textproduct: Chicago

KEY MESSAGES

- Dangerous heat is expected across the area next 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 114 PM CDT Sat Jun 27 2026

A weak area of low pressure, loosely analyzed in south central Illinois, is taking its sweet time moving east. A pseudo warm front along and just south of the southern portions of the forecast area is the main focus of showery precipitation this morning and afternoon. While the lingering moisture north of this area will continue to feature a wide swatch of cumulus clouds, most of the shower activity should remain confined to the US 24 corridor, and diminish as sunset approaches.

Recent forecasts have been monitoring an incoming EML plume out of the southern Plains tonight along an axis created by an amplifying ridge/trough. Models are consistently trending more hesitant with its northward progression due to lack of moisture and forcing. While the forecast was kept dry through the night, closer attention is being paid to what may transpire in South Dakota tonight. That previously mentioned EML plume will drive a fairly stout northwest to southeast oriented MUCAPE axis into Sunday. There has been a notable uptick in model activity developing a MCS in South Dakota tonight and have it moving eastward. While it is a small percentage of models, some have it riding the instability gradient southeastward into northwest Illinois. For now, confidence is low in the threat for showers or storms tomorrow afternoon. It would take a fairly strong MCS to hold itself together and fight the drier air mass expected over Illinois tomorrow, and even the models that show it entering Illinois have it arriving in a decaying state. But with plenty of instability and shear, PoPs were increased to at least have "silent" 10 to 14 percent chances in the northern half of the forecast area.

The main focus of the forecast will continue to be the first heat wave of the year. The aforementioned amplifying upper level pattern will continue today and tomorrow and feature a strong upper level low to the west, and an upper level high embedded in a ridge to the east. This pattern will advect around 25C temperatures at 850 mb into the area and turn the prevailing wind pattern to the southwest. This warmer, drier air mass is expected to remain for much of next week. Widespread temperatures in the 90s, and dew points well into the 70s will create hot and humid conditions. This will create afternoon heat index values around 105F each day starting on Monday. Little to no relief can be expected at the lakeshore with the offshore flow, and nighttime will not help either as overnight lows are only expected in the mid to upper 70s. It is becoming increasingly likely that a Heat Advisory will be needed across the forecast area, but for the time being, an Excessive Heat Watch was issued for Cook County from Monday afternoon through Wednesday evening.

So, if confidence in an Advisory is increasing, why was one not issued? Why did Cook County get a Watch? Is a Warning possible for the rest of the forecast area? First, Cook County has a different criteria for Heat watches and warnings, given the population size. It only takes heat indices of 105F for only two days or three days of 100F to justify a watch/warning. The rest of the forecast area needs heat indices of 110F for a watch/warning, and unless dew point temperatures over-perform, that appears unlikely. Lastly, there is uncertainty with the MCS tomorrow on its progression and if it lingers into Monday. Additionally, models are trending farther east with the upper level ridge and having it start to break down somewhere in the middle of the week. This further adds questions on the duration of the event, as a weaker and/or more eastern ridge would open the door to additional convection that could provide lower heat indices in the 90s and up to 100 (which is still hot, just sub- criteria). Meanwhile, a stronger ridge could make for a longer duration heat event that could last until the holiday weekend next week.

So the main message: the first heat wave of the year is expected next week. Take the hot temperatures into account when planning activities through the week.

DK

AVIATION /18Z TAFS THROUGH 00Z MONDAY/

Issued at 1216 PM CDT Sat Jun 27 2026

Northeasterly winds will prevail today and this evening, before shifting out of the southeast on Sunday, likely becoming sporadically gusty during the afternoon.

Largely VFR conditions are expected. Some MVFR cigs will be possible near GYY this afternoon, followed by some shallow VFR BR late tonight. Diurnal VFR cumulus will develop through the day on Sunday. At ORD/MDW: low stratus continues to hug the immediate lakeshore. There is a small chance this activity attempts to ooze inland this evening, but currently not seeing a significant signal for this and have maintained VFR conditions.

There is a very small (at this time less than 10 percent) chance for a complex of thunderstorms originating out of Minnesota to move into the region tomorrow afternoon. This does not appear to be the most likely scenario as the environment will become increasingly hostile to convection in the local area. Will continue to monitor trends, but chances are much too low to justify any precip mention in the extended ORD/MDW TAFs.

Carlaw

CLIMATE

Issued at 252 AM CDT Sat Jun 27 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...Extreme Heat Watch from Monday afternoon through Wednesday evening for ILZ103-ILZ104-ILZ105.

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