye on the Thursday evening timeframe as some guidance (mainly the NAM) suggests a brief window for a potentially more squall-like snow showers with notably steeper low-level lapse rates compared to the rest of the guidance suite, but note that this remains an outlier solution. Regardless, localized dustings of snow will be possible before activity diminishes late overnight into Friday morning. Wind chills by early Friday AM will fall into the +5 to -5 range.

Regarding winds: a substantial southerly LLJ will develop on Thursday, but not seeing any signs that this flow will manage to meaningfully work its way to the surface during most of the daytime hours. Once the main push of CAA arrives, steepening low-level lapse rates will allow for some deeper mixing, likely resulting in a period of 35 to 40 mph wind gusts before the strongest flow weakens towards daybreak Friday.

Carlaw

Friday through Tuesday:

A weakening mid-level ridge and associated surface ridge will cross the forecast area through the day on Friday. Stratus with some flurries (cloud layer in the DGZ) trapped under a strong subsidence inversion late Thursday night may linger into Friday morning, but substantial low-level dry air advection on gusty northwest winds in the morning should erode remaining clouds from west to east before the ridge arrives late in the afternoon. After highs in the mid to possibly upper 20s Friday afternoon, increasing WAA amid weak low-level stability and a 40+ knot LLJ spreading over the area will likely result in steady or even rising temps Friday night.

A low-amplitude wave will then cross far western Ontario on Saturday while keeping any appreciable forcing well north of the area. A brief warmup into the 40s and a ribbon of very light rain may precede a passing cold front Saturday afternoon and evening. Colder conditions with highs in the upper 20s and mostly sunny skies are expected Sunday as another high pressure crosses the region.

Moisture return ahead of the next storm system passing well to the north Monday into Tuesday appears to be insufficient for widespread precip over the area before a weak cold front passes through on Tuesday. Looking ahead to Christmas Eve and Christmas, a rapid return of WAA over the central CONUS is on track to bring unseasonably warm conditions over the area. With a long trajectory of moisture from the Atlantic and Gulf, abundant moisture surging over the region will likely bring periods of showers/drizzle and plenty of stratus.

Kluber

AVIATION /12Z TAFS THROUGH 18Z THURSDAY/

Issued at 527 AM CST Wed Dec 17 2025

A strong inversion between 1-2kft will remain in place over much of the region through tonight. Under this inversion, shallow MVFR stratus from southern MN to southern Lake MI has struggled to push southward during the early morning hours. However, daytime mixing from filtered sun through batches of cirrus may be sufficient to yield FEW/SCT shallow low-end MVFR stratus under the inversion later this morning into this afternoon. If these clouds do not materialize, VFR conditions can be expected through at least this evening.

WNW winds up to around 10 knots at TAF issuance will become VRB under 5 knots late morning into early afternoon as a surface ridge crosses the area. Winds will then settle SE/SSE around 10 knots by late afternoon. A 50-60 knot low-level will then develop within the inversion this evening. Given the strong low- level stability in place, surface gusts may struggle to develop until very late this evening or even overnight. Have therefore included LLWS at 50 knots down as low as 1.2kft in the event surface gusts do not occur or are only sporadic. Winds will eventually shift just west of south overnight, with gusts of 25 knots or higher likely becoming more prevalent as low-level stability weakens.

The low-level jet will produce a surge of moisture over the area late tonight as deep-layer forcing increases. This will result in rapid saturation and a quick transition from VFR to IFR ceilings and SHRADZ in just an hour or two (generally 09-11Z for ORD/MDW). These conditions should continue through much of the morning, with a cold front shifting winds SW, ending precip, and quickly lifting/scattered ceilings by early afternoon.

Kluber

MARINE

Issued at 407 AM CST Wed Dec 17 2025

A strong area of low pressure near 29.1 inches will move across the Great Lakes region on Thursday. In advance of this system, southerly winds will increase this evening over the Illinois and Indiana nearshore waters. While winds are forecast to increase to near 50 kts at 1500 ft very late tonight and into Thursday morning, strong stability over the lake is expected to limit much of this flow from reaching the surface, curtailing the frequency of 35 kt gales. A Small Craft Advisory is in effect from 9 PM this evening through 6 PM Thursday.

As the surface low moves across Lake Superior, a cold front will shift winds out of the west Thursday evening and overnight. The potential for 35 to 40 kt gales will increase during this period, although some uncertainty in the frequency of the strongest winds remains. As a result, a Gale Watch has been issued and is in effect from 6 PM Thursday through 6 AM Friday.

Winds will diminish through late Friday afternoon as an area of high pressure briefly builds across the southern portion of Lake Michigan.

Carlaw

LOT WATCHES/WARNINGS/ADVISORIES

IL...None. IN...None. LM...Small Craft Advisory from 9 PM this evening to 6 PM CST Thursday for the IL and IN nearshore waters.

Gale Watch from Thursday evening through late Thursday night for the IL and IN nearshore waters.