textproduct: Chicago

KEY MESSAGES

- Cold conditions will continue into the weekend with mainly subzero wind chills through Saturday morning.

- Threat for impactful lake effect snow continues to increase for northeast IL and northwest IN Thursday night through Saturday night.

DISCUSSION

Issued at 236 AM CST Wed Jan 28 2026

Mainly quiet and cold conditions will prevail today. Longwave troughing will remain entrenched across the Upper Midwest and Great Lakes today, guiding a fast-moving disturbance well to our south tonight into Thursday morning. West to northwesterly breezes will develop this morning and afternoon which will result in wind chills in the 10 to 20 below range this morning, meandering towards 0 this afternoon. By later this afternoon and overnight, expecting lake effect cloud cover to build westward across the lake a bit as boundary layer flow turns a bit more northwesterly. Have introduced some low precip chances for lake effect snow showers across far northeastern portions of Porter County tonight into early Thursday morning given progged increases in 0-2 km BL moisture in this region which could result in a dusting. The bulk of this LES will remain east of the CWA, however. Thursday looks pretty similar to today, with any lingering LES across NE Porter county ending through the morning.

The main focus of this forecast remains squarely on the late Thursday night through Sunday morning time frame with a long- duration lake effect snow event looking increasing likely across our lake adjacent counties. However, there are significant uncertainties related to the specific evolution of this LES episode, including the residence time of heavier rates with a large amount of band movement and wobbling expected, particularly in the initial stages through Friday night. This has huge implications on snowfall totals, with the main axis of persistent LES potentially setting up as far west as the NE Illinois shore to as far east as northern Porter/La Porte counties. The other consideration is what impact (if any) increased ice concentration in the first dozen miles or so of shore could have on lake fluxes, although based on Sunday's event at least, this may end up being a minimal impact. The long-duration aspect of this event will also end up being another issue, with many precip-free hours possible (a big messaging challenge). Given all of this: while we ultimately may need Winter Storm Watches for some or all of the lakeside counties, there is too much uncertainty to justify issuance at this time, especially considering the most organized/intense LES may not develop until Friday evening into Saturday.

Early Friday Morning through Friday Afternoon:

There is fairly good model agreement in the handling of the opening stages of this event late Thursday night/Friday morning at least. The combination of a westward-advancing land breeze, southward-pivoting upper low, and associated inverted surface trough is expected to drive an initial region of snow across northeast Illinois and parts of northwest Indiana on Friday morning. This first stage will feature the coldest 850 mb temperatures of the event, and resultantly the deepest lake- induced equilibrium levels (near 10 kft) and associated CAPE, but also likely the most transient and fast-moving features. Latest guidance depicts at least a moderately intense region of LES rolling ashore across NE Illinois, including the Chicago- area) around or just after daybreak Friday morning. Suspect given the degree of convergence and thermodynamics involved that current coarse-resolution guidance is underdoing the amount of QPF at this range, with the potential for one to two inch/hour type rates with this initial activity and a high ratio/powdery snow. Residence time of this first round may not be all that long, however, potentially limiting the accums to start. Another trend is towards more significant inland penetration given the more cohesive convergence associated with the aforementioned inverted trough and BL lapse rates increasing markedly through the morning. Some of the current guidance suggest a potential for robust snow showers or even bona fide squall-like activity pushing west and southward through the area into early afternoon. Have expanded PoPs across most of the forecast area to account for this, although am not currently as aggressive as some of the latest guidance would suggest (NAM/GFS).

Friday Evening Through Saturday Night:

The main area of lake effect is forecast to gradually organize into a single dominant band during this time, although may have some difficulty doing so initially given modeled shear within the lake boundary layer. However, surface convergence is forecast to gradually focus down the long axis of the lake, with ELs progged around 8-9 kft. The vast majority of guidance indicates the main band shifting steadily away from NE Illinois and across parts of NW Indiana Friday night/early Saturday morning. Intermittent 1"+/hour rates will be possible, with ratios likely to exceed 25:1. By Saturday morning, guidance spread increases with the handling of the core of the main upper low and orientation of the arctic high to our west, which in turn results in a slightly different orientation to the boundary-layer flow. GFS/CMC shift the flow NEly driving the main lake band back across NE Illinois/Chicagoland, while the rest of the global output more-or-less maintains a steady band placement across the Lake-Porter-La Porte county vicinity. This will be one of the main items to resolve over the coming forecast shifts. If LES becomes quasistationary, some double- digit snow totals would be in play.

Thermodynamic parameters will gradually become less supportive of intense LES through Saturday evening/overnight, with activity expected to gradually wane into Sunday morning.

Sunday night and beyond:

A very active upper pattern will continue next week with several additional chances for snow, the first arriving Sunday night - Monday, and another towards the middle/end of the week. By that time, guidance does suggest a general warming trend, although likely not enough to limit the wintry precip potential.

Carlaw

AVIATION /06Z TAFS THROUGH 12Z THURSDAY/

Issued at 1112 PM CST Tue Jan 27 2026

There are no aviation concerns through the TAF period. Winds will generally be west to northwest (280 to 300 deg) with occasional gusts to about 20kt during daylight hours. A few clouds based near 2500 ft may develop during the afternoon hours beneath scattered high clouds.

Borchardt

HYDROLOGY

Issued at 230 PM CST Fri Jan 23 2026

River gauges, reports from local officials, and reports from trained ice spotters continue to indicate a few ice jams across the area due to the significant increase in river ice over recent days. With continued cold weather into next week, conditions will remain favorable for river ice formation. While no widespread flooding is expected, localized ice jams may occur.

- NWS Chicago

LOT WATCHES/WARNINGS/ADVISORIES

IL...None. IN...None. LM...None.