textproduct: Chicago

KEY MESSAGES

- Dangerous heat is expected across the area this week with afternoon heat index values forecasted over 100 degrees. Little to no relief is expected at the lakeshore or at night.

- A return to a more active pattern and thunderstorm chances is likely by the weekend.

DISCUSSION

Issued at 159 AM CDT Mon Jun 29 2026

No changes made to ongoing heat headlines, with an Extreme Heat Warning for Cook County and a Heat Advisory for all other counties running from noon today through 10 PM CDT Wednesday evening.

Evening upper air analysis depicts an unseasonably deep long- wave upper trough and embedded closed low over the western CONUS, and downstream upper ridging from the lower Mississippi Valley into the Midwest. At the surface, deep low pressure around 996 mb was over northeast Colorado, with a warm front arcing into northwest IA and then southeast along roughly a KMLI-KCMI-KHUF line across central Illinois and southwest Indiana as of midnight. This warm front is expected to lift north across the WFO LOT forecast area this morning, as the low develops north-northeastward across the Plains in response to a strong mid-level short wave rounding the base of the aforementioned upper low in the west.

Meanwhile, the upper ridge is progged to build north-northeast into the Ohio Valley today through Wednesday, strengthening to nearly 6000 meters. Model 850 mb temperatures of +20/22C today and +23/25C across the forecast area Tuesday and Wednesday will support unseasonably warm surface temperatures in the lower and middle 90s (perhaps a few upper 90s in spots), as well as relatively strong capping with 700 mb temperatures +12C or higher. Despite the strength of this capping inversion, breezy southwest low-level flow and dry air aloft is likely to aid in mixing out dew points during the warmest parts of each day, with guidance generally indicating mid-70s afternoon Td values (slightly lower across the urbanized Chicago metro area). Forecast peak heat indices therefore remain around 105 or just a bit higher for the most part.

With the relatively warm mid-level temperatures and associated capping, the main axis of thunderstorm potential today through Wednesday is expected to remain northwest of the forecast area across parts of IA/MN and WI. Some guidance does indicate that convective complexes will occur to our northwest both tonight and Tuesday night, which could push decaying convection, or more likely, associated outflow boundaries into northern IL during the early parts of both Tuesday and Wednesday. While these outflows could potentially impact heat indices and would need to be monitored, the breezy southwest flow across the region would most likely mix out the effects on surface temps/dew points during the day.

By Thursday and Friday, the center of the upper ridge is forecast to shift slightly southeast across Kentucky and Tennessee, flattening across the upper Midwest and Great Lakes regions as the western CONUS upper low lifts out across Ontario. Cooler mid-level temps result in weaker capping across the upper Midwest, and increase the likelihood of thunderstorm complexes (or their remains) working into the forecast area at times. While still warm and muggy even into next weekend, peak daytime heat indices are forecast more in the 90s to around 100 degree range even without accounting for direct impacts of storms or convective outflows. Thus the timing of current heat headlines, which run through Wednesday evening, continues to look very reasonable. In fact, over the weekend, current guidance and ensembles eventually flatten/break down the ridge over the southeastern U.S., and indicate a return to a potentially more active (storm-wise), zonal jet pattern.

Ratzer

AVIATION /06Z TAFS THROUGH 12Z TUESDAY/

Issued at 1243 AM CDT Mon Jun 29 2026

An axis of low-level moisture southwest of the terminals early this morning will lift northeast over the next several hours. Periods of BKN MVFR ceilings are likely during this time. Otherwise, VFR conditions are expected through tonight.

SE/SSE winds up to around 10 knots early this morning will gradually veer SSW through 12Z. Winds will then remain SSW/SW through the tonight, with gusts to 20 knots in the morning and around 25 knots during the afternoon. Though surface winds will diminish after sunset, a 40 to 45 knot low-level jet late this evening and overnight will either produce sporadic surface gusts to around 20 knots or lead to LLWS. Expectation is that some gusts will persist at ORD/MDW due to weaker near surface stability over the urban heat island. However, have included LLWS at RFD where the jet will be slightly stronger by late evening. Will likely need to add this to DPA/GYY for the next TAF cycle.

Kluber

CLIMATE

Issued at 252 AM CDT Sun Jun 28 2026

.Daily Records for the Upcoming Week........

Chicago (KORD) Record High Record Warm Minimum June 29 97 (1954) 80 (1931) June 30 99 (1953) 80 (2018) July 1 103 (1956) 78 (1931) July 2 99 (1970) 79 (1911) July 3 102 (1949) 78 (1911) July 4 102 (2012) 80 (1921)

Rockford (KRFD) Record High Record Warm Minimum June 29 103 (1931) 75 (1931) June 30 102 (1931) 76 (2018) July 1 101 (1931) 76 (1970) July 2 99 (1910) 74 (1970) July 3 99 (1925) 75 (2012) July 4 102 (2012) 77 (1977)

NWS Chicago

LOT WATCHES/WARNINGS/ADVISORIES

IL...Heat Advisory from noon today to 10 PM CDT Wednesday for ILZ003-ILZ004-ILZ005-ILZ006-ILZ008-ILZ010-ILZ011-ILZ012- ILZ013-ILZ019-ILZ020-ILZ021-ILZ023-ILZ032-ILZ033-ILZ039- ILZ106-ILZ107-ILZ108.

Extreme Heat Warning from noon today to 10 PM CDT Wednesday for ILZ103-ILZ104-ILZ105.

IN...Heat Advisory from noon CDT /1 PM EDT/ today to 10 PM CDT /11 PM EDT/ Wednesday for INZ001-INZ002-INZ010-INZ011-INZ019.

LM...Small Craft Advisory from noon today to 10 PM CDT this evening for the IL nearshore waters.