textproduct: Chicago

KEY MESSAGES

- Strong southerly, then westerly winds are expected today as a multi-faceted weather system moves through the region.

- Thin line of showers may result in some sporadic wind gusts of 50+ mph between about 11 AM and 3 PM (earliest west).

- Scattered wind-shipped snow showers with reduced visibilities and rapidly falling temperatures could lead to some slippery travel conditions this evening.

- Wind chills of 0 to 10 below expected early Friday morning.

- The blustery and briefly colder conditions tonight into Friday will be followed by variable, but generally above normal temperatures into next week.

DISCUSSION

Issued at 310 AM CST Thu Dec 18 2025

Through Tonight:

Moisture is surging northward across the region early this morning with low stratus rapidly expanding overhead. Cloud depths will increase beyond 5-6 kft through 5 AM, and this will result in a rapid expansion of showers and drizzle through the morning hours. Earlier this evening, we had noted a fair amount of frost/ice development on untreated surfaces, even with above- freezing air temperatures. This was likely tied to fairly deep frost development coming out of the recent cold stretch. Thankfully, road temperature sensors at this hour have continually warmed, with all available data showing temperatures now above freezing. With air temperatures continuing to warm, we don't have any concerns with icing threats this morning.

A robust--currently roughly 980 mb--surface low will shift east across the Minnesota Arrowhead Region this morning and will eventually drag not one, but two, cold fronts through the region--one this afternoon, followed by the main cold push this evening. The Omaha, NE radar is nicely picking up this first front which is slicing into western Iowa at this hour. Expecting the development of a thin, strongly-forced line of shallow convection later this morning as the front begins to intercept deeper moisture with dewpoints pushing into the mid and upper 40s. A very narrow axis of non-zero 0-3 km CAPE is forecast to develop and push into parts of central Illinois along the front. At this time, it looks like the bulk of this still meager low- level instability will remain well to our south, but a very narrow ribbon of reduced static stability may accompany the line of convection through our forecast area. While the severe threat remains relatively low, such strong forcing coincident with an intensifying southerly LLJ overhead suggests some potential for stronger wind gusts being transported to the surface withing any taller cores. In fact, can't rule out a few gusts near 50-60 mph as this convective line marches eastward (either side of 11 AM near I-39, Noon-2 PM through the Chicago Metro, before clearing our NW Indiana counties around 3-4 PM). In the wake of this activity/cold front, temperatures will fall quickly into the 30s.

Out ahead of this line, a ripping low-level jet just off the deck will result in increasing southerly synoptic winds. Low- level inversion looks formidable, and likely will curtail the more significant winds from mixing down to the surface, but a few 40-45 mph gusts will be possible before the convective line passes.

A secondary narrow corridor of low-level instability is forecast to develop immediately ahead of the secondary cold front, focused primarily across central Illinois. Could envision some additional low-topped showers developing in this region towards 4-7 PM south of the Kankakee River, but this activity seems like it'd be occurring with above-freezing wetbulb temperatures.

The secondary cold front with the main push of CAA will arrive near I-39 towards 5-6 PM and will surge through the rest of the forecast area through the late evening hours. Temperatures are expected to fall quickly into the teens and 20s through midnight. Snow squall parameter output remains pretty muted, even on the previously-aggressive NAM. This seems to be tied to fairly shallow low-level instability (0-2 km lapse rates generally less than 5-6 C/km, with most of the steeper lapse rates confined to the 0-1 km layer). This will still support the development of some gustier, wind-whipped snow showers which may knock visibilities down briefly under a mile this evening with localized coatings of snow.

As the previous shift mentioned, we'll likely see another "pop" of winds as this front arrives, with strong westerly gusts developing through the evening and overnight. While the peak gusts map currently across the Dakotas shows quite a few 50+ kt reports, guidance continues to suggest the strongest winds will have a very difficult time pushing past the Mississippi River. 3-hr pressure rises are forecast to ease significantly with eastward extent, and it looks like the main core of stronger 925-850 mb winds will end up passing off to our south and west. As a result, while a few 45 mph gusts will be possible this evening, not seeing enough of a signal that these will be widespread and long enough duration to warrant a Wind Advisory at this point.

