textproduct: Chicago

KEY MESSAGES

- Chance of wind-whipped snow showers Wednesday morning across far eastern IL into northwest Indiana.

- Unseasonably mild temperatures will continue until a strong cold front sends temperatures tumbling starting late tonight; seasonably cold temperatures expected Wednesday through Thursday.

- Winter weather advisory issued for northern and eastern Porter County for lake effect snow showers Wednesday and Wednesday night, localized accumulations of 2-5 inches expected, especially northeast Porter County.

- A "Clipper Express" pattern may materialize from the end of this week into early next week with periodic opportunities for snow, reinforcing shots of cold air, and blustery northwest winds.

DISCUSSION

Issued at 236 PM CST Tue Jan 13 2026

Through Thursday:

Water vapor imagery early this afternoon shows an amplifying trough digging south into the Upper Mississippi Valley and western Great Lakes. Fascinating to see all of the smaller embedded vorticity maximums embedded within the larger developing trough, the most significant being the one over extreme southwest Ontario. This Ontario vort max is progged to dive south-southeastward into southern WI by 12z Wednesday and could bring some precipitation to the area very late tonight into Wednesday morning, but there are some pretty big uncertainties...

Guidance is pretty split with GFS and NAM dry, while ECMWF and GEM do squeeze out some light QPF over mainly eastern portions of the CWA, including the Chicago Metro area, Wednesday morning. While vort max of interest will be quite strong, the associated ascent is mostly progged to occur in dry air above a strong low level frontal inversion. The difference between the guidance that produces QPF and those that don't seem to lie in the height of the progged frontal inversion and depth of the subsequent convective cloud depth under the inversion. Guidance with lower inversion heights are dry while those with slightly higher inversion heights do have some QPF. Hard to say with high degree of confidence which scenario is more likely at this point, so opted to hang onto the chance pops late tonight and especially Wed morning. The low probability, worst case scenario seems to be some more robust wind driven snow showers, leading to a coating to a half inch of snow and potentially hazardous travel. The chances of that occurring appear quite low, so not planning to hit the messaging any harder at this time. Worth noting the HRRR does break out some light rain showers ahead of the front overnight, but forecast soundings from the HRRR look suspiciously moist and not realistic. Just kept some slight chance pops in the grids as a nod to the HRRR, but this scenario seems unlikely.

Regarding the lake effect snow...

Strong cold air advection over a fairly warm lake (SSTs in the lower 40s over most of the open waters) should result in moderately strong lake induced instability Wednesday into Wednesday night. Inversion heights are progged to be extreme, topping out 6-8kft, but plenty high to support some heavy lake effect snow showers, particularly given the DGZ embedded within the convective clouds.

Expecting mostly multi-banded lake effect set-up during the daytime hours Wednesday, effecting northern and perhaps eastern Porter County. Wednesday night, land breeze convergence could lead to a more dominant, strong lake effect band, which would likely take aim on areas east of Porter County. Some CAM guidance suggests the there could be a weaker secondary band to the west over Porter County or even just some weaker multi-band structure snow showers. Lake effect chances for Porter County look to decrease early Thursday morning as winds back and push the band east into WFO IWX's area. Given the strong winds and the potential for at least a couple/few inches of snow, opted to issue a winter wx advisory for northern and eastern Porter County. As is typically the case with lake effect, totals could vary significantly over a short distance.

- Izzi

Thursday Night through Tuesday:

A deep long-wave upper level trough is forecast to take up residence across eastern North America through the period, reinforced by a series of short waves digging into the western periphery of the trough from the Canadian prairies to the Midwest. Global ensembles (EPS/GEFS and CMCE) continue to be in good agreement with a potent wave rotating through the region Thursday night into Friday, with an associated surface low of around 995 mb tracking across the northern Lakes. This would support a wind-driven accumulating snowfall developing across the forecast area from Thursday evening into Friday morning as the vort pivots overhead. Deep forcing eventually weakens behind the initial vort Friday, though steepening lapse rates driven by cold advection, the continued deepening of the upper trough, and lingering low-mid level saturation should support additional snow showers (if not snow squalls) through the day. Guidance is also in good agreement in a reinforcing push of colder low- level air Friday night.

Models then remain in good agreement in keeping the long-wave trough axis near or just east of the area though the weekend and into early next week placing our area in the "cone of uncertainty" for additional short waves/clipper systems to swing into the area. While forecast confidence is naturally low on the exact cadence and track of each clipper, confidence remains high that we will experience a prolonged period of cold and occasionally blustery weather into early next week with wind chills spending considerable time in the single digits either side of zero. This includes during the upcoming weekend.

Ratzer/Borchardt

AVIATION /00Z TAFS THROUGH 06Z THURSDAY/

Issued at 543 PM CST Tue Jan 13 2026

Forecast concerns include...

Wind shift to north/northwest tonight. Strong/gusty winds overnight/Wednesday. Chance of snow showers Wednesday morning.

A cold front will move across the area early Wednesday morning. Westerly winds this evening will slowly become northwest this evening and then shift to the north/northwest with the cold front early Wednesday morning. Gusts into the lower 30kt range will be possible with and behind the cold front and then gusts will diminish into the mid 20kt range by midday Wednesday and continue into Wednesday evening.

There is a chance for a few sprinkles later this evening and then there will be a chance for snow showers behind the front through mid morning. Confidence is low for coverage for any possible snow showers and current prob mention seems on track. Some flurries may persist into Wednesday afternoon. cms

LOT WATCHES/WARNINGS/ADVISORIES

IL...None. IN...Winter Weather Advisory from 8 AM Wednesday to 4 AM CST Thursday for INZ002.

LM...Small Craft Advisory until 6 PM CST this evening for the IL and IN nearshore waters.

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

Gale Warning from 3 AM Wednesday to 3 AM CST Thursday for Gary to Burns Harbor IN-Burns Harbor to Michigan City IN.