textproduct: Chicago

KEY MESSAGES

- A decaying cluster of thunderstorms may affect portions of northern Illinois this afternoon.

- Dangerous heat is expected across the area this week.

- Afternoon heat index values forecasted over 100 degrees each day next week, with little to no relief at the lakeshore and at night.

DISCUSSION

Issued at 252 AM CDT Sun Jun 28 2026

A weak 925-850 hPa warm front stretching WNW to ESE from around Lee County to just south of the Kankakee River Valley in northwest Indiana continues to provide a focus for spotty light showers early this morning. Meanwhile, a developing surface theta-e gradient associated with notably higher moisture just southwest of the CWA will lift northward through the day. Weak low-level isentropic ascent associated with these features should allow for an axis of increased cloud cover and perhaps a few light showers/sprinkles late this morning into this afternoon.

Severe convection tracking along and north of the SD/NE line early this morning will remain the primary focus for the forecast area this afternoon. This cluster of convection has remained fairly strong through much of the night, with an associated MCV now evident. Renewed convective cells over northeast Nebraska ahead of this complex indicate that the environment is not entirely capped farther south. The convection will begin to encounter substantial low to mid-level warming with an incoming EML lifting northeast across the Mississippi River Valley late this morning into the afternoon. However, the combination of steepening mid-level lapse rates, low-level moisture advection, and local weakening of capping near any mature MCV does warrant some concern for convection to persist across the Mississippi River and into the CWA. With modestly strong mid-level flow across southern Wisconsin and a strong instability gradient bifurcating the CWA, there is a sweet spot where convection, possibly severe, could event remain well formed and track well into (or even across) the CWA. Have introduced chance PoPs for showers and storms west of the Chicago metro, and slight chance PoPs elsewhere, for the afternoon. Additionally, some CAMs indicate that any decaying MCV could produce a wake low and resultant stronger southeast winds over portions of the area this afternoon.

Regardless of potential convection, heat and humidity will begin to build today with heat index values rising to around 90 inland during the afternoon and persistent onshore flow maintaining cooler conditions along the shore.

The well-advertised period of heat this week remains on track, particularly Monday through Wednesday. An anomalously strong mid-level ridge (heights very near 600dam) drifting along the Ohio River Valley and H850/H700 temps of over 25C/12C Monday through Wednesday will yield unseasonably warm conditions with daytime highs well into the 90s while building an incredibly strong cap over the local area. Combined with persistent southwest low-level flow, there looks to be no relief from lake breezes or upstream convective outflow boundaries during this time. Though capping will be quite strong for Monday and Tuesday (and possibly into Wednesday), persistent strong southwesterly gradient flow will likely allow for modest mixing into incredibly dry air across the base of the EML. Though this may lower dew points by just a couple degrees (e.g. mid 70s vs upper 70s), the difference may be enough to keep heat index values in check.

Have maintained the Extreme Heat Watch for Cook County from Monday through Wednesday, and issued a Heat Advisory for the remainder of the forecast area for the same period. Expectations are that Cook County will need to be upgraded to an Extreme Heat Warning (criteria of heat index values of 100 or higher for three days). Outside of Cook County, this looks like a solid long duration advisory event with afternoon heat index values hovering around 105 degrees (Heat Advisory criteria). A case can be made that the longevity of the heat may require an eventual upgrade to an Extreme Heat Warning for areas outside of Cook County if convection remains north of the area on Thursday and Friday.

Regarding the potential for convection this week, the strong cap early in the week should deflect convection well to the northwest (Minnesota through Upper Michigan) through at least Wednesday. An expected flattening of the ridge and eventual SW to NE orientation of the ridge axis toward the western Great Lakes would favor increasing chances for convective episodes to affect some or all of the forecast area as early as Wednesday night. And with guidance often too slow with breaking down the periphery of summertime ridges in the Great Lakes, direct or indirect (outflow) impacts from convection lowers confidence on headlines beyond Wednesday. The overall message is for heat to potentially persist into the Independence Day holiday, but for increasing chance for convection with each day.

Kluber

AVIATION /12Z TAFS THROUGH 18Z MONDAY/

Issued at 637 AM CDT Sun Jun 28 2026

- Period of MVFR ceilings likely through midday.

- Chance (~30%) of a period of SHRA/TSRA this afternoon with highest chances TS at KRFD after ~19Z.

Deep surface low pressure was located over western NE early this morning. with a northward-lifting warm front extending east and southeast from the low in to the lower MO/OH Valleys. A moist low-level air mass is in place north of the warm front into northern IL/IN, which will likely support development of an MVFR deck early this morning (along with northward advection of an existent MVFR deck currently along/south of a KSQI-KJOT-KOXI line). Once developed, MVFR may persist through midday before eventually rising to VFR.

Farther to the northwest, a well-developed thunderstorm complex was evident in radar imagery across southwest MN/northwest IA this morning. High-res CAM guidance continues to indicate the southern portion of this complex may spread into northern IL this afternoon, at least in a weakening fashion, on the nose of a low-level jet and region of steep mid-level lapse rates. Models continue to indicate the greatest potential (~30%) for TSRA at KRFD and have maintained the PROB30 there after 19Z. Indications of weakening and a decreasing footprint remain for areas farther east/southeast into the Chicago terminals, though support remains for at least a PROB30 mention for SHRA after 20Z, and potentially may need TS included if trends dictate with later updates.

As for winds, light variable/northeast winds should turn more east-southeast as we mix this morning, and look to gradually increase this afternoon as the gradient tightens in response to the aforementioned surface low lifting slowly northeast toward the upper Midwest. There has also been a signal in some CAM guidance for a weakening wake-low induced wind field associated with the storm complex to spread into southern WI/northern IL later this afternoon. Have thus bumped up winds (including in the PROB30 groups) for potentially breezier conditions. Winds should settle into a southeast direction later today/tonight and may become breezy again overnight in response to a developing 30 kt low level jet, while veering south with the approach of the surface warm front. Winds should then shift southwest and remain somewhat breezy after daybreak Monday as the warm front lifts north of the terminals.

Ratzer

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 Monday 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 Watch from Monday afternoon through Wednesday evening for ILZ103-ILZ104-ILZ105.

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

LM...None.