Scattered snow showers/flurries will probably persist through much of the night, at least until the main 700 mb vort lobe passes overhead. Wind chills by daybreak Friday are forecast to be in the 0 to 10 below range.

Carlaw

Friday through Wednesday:

Stratus with some flurries (cloud layer in the DGZ) trapped under a strong subsidence inversion late tonight may linger into Friday morning, but substantial low-level dry air advection should erode clouds from west to east as a surface ridge crosses the area Friday afternoon. Expect a much colder day on Friday with highs in the mid 20s. Only a small drop in temperatures is expected Friday evening as a 50 knot LLJ spreads over the area and results in steady WAA through the night.

A low-amplitude wave will then cross far western Ontario on Saturday. A brief warmup into the 40s is expected early in the day before temps fall with the passage of a cold front during the afternoon. Marginal moisture profiles may support a narrow band of light mixed precip along and slightly behind the front mainly north of I-80. Colder conditions with highs in the upper 20s and mostly sunny skies will follow on 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 on Tuesday. Guidance continues to favor the development of a broad ridge across the central CONUS by early Christmas. Expansive WAA with steady low-level moisture advection is expected across the mid-Mississippi River Valley by Christmas Eve, with a likely area of stratus and drizzle/light rain in the region. Placement is still a bit far out to pin point, but a mild and damp Christmas Eve into Christmas is possible.

Kluber

AVIATION /12Z TAFS THROUGH 18Z FRIDAY/

Issued at 543 AM CST Thu Dec 18 2025

Conditions have rapidly deteriorated over the past couple hours as a 60+ knot low-level jet advects abundant low-level moisture over the area. Even with ongoing S gusts over 25 knots and expected SSW gusts to 30 knots through the morning, have maintained LLWS as KLOT VWP data indicates winds of 60-65 knots are located as low as 1500kt. Meanwhile, areas of SHRADZ will persist through the morning, but likely be more prevalent through mid-morning.

Ceilings/visibilities range from VLIFR/LIFR west of the Fox Valley to IFR closer to Lake Michigan. Existing snowpack at DPA/RFD is likely contributing to the LIFR or lower conditions, which should persist for at least a few more hours before slowly improving late this morning. Ceilings and visibility should remain in IFR levels at ORD/MDW/GYY through the morning.

A cold front will bring a thin line of strongly forced showers (almost convective in nature) across the terminals early this afternoon. Squall-like conditions from west gusts over 40 knots and visibility as low as 1SM are possible for 30 min or less in the 18- 20Z window for ORD/MDW.

Winds will turn west and gust up to 30 knots behind the front while ceilings lift into VFR levels later this afternoon. A secondary cold front will then bring a period of scattered SHSN, westerly gusts over 30 knots, and a return of MVFR ceilings early this evening and overnight. Brief periods of IFR visibility are possible for the first few hours behind the front (generally 03-06Z), with SHSN waning in intensity the overnight.

Kluber

MARINE

Issued at 435 AM CST Thu Dec 18 2025

A strong area of low pressure near 29.1 inches will move across the Great Lakes region today and tonight. In advance of this system, southerly winds will continue to gradually increase this morning, with frequent gusts around 30 kts expected. A strong low-level jet with winds near 50 kts is forecast to develop later this morning and afternoon. Strong stability over the colder lake waters is expected to limit the frequency of stronger gusts, but some intermittent gales into the 35-40 kt range will be possible during a brief window between about 10 AM and 3 PM, mainly at elevated/higher-level platforms. Thereafter, winds may briefly ease into the 25-30 kt range through the rest of the afternoon.

A thin line of fast-moving showers is expected to push across the lake, approximately during the 1 to 4 PM timeframe. Some wind gusts near 45 kts will be possible with any stronger/deeper showers. Special Marine Warnings may be needed for this activity.

A second cold front will move across the southern portions of Lake Michigan this evening. Westerly winds around 35 kt are expected, and a Gale Warning remains in effect this evening and tonight to cover this potential. 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 until 6 PM CST this evening for the IL and IN nearshore waters.

Gale Warning from 6 PM this evening to 6 AM CST Friday for the IL and IN nearshore waters.