textproduct: Chicago

KEY MESSAGES

- Today will be another warm day with highs ranging from the upper 70s (north) to mid 80s (south). A sprinkle or two cannot be ruled out mainly this morning.

- Showers and thunderstorms will move through the region this evening and tonight especially near and south of Interstate 88 No severe weather is expected at this time.

- Additional showers and a few thunderstorms will move through the region on Thursday tied to a fall storm system.

- Friday will feature blustery northwest winds and highs in the 50s. Frosty and locally freezing conditions are favored away from Lake Michigan Friday night into Saturday.

- After the brief period of chilly weather, there will be a warming trend this weekend into early next week.

DISCUSSION

Issued at 208 AM CDT Tue Oct 13 2026

Though Tonight:

A recent hand surface analysis revealed a 1003mb surface low pressure system centered over southern Wisconsin. A sharp cold front extends southwestward through central Iowa and into the central Plains toward a secondary surface low centered near the Colorado and Kansas border. In the warm sector of the Great Lakes cyclone, it is an unseasonably warm night with current temperatures at press time in the mid 70s (some 10 degrees warmer than typical high temperatures this time of year). A strong low-level jet (50-55kt at about 3000 feet AGL per the KLOT VWP) is maintaining a few southwesterly surface gusts of 15 to 20 mph early this morning, as well (the Chicago Crib is gusting to nearly 40 mph). Taking a step back, the large-scale pattern is being driven by a strong Pacific trough digging toward the four Corners Region, leading to quasi-zonal flow across the central and eastern United States. Over the next 72 hours, the local weather will be influenced by the trough as it evolves eastward with time.

Based on the current location and speed of the cold front, it should move across our local area just after daybreak. Low-level cold air advection doesn't look particularly noteworthy in our area behind the front, a consequence of a low-level pressure col developing between the departing low pressure system toward the northeastern US and the surface pressure falls in the Plains. In fact, the front may very well stall somewhere near US-24 by early afternoon, transitioning into more of a stationary front ahead of the trough moving through the southwestern United States. Nevertheless, today is still shaping up to be another unseasonably warm day with highs in the upper 70s near the Wisconsin state line, around 80 near the I-88 and I-80 corridors, and mid to locally upper 80s in closer proximity to the frontal boundary near US-24. Will have to watch for a sprinkle or two from mid-morning to early afternoon near the front as a plume of modestly steep mid-level lapse rates (around 7 K/km per the 00Z OAX RAOB) coincident with a pocket of mid- level moisture (area of ACCAS currently over eastern Nebraska and western Iowa) slides through northern Illinois. Given a residual pocket of dry low-level air across the area (notably below 700 mb, see the 00Z DVN RAOB), will keep the chance of measurable precipitation in the 5 to 10% range, again largely south of I-80.

This evening and tonight, the dominant Pacific upper-level trough will propagate toward the Four Corners region establishing southwesterly low- to mid-level flow from the southern Plains toward the Lower Great Lakes. The tightening low-level baroclinic zone across the middle Mississippi River Valley (partially forced by a building surface high in the northern Plains) beneath a parade of upper-level shortwaves will support the development of several clusters of showers and thunderstorms. The current expectation is for the most robust activity to remain tied to the highest-quality low-level moisture near and south of the front generally from central Kansas to central Illinois, through still do expect a healthy coverage of showers and storms as far north as the I-88 corridor from the evening through the overnight hours. Relatively high freezing levels (near 14,500 ft AGL) and modest/skinny CAPE profiles to limit updraft intensity (weak lapse-rates) should otherwise offset a favorable kinematic/shear profile for hail. And while the upper-level flow does look extremely unbalanced, any synoptic-scale gravity wave formation should tend to be offset well north of the instability axis likely mitigating a GWAC/damaging wind threat. As a result, current expectations are for the thunderstorms tonight to be fairly generic with the common threats of downpours, occasional lightning strikes, and pea to dime size hail. Coverage of activity should wane markedly by daybreak Wednesday.

Wednesday into Wednesday night:

On Wednesday, the northern Plains high pressure system will slide eastward beneath rising mid-level heights as the Pacific trough moves toward the central Rocky Mountains. An increase in east-northeasterly flow on the southern side of the surface high pressure system will shunt the low-level baroclinic zone southward. Trapped low-level moisture beneath a persistent inversion as well as a steady stream of high clouds from convection to our south will maintain cloudy skies throughout the day. In fact, would not be surprised to see some areas of drizzle particularly in the morning before dry air advection allows for the low-level moisture depth to shrink through the afternoon hours. Taken altogether, opted to undercut NBM temperatures for Wednesday and instead advertise highs in the low to mid 60s to perhaps around 70 near the US-24 corridor.

