textproduct: Chicago

KEY MESSAGES

- After unseasonably mild temperatures today through this evening, expect a return to generally much colder temperatures Wednesday through the upcoming weekend.

- Chance of wind whipped snow showers/flurries late tonight into Wednesday morning, greatest coverage from northeast Illinois and east central IL and points eastward into northwest Indiana.

- Wind-blown accumulating lake effect snow expected downwind of Lake Michigan Wednesday afternoon through Thursday morning. The highest amounts are expected to occur east of Porter County, but hazardous travel appears likely in particularly northeast Porter County due to a combination of light to moderate lake effect snow and strong northwesterly winds.

- Another round of accumulating wind-driven snow appears likely Thursday night into Friday. An unsettled weather pattern then continues through the weekend and into next week with periods of gusty winds and occasional chances for light snow/flurries.

DISCUSSION

Issued at 345 AM CST Tue Jan 13 2026

Through Thursday:

The main concern is the anticipated lake effect snow event for portions of northwest Indiana, north central Indiana, and far southwest lower Michigan. Bottom line up front for this issuance is that no winter weather headlines were issued yet for Porter County, but expecting that we'll need to hoist a Winter Weather Advisory on the day shift.

Today will be breezy/windy and mild, especially in the afternoon as some sunshine appears probable. Looking at highs in the upper 40s to lower 50s, which will be the last non-winter-like day for a while. As noted in yesterday's discussion, can't rule out some mid 50s in spots. West-southwest winds will gust up to 30-35 mph, and if mixing heights are deeper/maximized, sporadic gusts to 40 mph are in the realm of plausibility.

The advertised abrupt (and unwelcome for some) change in our weather is on track for prior to daybreak Wednesday as a strong cold front rips through the area. A potent 500 mb vort lobe is progged to pivot southeast across the area from the pre-dawn hours through the mid morning. In the overnight guidance suite, the corridor of maximized forcing over the eastern 1/3 to 1/2 or so of the area exhibited a slightly deeper convective layer on forecast soundings than noted in the previous discussion. For this reason, right behind the fro-pa, there could be scattered more robust wind-whipped snow showers (outside chance for a few snow squalls) for a short period (between ~4 and 10 AM CST). West of this area (northwest and north central IL), expect primarily flurries and perhaps a few isolated snow showers.

Temperatures will quickly cool into the 20s from north to south behind the front and paired with strong north-northwest winds gusting to 30-40 mph, daytime wind chills will range from +5 to +15F. So after any morning snow showers/flurries, the main story for the areas outside the lake effect threat zone (which is most of the CWA), is that it will be decidedly back to winter, so bring the winter clothes back out!

As the column dries out west of Lake Michigan by Wednesday afternoon (ending lingering flurries/snow showers), the lake effect machine will roar to life, with multi-banded snow showers likely streaming into Porter County (and even portions of Lake County). Fairly impressive lake effect parameters with top of the synoptic frontal inversion up to around 7-8kft or more with convective cloud layer well within the DGZ. By Wednesday evening, LES should organize into a a strong single band with the million dollar question being where will that band set up. Synoptic flow continues to favor LaPorte County and points east, but it still could be close for especially northeast Porter County.

Even after the more intense single band develops during the evening, recent CAM depictions favor light to moderate snow showers still streaming into the northeast 1/2 or 1/3 of Porter County, with some accumulations likely. Still can't rule out a trend to a bit more veered with the flow that would potentially open the door to the intense single band and heavy snowfall rates getting into northeast Porter County for a time Wednesday evening. Our latest forecast update favors fairly continuous periods of snow Wednesday through early Thursday over northern and northeast Porter County gradually adding up to a few to perhaps several inches of accumulation. Localized 4-6" amounts appear plausible in the far northeast portion of the county. Paired with strong northwest winds enhanced by lake-induced convergence processes, falling and blowing and drifting snow will likely cause hazardous travel. Feel confident we'll need a Winter Weather Advisory for Porter County, possibly needing to start as early as Wednesday morning for the initial lake enhanced wind-whipped snow showers.

Lows will fall into the teens area-wide on Wednesday night, except perhaps right on the northwest Indiana shore, and with northwesterly winds slowly easing, expect wind chills to bottom out around 0 degrees (plus or minus a few degrees either side). After a mostly sunny start to Thursday away from northwest Indiana, clouds will increase in advance of the next system expected to bring periods of snow Thursday night into Friday (detailed below). Expect highs in the mid 20s, a few degrees below normal for the date.

Castro

Thursday Night through Monday:

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 light snow/snow showers 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. However, ensembles continue to show spread in the timing of additional short waves rotating through the trough in the Saturday through Monday time frame. While periods of snow/snow showers will likely occur, confidence in timing the impacts of individual short waves is quite low at this distance and have made no changes to NBM pops. Of higher confidence is that we'll see 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.

Ratzer

AVIATION /12Z TAFS THROUGH 18Z WEDNESDAY/

Issued at 539 AM CST Tue Jan 13 2026

Aviation forecast concerns for the 12Z TAFs:

- Strengthening southwest to west winds today become northwest tonight. Gusts around 30 kt at times this afternoon as well as late tonight/Wednesday.

- MVFR ceilings develop late this evening and persist overnight.

- A period of snow showers possible early Wednesday morning. Brief IFR possible.

Deep surface low pressure will pass well north of the forecast area today, though the effects will be felt locally with southwest winds becoming at least sporadically gusty 20-25 kts this morning. Winds will turn westerly and increase early this afternoon, with gusts around 30 kt behind a cold front. Winds may ease a bit early this evening, before a secondary cold front arrives predawn and shifts winds to the north-northwest with gusts again around 30 kts.

VFR conditions are expected through early this evening, after which an MVFR stratus deck is expected to spread across the terminals. Scattered snow showers, capable of producing brief IFR conditions, are possible for mainly the Chicago terminals in roughly the 10-14Z timeframe behind the second cold front. A few flurries will likely linger beyond that.

Ratzer

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 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.