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 /06Z TAFS THROUGH 12Z SATURDAY/

Issued at 1226 PM CDT Fri Oct 16 2026

Key Aviation Messages for the current TAF period:

- Prevailing MVFR is expected through much of today.

- Strong northwest wind gusts 25 to 30 knots overnight, strengthening in excess of 30 knots this afternoon.

- Widely scattered showers are expected this afternoon, with best chances around KGYY and northwest Indiana.

- Wind gusts diminish after 00Z with improving/clearing sky conditions.

With surface low pressure moving east of Chicago, cigs have lifted slightly to MVFR levels. These MVFR cigs are expected to prevail through much of the day, though slowly lifting and breaking up closer to 00Z this evening.

Isolated showers still fester over the region, but as the low continues to exit to the east, drier conditions are expected through the overnight. Most impactful will be the strengthening northwest wind gusts through the overnight. Gusts are already being observed between 20 and 25 knots, and that is expected to increase to 25 to 30 knots through daybreak. As deeper mixing occurs during the day, gusts in excess of 30 knots are expected. Isolated gusts between 35 to 40 knots are possible this afternoon, though the better chances remain just over 30 knots.

Redeveloping showers in the wake of the surface low is expected this afternoon. There is lower confidence in coverage, though expected to be widely scattered over much of the area, with best chances in northwest Indiana as Lake Michigan will help enhance shower development this afternoon. No changes were made to the PROB30/TEMPO groups, except adjusting for timing expectations.

The pressure gradient will gradually weaken after 00Z, winds will turn more northerly and cloud cover will begin to clear. Wind gusts will diminish through the evening with a chance that they end all together after midnight. For now, gusts were maintained at ORD/MDW through 12Z Saturday given the lower confidence on exact timing for gusts ending, but will be re- evaluated at later TAF packages.

DK

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.