textproduct: Chicago

KEY MESSAGES

- A fall storm system will move through the area through Friday bringing periods of rain showers and gusty northwesterly winds (the latter primarily on Friday).

- Frosty and near-freezing conditions favored away from Lake Michigan Friday night into Saturday and again Saturday night. - After the brief period of chilly weather, there will be a warming trend this weekend into early next week.

DISCUSSION

Issued at 305 PM CDT Thu Oct 15 2026

Through Friday night:

An expansive fall storm system centered over the mid- Mississippi Valley is lifting a warm front into Illinois and Indiana this afternoon. Deep atmospheric moisture is being drawn northward ahead of this system, yielding precipitable water (PWAT) values up to 1.75-2.0 inches (near record highs) for this time of year. This moisture will support narrow corridors of torrential rain late this afternoon and evening, particularly where convective elements train east-northeastward. While not everyone will experience torrential downpours, scattered to numerous rain showers are expected area-wide through early this evening. The threat for isolated thunderstorms remains low, but with the primary MUCAPE reservoir lifting across east-central Illinois into northern Indiana by early evening, any thunderstorm activity should remain primarily south of I-80. A strong storm or two cannot be ruled out through early evening, but that threat will be confined even farther south (generally south of a line from Gibson City, IL to Remington, IN), aligning well with the SPC Day 1 outlook.

The primary low-pressure system will track directly over northern Illinois early this evening, then deepen as it shifts northeast into lower Michigan tonight. Expect a brief period of lower ceilings and reduced visibility for a few hours as the low moves overhead. Conditions will improve rapidly later tonight following a sharp cold front. In its wake, northwest winds will become gusty (3035 mph) overnight, while strong cold air advection drops temperatures into the 40s to low 50s by early Friday morning. Breezy conditions (gusts of 35 to 40 mph) will persist through Friday due to strong pressure rises behind the departing low and ahead of a 1035+ mb surface high building in from the Plains. Even stronger northerly winds (~45 mph) are projected over southern Lake Michigan Friday afternoon and evening. Onshore flow will push some of these higher winds inland across far northern Porter County, Indiana, where a Wind Advisory is in effect from 1:00 PM to 10:00 PM CDT Friday.

Much cooler weather is expected Friday, with temperatures hovering in the low to mid-50s as the coldest airmass of the season arrives under a sharp trough moving across the western Great Lakes. Steepening low- to mid-level lapse rates will support a diurnal increase in scattered showers, especially Friday afternoon. While showers will remain hit-or-miss across interior northern Illinois, favorable lake-effect conditions will bring widespread showers and potentially isolated thunderstorms to southern Lake Michigan and adjacent shorelines Friday afternoon and evening, supported by lake-induced equilibrium heights exceeding 20,000 feet.

KJB

Friday night into Saturday:

The progressive pattern continues as the surface low departs and surface high pressure moves toward the middle Mississippi River Valley. Lingering lake-effect clouds and persistent cold air advection will keep winds slower to diminish Friday night across eastern Illinois and northwest Indiana. Farther west, clearing skies and light winds near the approaching surface ridge axis across northwest Illinois will set the stage for a potential freeze. In collaboration with WFOs MKX and DVN, a Freeze Watch was issued for Boone, Winnebago, and Ogle counties, where lows near 30F are expected early Saturday morning. Widespread frost is also likely to extend farther east and south, where Frost Advisories may eventually be needed, potentially reaching as far east as some of the western Chicago suburbs.

Saturday will bring full sunshine and weak winds as high pressure moves directly overhead, though the chilly start will cap afternoon highs in the 50s. Saturday night will be chilly again, with the coldest temperatures favored east of I-55 beneath the lightest winds near the high's center, likely requiring additional frost headlines.

Southwesterly return flow will establish by Sunday on the backside of the departing high, initiating a warming trend into early next week.

KJB/Borchardt

AVIATION /00Z TAFS THROUGH 06Z SATURDAY/

Issued at 637 PM CDT Thu Oct 15 2026

Key aviation forecast messages:

- Rain/showers with periods of IFR/patchy LIFR conditions this evening.

- Improvement to MVFR ceilings late this evening, and prevailing through much of Friday.

- Surface winds strengthening and becoming west-northwest late this evening and northwest Friday. Gusts 25-30 kt overnight and around 30 kts (possibly slightly higher Friday afternoon/early evening).

A strong autumn storm system was moving across the region early this evening, with surface low pressure just west/northwest of Chicago at issuance time. This low will continue to track northeast across lower Michigan overnight. A combination of light rain/drizzle and scattered heavier showers will linger in the vicinity of the low early this evening, eventually ending by midnight or so as the system pulls away. Periods of IFR CIG/VIS early should improve to MVFR CIGs overnight as the rain ends and drier west-northwest winds develop behind the low. These winds will strengthen and become blustery late this evening with gusts 25-30 kit overnight and around 30 kts on Friday. A few gusts over 30 kts are possible Friday afternoon/early evening.

While the surface low center departs tonight, the main upper- level trough axis will move across the area Friday. This looks to support redevelopment of scattered showers by midday, especially over Lake Michigan and into northern IN. Have maintained the VCSH mention for the IL TAF sites Friday afternoon, but did add PROB30 mentions for the Chicago area sites where shower coverage will likely be a bit higher and could produce brief lower-MVFR conditions. For KGYY, went prevailing SHRA, with a tempo for lower conditions in the afternoon with higher coverage expected near/downwind of the lake.

Showers will move east of the area Friday evening, with clearing skies and decreasing winds overnight.

Ratzer

MARINE

Issued at 305 PM CDT Thu Oct 15 2026

The forecast remains on track with the fall storm system set to impact the area through Friday.

Over the next 18 to 36 hours, the first fall storm system of the season will move through the Great Lakes. As the low lifts into northern IL early this evening, breezy east-southeasterly winds will create waves generally of 3 to 5 feet along the Illinois shoreline. Westerly winds will increase in magnitude later this evening on the immediate backside of the system and behind a cold front with frequent gusts of 25 to 30 kt and a few gale force gusts to 35 kt.

Friday afternoon, a secondary cold front will surge down Lake Michigan leading to a change in wind direction to northerly and rapid increase in wind speeds. Sustained winds of 30 to 35 kt with frequent gusts to 40 kt are expected, with locally higher gusts in excess of 45 kt possible on the west side of lake effect rain showers. The strongest wind gusts are expected Friday afternoon and evening before gradually tapering after midnight early Saturday morning. A Gale Warning is in effect from 1 PM to 1 AM Friday night into Saturday morning.

Finally, strong lake induced instability may support waterspouts near lake effect showers Friday afternoon and evening.

Borchardt/KJB

LOT WATCHES/WARNINGS/ADVISORIES

IL...Freeze Watch from late Friday night through Saturday morning for ILZ003-ILZ004-ILZ008.

IN...Wind Advisory from 1 PM to 10 PM CDT Friday for INZ002.

LM...Small Craft Advisory until 1 PM CDT Friday for the IL and IN nearshore waters.

Gale Warning from 1 PM Friday to 1 AM CDT Saturday for the IL and IN nearshore waters.