textproduct: Chicago

KEY MESSAGES

- Scattered showers and thunderstorms are expected late this evening and overnight, especially south of Interstate 88. The strongest storms may be capable of hail. - Additional showers and a few thunderstorms will move through the region on Thursday.

- Friday will feature blustery northwest winds and highs in the 50s. Areas of frost and locally near-freezing conditions are favored away from Lake Michigan Friday night.

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

UPDATE

Issued at 753 PM CDT Tue Oct 13 2026

No significant changes to the going forecast this evening, with scattered showers and thunderstorms expected to develop late this evening and increase in coverage across the forecast area into the overnight hours.

Latest surface analysis places a slow-moving cold front roughly south of KBMI but just north of KTIP/KLAF (Rantoul, IL and Lafayette, IN) across our far southeastern cwa. 00Z ILX sounding and latest SPC/RAP mesoanalysis indicates in excess of 100 J/kg of MLCIN along the boundary, with weaker capping farther west/southwest along the frontal boundary into far western IL and MO. This supports lack of near-surface based convective development along the front as depicted by most CAM guidance. Farther northwest however, the DVN 00Z RAOB depicts steep mid- level lapse rates above the stable near- surface layer. Elevated thunderstorms were already occurring over parts of southwest/south-central WI, on the nose of a ~60kt mid- level speed max/weak vort lobe. This activity is expected to continue to translate east-northeastward across southern WI over the next few hours in association with this disturbance, though the southwestern periphery of this activity will be close to the IL/WI border. However, more widespread shower and thunderstorm development is expected across our forecast area from mid-late evening into the overnight hours, as additional sheared mid- level energy propagates northeast from eastern KS/northern MO and enhances isentropic ascent over the low-level frontal zone. Eventual translation of the divergent right entrance region of a 110 kt upper level jet streak across the region later tonight should also increase overall forcing for ascent. Some of the resulting storms may be strong to severe, with the increasing mid-level flow resulting in effective shear in the 40-50 kt range overnight. The main severe threats appear to be large hail, though isolated gusty surface winds would also be possible with the strongest storms. Precipitable water values near/slightly above 1.75 inches (250% of normal for mid-October) will also support heavy downpours and local rainfall amounts in excess of a half inch.

Going forecast accounts for this pretty well, and other than adjusting early evening pops based on observed radar trends, have not made significant changes. Once showers and storms set up, high-res guidance appears to focus on the I-88 and I- 80 corridors for the highest pops/coverage before eventually shifting south into Wednesday morning.

Ratzer

DISCUSSION

Issued at 315 PM CDT Tue Oct 13 2026

Today was yet another unseasonably warm mid-October day across Chicagoland. The weak cold front that moved into the area earlier this morning has since stalled out south of I-80 which has resulted in a notable temperature gradient from northwest (mid 70s) to southeast (upper 80s). A couple of areas of altocumulus (one overhead and another crossing the Mississippi River) associated with narrow pockets of mid-level moisture have managed to produce isolated showers/sprinkles. Think this activity will remain fairly isolated through the early evening hours but can't rule out a few showers or a rumble of thunder.

Focus continues to be on the potential for scattered showers and thunderstorms later this evening through the overnight hours. Model guidance has remained varied on the timing and placement of the greatest coverage of showers, likely owing to differences in the handling of the timing and placement of subtle vort lobes and areas of drier air between plumes of increasing moisture. This is especially evident in regional satellite and radar imagery showing large gaps between ongoing convection and areas of ACCAS. Nevertheless, the presence of sufficient synoptic forcing within the right entrance of the upper jet and moist air lifting atop a stalled surface boundary support an eventual increase in shower and storm coverage overnight, and potentially developing as early as 8-9 PM if some of the earlier hi-res guidance verifies. Overall weaker mid-level lapse rates should limit storm strength overall even in the presence of favorable shear for hail. Thus, will continue to message the potential for small hail with the strongest storms with the severe potential remaining mainly southwest of the area.

The threat of lightning quickly wanes toward daybreak Wednesday with shower coverage beginning to focus south of the area. However, residual low to mid-level moisture in the presence of weak ascent may support lingering light rain or drizzle through part of the morning. Expanding high pressure across the western Great Lakes will eventually shift this south of the area toward midday, with dry conditions forecast through the remainder of the afternoon and evening hours. Persistent north to northeasterly winds will bring temperatures back to more seasonable levels with highs in the 60s for most of the area and lower 70s well south of I-80.

This brief break in precipitation will be fairly short lived as energy ejecting off the Rockies will phase with a wave diving southeast toward the Upper Midwest late Wednesday night into Thursday. An associated developing low pressure system will move across the area bringing widespread showers and a chance of thunderstorms on Thursday.

Petr

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 /06Z TAFS THROUGH 12Z THURSDAY/

Issued at 1249 AM CDT Wed Oct 14 2026

Forecast concerns for the terminals...

- Scattered showers and thunderstorms will linger at the Chicago area terminals and points south through 10-11z.

- MVFR ceilings will develop late tonight and persist through at least Wednesday evening, especially at the Chicago area terminals.

- Non-zero chance (15-20%) for a period of light rain/drizzle Wednesday morning at the Chicago terminals which would result in IFR ceilings and/or visibilities.

Scattered showers and thunderstorms continue to fester along a cold front stretching from southern MI to central MO. Locally, the favored axis of showers/storms is expected to keep shifting southward through the night before exiting the area between 10-11z. In the meantime, expect some showers and occasional lightning to linger in and around the Chicago area terminals through 08z with generally dry conditions expected at RFD.

In the wake of the rain, ceilings should gradually lower to MVFR with bases generally around 2000-2500 ft where they are forecast to remain through the rest of the TAF period. That said, there is a chance (15-20%) for a period of light rain and drizzle to develop at the Chicago area terminals and points south Wednesday morning as low-level moisture pools beneath a building inversion. Given that coverage of drizzle is still uncertain, have opted to introduce 3-4 hour PROB30s in the Chicago terminals' TAFs to highlight this potential. If drizzle does materialize then IFR ceilings and/or visibilities would also accompany the drizzle.

As for winds, expect wind directions to become more northeasterly tonight as rain ends with speeds around 5 kts before increasing to around 10 kts Wednesday morning. These northeast winds will persist through Wednesday evening before directions turn more east-southeast after midnight Wednesday night.

Yack

LOT WATCHES/WARNINGS/ADVISORIES

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