textproduct: Chicago

KEY MESSAGES

- An Arctic cold front will move across the area late this evening with west northwest winds gusting to 40 mph and tumbling temperatures. Scattered wind-whipped snow showers will accompany the front eastward, with localized but quick additional accumulations near one half to one inch possible. - 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 /06Z TAFS THROUGH 12Z TUESDAY/

Issued at 1105 PM CST Sun Jan 18 2026

Aviation messages:

- Arctic front moves across the terminals through 07-09z with strong northwest wind shift and gusts around 30-35 kts.

- Scattered wind-whipped snow showers expected as the front passes. Threat for localized sharp vsby reductions and quick additional accumulations near a half inch.

- MVFR cigs and intermittent snow showers/flurries through the night and likely through midday Monday.

- Low confidence on return to VFR conditions Monday afternoon. Small chance for flurries/light snow Monday afternoon and evening.

A strong arctic cold front will be pushing through the Chicago- area terminals through 07-09z resulting in increasing northwesterly winds gusting around 30-35 kts. Accompanying the front will be an area of scattered wind-whipped snow showers, the most robust of which are resulting in sharp but brief vsby reductions down to around a half mile, gusts up to 45 mph, and a quick half inch or so of snow accumulation. Coverage of these showers is such that the most intense portions may not directly impact the airfields, but is sufficient to justify TEMPO groups with IFR vsbys.

Once this activity passes, light snow showers/flurries will likely persist intermittently through much of Monday morning with periods of 4-5 sm vsbys possible as the arctic airmass settles overhead. While there is some potential for BLSN, given the limited snowpack and snowfall from earlier this evening, confidence in prevailing BLSN at the terminals was too low to include in the TAFs.

VFR cigs may return sometime Monday afternoon, although confidence in this is low, with some potential that MVFR cigs just linger through the afternoon and evening. Winds will remain blustery through Monday evening but will gradually diminish with time. There is a small potential for another period of flurries late Monday afternoon and evening, but the main signal for this remains north of the Wisconsin state line, with no mention in the current TAFs.

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 until noon CST Monday for Winthrop Harbor IL to Gary IN.

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

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