textproduct: Chicago

KEY MESSAGES

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

- Frosty and near-freezing conditions are 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 308 AM CDT Thu Oct 15 2026

Though Tonight:

Per recent GOES-19 water vapor imagery, an upper-level trough is lifting northeastward across the Rocky Mountains toward the central Plains ahead of a vigorous northern-stream trough digging southeastward into the Dakotas. Pressure falls on the order of 1 to 2 mb/hr have commenced in central Oklahoma (likely augmented by convection), which will spread northeastward with time as the lead upper-level trough lifts atop a southwest- to northeastward-oriented low-level baroclinic zone stretching into the Great Lakes. Generally speaking, the forecast for the next 24 to 48 hours remains on track with the expectation for the first fall storm of the season to move through the Great Lakes region. As this occurs, a myriad of weather conditions will occur in our local area.

After daybreak, showers and areas of drizzle (already developing at press time) will gradually expand across northern Illinois and northwestern Indiana. As our area will be somewhat removed from the strongest forcing (warm conveyer belt focused just south of our area, and increasing frontogenetical forcing focused to our north), am not expecting hefty precipitation totals through the first half of the day. Coverage of showers and drizzle should maximize this afternoon as the surface low moves directly over our area, though again coverage may be more scattered than widespread owing to somewhat disjointed forcing. Nevertheless, seasonably high PWATs (nearing 2 inches) will support torrential downpours where showers are most persistent this afternoon.

Am seeing quite a bit of variance in the exact track of the surface low in high-res guidance early this morning, so will hedge toward global guidance (which often does not suffer impacts of convective effects). Such a hedge favors the track of the low passing directly from LaSalle to Chicago this evening while deepening a mb or two per hour. With this in mind, will have to watch for a narrow window for moisture-driven surface- based instability to sneak along and east of I-55 this evening (5 to 9 PM) to support low-topped convection along or just ahead of the synoptic cold front. The overlap of strongly veering low-level wind profiles within the low-centroid and high moisture-driven convective layer does raise concern for low- topped supercell structures, though would have to think they would be harmless given limited opportunity for breaks in cloud cover to steepen low-level lapse rates (effectively limiting the opportunity for truly surface-based convection). In the off chance that the surface low moves even further north than expected AND there is partial sunshine in the narrow warm sector to steepen low-level lapse rates, cannot discount a threat for a brief tornado. At this point, we assess the chance of such an outcome to be very low (less than 5%), a sentiment that is shared by SPC (we did check in with them this morning). So, will continue to withhold mentioning a threat for severe weather today in the HWO and graphical products and allow for observations this afternoon guide adjustments in strategy, if needed.

In the immediate wake of the cold front this evening, initial pressure rises on the order of 2 to 3 mb/hr will drive a period of westerly winds gusting 30 to 35 mph. Robust cold air advection will send temperatures falling into the upper 40s to lower 50s overnight with wind chills by daybreak Friday solidly in the low to mid 40s. The overnight period is favored to be dry for most areas, though can envision a few showers continuing near and southeast of I-57 near and just behind the cold front.

Friday:

On Friday, the northern-stream trough will whip through the Great Lakes anchored by a seasonably cool 500mb temperatures near -25C. A low-level trowel-like structure within the cyclonic shear axis of the trough will support a diurnal flare of showers with peak coverage during the afternoon hours across the area. Pressure rises will actually be greatest during the afternoon (+10mb/6hr) as a 1040mb surface high builds into the plains. As a result, the expectation is for a period of strong north to northwesterly winds Friday afternoon, peaking in the 35 to 40 mph range. Locally higher gusts of 45 to 55 mph cannot be ruled out in the tallest showers. Extensive stratus and continued cold air advection will limit highs to the lower 50s, though would not be surprised if temperatures remain in the upper 40s as hinted by more aggressive deterministic guidance.

As the upper-level trough whips overhead, tumbling temperatures atop the warm waters of Lake Michigan will set the stage for the blossoming of lake effect rain showers. With inversion heights progged to climb to nearly 20kft and above the minus 20C level, cannot rule out a few lightning strikes and graupel (soft hail) in the deepest lake effect convective cores. The northerly wind direction will point the stream of lake effect showers into northwestern Indiana during the afternoon, though any mesolows may tend to push showers into Cook County, as well. Wind gusts along the immediate shoreline may also routinely poke above 45 mph (stronger near any mesolows). Will brief the day shift to consider hoisting a Wind Advisory for Lake and Porter counties in northwestern Indiana centered on the Friday afternoon period (typically like to wait until within 24 hours of an event to pull the trigger on a headline).