Wednesday night into Thursday night:

Wednesday night into Thursday, ensemble model guidance is in fair agreement that the trough in the central Plains will phase with a northern-stream trough diving southeastward out of Canada. Even with the usual variance in exactly how the systems phase, there remains a persistent signal for disorganized pressure falls in the Plains to merge into a coherent surface low which would be poised to rapidly deepen while lifting northeastward close to or directly over our area on a path toward southeastern Canada. Taken together, do expect periods of showers and a few thunderstorms focused on Thursday as the surface low moves through the area, which is reflected in NBM PoPs of 80 to 90% areawide. Did opt to again undercut NBM highs particularly with northward extent to be closer aligned to ensemble means, though will note there could be a fairly tight gradient locally as the warm sector attempts to lift along the I-57 corridor. Nevertheless, will advertise highs in the upper 50s near the Wisconsin state line to the mid 60s south of I-80. Finally, am not seeing any obvious warning signs for severe thunderstorms locally with the cyclone, but it's still a bit to early to say for sure given the aforementioned variance in phasing within the ensemble envelope.

Friday Onward:

As the surface low pulls away (and deepens toward 990 mb in southern Ontario), the arrival of a strong (nearly 1040 mb) surface high pressure system into the central Plains will set the stage for an impressive pressure gradient across the Great Lakes. Accordingly, do expect a windy day on Friday with early BUFKIT mixing profiles supporting frequent gusts of 30 to 40 mph. Such strong winds will facilitate aggressive low-level cold air advection in the wake of the cyclone with mean 850mb temperatures plummeting 10 to 15 degrees C in 24 hours (to near or below 0C). Taken together, Friday will feel every bit like October with highs in the 50s, blustery northwest winds, and showers of leaves (not water) flying through the air. With that said, a notable temperature difference between Lake Michigan water and the fall airmass will support lake effect rain showers mainly east of Gary.

There remains ensemble agreement that the surface high will slide directly over the Great Lakes Friday night into Saturday morning setting the stage for ideal radiational cooling and the coldest night of the autumn season so far. Assuming no major changes in forecast guidance, would expect areas of frost and even locally freezing temperatures across the region right in line with climatology (the first freezing temperatures away from Lake Michigan typically occur in the October 11 to 20 timeframe).

Thereafter, ensemble guidance advertises the surface high stalling over the Atlantic Seaboard and the northward-shift of the dominant upper-level jet into Canada supporting gradually rising temperatures into next week.

Borchardt

AVIATION /12Z TAFS THROUGH 18Z WEDNESDAY/

Issued at 622 AM CDT Tue Oct 13 2026

Forecast concerns for the terminals...

- Period of scattered showers and thunderstorms with a cold front late this evening through early Wednesday morning.

- MVFR ceilings to develop behind the front tonight and linger through at most of the day Wednesday.

Quiet weather conditions are expected through this afternoon as mid-level ridging remains in place overhead. Winds will become westerly by mid-morning with speeds remaining around 10 kts. Skies will start the day mostly clear, but become partly cloudy (SCT) with VFR clouds around 9000-1000 ft this afternoon.

Heading into this evening, a cold front will move into the area and bring with it a period of scattered showers and thunderstorms. While guidance has continued to vary on the exact timing of the showers/storms tonight, the general consensus continues to favor showers/storms arriving around 03/05z and then lingering through 08/10z Wednesday morning at RFD and the Chicago terminals respectively. The current forecast continues to be more broad in timing due to the aforementioned timing uncertainty, but it seems that showers/storms should last about 3-4 hours at any one location.

The cold front will also result in a northeasterly wind shift at the terminals tonight with directions expected to remain as such through the rest of the period. Speeds though will remain around 8-10 kts. Finally, ceilings are expected to lower to MVFR in the wake of the front and showers/storms tonight. Once the MVFR arrives it is forecast to persist through most of the day on Wednesday, but some slight improvement may occur Wednesday afternoon. There is also a low chance (10-15%) for some patchy drizzle to develop after daybreak near the Chicago terminals though confidence in coverage is too low to warrant a formal mention at this time.

Yack

LOT WATCHES/WARNINGS/ADVISORIES

IL...None. IN...None. LM...Small Craft Advisory until 10 AM CDT this morning for the IL nearshore waters.