textproduct: Chicago

KEY MESSAGES

- Fluffy steady snow will spread over the area late this afternoon and end before midnight. Untreated surfaces may become slick.

- An Arctic cold front will move across the area late this evening with west northwest winds gusting to 40 mph and tumbling temperatures.

- Bitterly cold conditions are expected Monday morning with wind chills of 20 to 25 below zero.

- A clipper system may bring accumulating snow to parts of the area Tuesday night into Wednesday morning.

- Another period of bitterly cold temperatures may arrive by Friday.

DISCUSSION

Issued at 225 PM CST Sun Jan 18 2026

Tonight through Monday:

A potent mid-level wave currently over southeast Minnesota will track southeastward late this afternoon and evening and bring a round of light snow accumulations across the area. Outside of a slight southward shift in the best forcing, no substantial changes were made to the forecast through this evening.

Modest deep-layer forcing via the periphery of the left exit of an upper-level jet and appreciable mid-level diffluence have contributed to a decent snow shield aloft over the Upper Mississippi River Valley early this afternoon. An embedded N/S axis of weak isentropic ascent is also locally enhancing snowfall rates aloft. However, moisture available remains a limiting factor for a more impactful snow event, with PWAT values as high as only 0.2". Upstream obs/radar support the notion that existing dry air is eroding the leading edge of the snow shield, but eventual top-down saturation is expected to occur and result in a quick change in conditions at onset. Thermo profiles in the bulk of the forced layer are on the cold side of the DGZ, which should favor smaller and drier snowflake generation through much of the event. Utilizing a SLR of 15-20:1 and consensus QPF of around 0.05" yields fluffy snow accumulations of around 1" for much of the area. Given the decent forcing in place, could still see some 2" amounts, particularly if saturation occurs earlier and/or SLR values surpass 20:1. The bulk of the snow will fall in the 4-10pm window, with a given location likely seeing a 3-4 hour period of more steady snow.

On the heels of the synoptic snow exiting to the east late this evening, an arctic cold front will sweep across the area generally in the 11pm-1am window. This will usher in rapidly falling temps with WNW/NW gusts to 40-45mph. Some gusty snow showers will also be possible with the front. With little existing snow cover (around 2" along the WI state line and Porter County to around 1" or less elsewhere) combined with expected snowfall this evening of under 2" should limit the availability for impactful blowing snow, but shallow blowing and drifting snow are still expected.

Overnight, lingering stratus and strong CAA should produce flurries with embedded narrow ribbons of snow showers efficient in sharply knocking down visibility.

By daybreak Monday, the combination of temps around or slightly above zero combined with the strong winds will result in wind chill readings ranging from around -25F west to -15F to -20F east. A Cold Weather Advisory is now in effect for the entire area. Highs Monday will struggle to reach 10 degrees, with maximum wind chills remaining solidly below zero even under mostly sunny to partly cloudy skies.

Kluber

Monday night through next Sunday:

The persistent upper-level pattern characterized by deep troughing across eastern North America and ridging along the Pacific West will persist through the middle to end of the week, if not longer. Our area will remain on the western side of the aggregate eastern troughing and indeed within the highway for periodic clipper systems and shots of Arctic air.

Ensemble model guidance remains in agreement that the next clipper system will move into the Midwest in the Tuesday night to Wednesday timeframe. At this point, ensemble guidance favors the clipper tracking either directly over our area or just to our north. Assuming this occurs, a quick hit of warm-air advection should help temperatures climb toward the freezing mark early on Wednesday before a swath of snow falls generally near and north of the Wisconsin state line. Will note however that confidence in the track of any clipper system more than 24 hours out is low, so adjustments in the forecast are all but certain going forward.

Looking toward the end of the week, ensemble guidance is trending toward another clipper system moving into the region followed by the southward surge of the Polar Vortex into the Great Lakes. The EPS in particular is bullish with 51-member mean low temperatures reaching double-digit below zero values near the Wisconsin state line Saturday morning, some 30 to 40 degrees below normal for this time of year. Would have to think the southward displacement of the low- level baroclinic zone would shift any big precipitation-makers well south of our area by the weekend, though we're not out of the woods, yet. Regardless, the Friday through Sunday timeframe is trending toward being bitterly to even dangerously cold across the Midwest/Great Lakes region.