Taken altogether, Friday is shaping up to be a rather raw and chilly day out there. The tree canopy will likely lose a lot of colorful leaves.

Friday night into Saturday:

The pattern will remain fairly progressive as the surface low pulls away from our area and the surface high pressure system slides toward the middle Mississippi River Valley. Am not convinced winds will crash quickly Friday night owing to a continued pressure gradient in our local area ahead of the approaching surface high pressure system until literally daybreak. Even so, clearing skies (by way of tumbling inversion heights) and continued cold air advection will still push temperatures into the low to mid 30s overnight. The exception will be across northwestern Indiana where lake effect stratus will likely continue through the night. This period continues to be a focus for potential frost/freeze headlines primarily across portions of Illinois.

While Saturday will feature full sunshine and weak winds (as the surface high slides directly overhead), the very chilly start to the day will serve as a low starting point for temperatures leading to afternoon highs generally in the 50s. Saturday night will be chilly once again, though the coldest temperatures will favor areas east of I-55 where winds will be weakest along the backside of the surface high. Can envision a need for frost headlines again where temperatures are coldest.

Southwesterly return flow will become established by Sunday along the backside of the surface high pressure system marking the start of a warming trend into next week.

Borchardt

AVIATION /12Z TAFS THROUGH 18Z FRIDAY/

Issued at 627 AM CDT Thu Oct 15 2026

Forecast concerns for the terminals...

- Increasing rain chances this morning with showers expected to persist through this evening.

- Ceilings gradually lowering to IFR through the morning. A period of LIFR conditions is expected as the surface low passes overhead late this afternoon through early evening.

- Gusty northwest winds developing after midnight with peak gusts around 20-25 kts, but locally higher gusts are possible.

A storm system continues to develop in the southern Plains this morning and will be moving into northern IL and northwest IN later today. Ahead of the developing system, widespread showers have developed across IA and MO and these showers will spread into the terminals after 15z (13z at RFD) this morning and persist off and on through this evening. As rain begins, ceilings are expected to gradually lower to IFR around midday with visibilities generally remaining in the MVFR (3-5 mile) range. However, a period of LIFR conditions is expected as the surface low passes directly overhead and leads to the development of some drizzle later this afternoon into the early evening hours. Given that the LIFR conditions should be tied to the core of the surface low (as noted in upstream observations) have opted to handle these conditions with TEMPO groups in the TAFs centered around when the center of the low will be over each terminal. As the surface low departs, the systems cold front will swing through and should allow ceilings to slowly begin to improve though some IFR ceilings may linger until rain fully subsides after midnight. Once the rain tapers, ceilings will rise to MVFR and remain as such through the rest of the TAF period.

As for the winds, easterly winds around 10 kts will prevail through the morning before directions turn more south-southeast this afternoon beneath the surface low. In fact, winds may even become light and variable for a short time as the low passes directly over the terminals. Winds will then turn west-southwest behind the low before settling into a northwest direction behind the system's cold front. Speeds will also quickly increase behind the front with gusts peaking in the 20-25 kt range tonight into Friday morning. However, if deeper mixing is achieved there is a chance for some locally higher gusts around 30 kts.

Finally, an additional period of scattered showers is forecast for Friday as the upper trough dives through the Great Lakes. Right now it appears that most of these showers should hold off until after the end of the 30-hour TAF so have maintained dry conditions after midnight tonight. That said, a formal mention for these showers will likely be needed in future TAFs.

Yack

MARINE

Issued at 308 AM CDT Thu Oct 15 2026

Over the next 24 to 48 hours, the first fall storm system of the season will move through the Great Lakes. As the low lifts from near Kansas City to Chicago today, east-southeasterly winds will increase across southern Lake Michigan leading to waves generally of 3 to 5 feet along the Illinois shoreline. Westerly winds will increase in magnitude after sunset 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

LOT WATCHES/WARNINGS/ADVISORIES

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