Borchardt

AVIATION /00Z TAFS THROUGH 06Z TUESDAY/

Issued at 535 PM CST Sun Jan 18 2026

Key aviation messages:

- Period of steady, but mainly light, snow this evening. Periodic dips to IFR vsbys. Activity diminishes through about 06z.

- Arctic front arrives with a strong and gusty northwest wind shift overnight and tumbling temperatures.

- Increasing concern for convective snow showers along the front. Low confidence on exact placement, but potential exists for brief but sharp vsby reductions under 1 sm and wind gusts near 35-40 kts.

- Intermittent flurries/shsn likely to persist overnight and possibly through Monday morning.

Steady, light snow will continue this evening with periodic IFR vsbys. This activity is expected to diminish through late this evening. An arctic cold front will sweep across the region later tonight (about 04-05z at RFD, 06-08z through the Chicago-area terminals) with a notable northwest wind shift with gusts near 30-35 kts. Temperatures will plummet into the single digits through the late overnight behind the front.

Upstream radars indicate the presence of convective snow showers along the front stretching from near Minneapolis, southeastward into eastern Iowa. This activity may push across the terminals later this evening, and have introduced TEMPO groups with lower vsbys to account for this. Brief vsby reductions (30 minutes or less) near or under 3/4 sm will be possible at times if more robust individual SHSN move across a particular area, but this is not possible to indicate with confidence and specificity at this range.

Otherwise, flurries/light snow showers will continue off-and-on through the night and potentially through Monday morning. Current TAFs indicate a return to VFR around midday, but it's entirely possible that MVFR cigs remain entrenched or redevelop through the day.

Carlaw

MARINE

Issued at 225 PM CST Sun Jan 18 2026

The gale warning remains in effect from midnight tonight into Monday morning for all of the IL and IN nearshore waters.

A heavy freezing spray warning remains in effect for the nearshore waters east of Gary, IN.

A strong arctic cold front will move across the area tonight, bringing a period of gale force winds between 35 and 40 knots. These strong winds will slowly diminish on Monday with gales ending across the IL nearshore Monday morning and then across the IN nearshore Monday afternoon. Very cold temperatures will spread across the area behind this cold front and combined with the high waves, heavy freezing spray is expected across parts of the IN nearshore waters, mainly east of Gary, IN. Offshore winds and a decent amount of ice formation along the Illinois nearshore over the past day precludes the inclusion of a Heavy Freezing Spray Warning west of Gary, IN.

cms/Kluber

HYDROLOGY

Issued at 126 PM CST Sun Jan 18 2026

Area hydrographs and ice spotter reports confirm that ice continues to develop and spread across areal rivers. Localized ice jams are already present along the Rock and Des Plaines rivers. Continued cold temperatures tonight and Tuesday will cause ice to continue growing.

After a brief warm-up in temperatures to above 20 degrees on Wednesday, another push of bitterly cold temperatures may arrive as early as Friday. As a result, the threat for ice jams including localized flooding will increase further toward the end of the week.

- NWS Chicago

LOT WATCHES/WARNINGS/ADVISORIES

IL...Cold Weather Advisory from 3 AM to noon CST Monday for ILZ003- ILZ004-ILZ005-ILZ006-ILZ008-ILZ010-ILZ011-ILZ012-ILZ013- ILZ019-ILZ020-ILZ021-ILZ023-ILZ032-ILZ033-ILZ039-ILZ103- ILZ104-ILZ105-ILZ106-ILZ107-ILZ108.

IN...Cold Weather Advisory from 3 AM CST /4 AM EST/ to noon CST /1 PM EST/ Monday for INZ001-INZ002-INZ010-INZ011-INZ019.

LM...Gale Warning from midnight tonight to noon CST Monday for Winthrop Harbor IL to Gary IN.

Gale Warning from midnight tonight to 3 PM CST Monday for Gary to Burns Harbor IN-Burns Harbor to Michigan City IN.

Heavy Freezing Spray Warning from midnight tonight to 9 PM CST Monday for Gary to Burns Harbor IN-Burns Harbor to Michigan City IN.