textproduct: Chicago

KEY MESSAGES

- Patchy fog possible (20% chance) tonight, mainly south of the Kankakee River Valley.

- 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.

UPDATE

Issued at 918 PM CDT Sat Jun 27 2026

A baggy area of lower pressure continues to slowly meander across central IL and west-central IN this evening with most of northern IL and northwest IN residing under the influence of the broad surface high over the northern Great Lakes. While this high will make for generally tranquil weather conditions tonight, a couple of isolated showers may brush portions of Ford, Iroquois, and Benton Counties through 2 AM CDT or so as the aforementioned low and its associated warm front drift by. Otherwise, expect partly to mostly cloudy skies to persist through the night which will keep temperatures on the mild side in the mid to upper 60s (warmest south of I-80). Despite the cloud cover, the area of higher moisture near the warm front may also support some patchy fog development after midnight into early Sunday morning. Since confidence in low-level saturation being reached remains on the low side, especially with northward extent, have opted to just introduce some 20% chances for fog south of the Kankakee River for now.

Heading into Sunday, any fog that develops overnight will erode by 8 AM CDT Sunday morning leaving us with partly cloud skies for the rest of the day. While mostly dry conditions are currently being forecast for Sunday, there remains a non-zero (~10%) chance for the cluster of storms in the Dakotas to drift into portions of northwest and northern IL Sunday afternoon. Recent 00z CAM guidance continues to vary on whether or not the cluster will get far enough east to remain in WI before the steering flow turns northwesterly (pushing storms towards the southeast) early Sunday afternoon which continues to make this part of the forecast low confidence. If the storm cluster does not dissipate and is able to take advantage of the northwest flow, the environment over northern IL will be supportive of showers and thunderstorms with steep mid-level lapse rates (around 7-8 C/km) and decent moisture courtesy of lower 70s dewpoints. The caveat however, will be that the warming mid- levels could provide a cap to surface based thunderstorms which in turn could limit the coverage and southward progress of the storm cluster especially if it arrives in a decaying state. Though, if the cluster is stronger the 30-40 kts of shear and aforementioned lapse rates could result in a period of strong to severe storms for portions of the area.

At this time, we still think the more likely scenario for Sunday is that the storm cluster either stays in WI or dissipates prior to reaching northern IL keeping dry conditions locally. Therefore, our current forecast remains dry with just some "silent" (10-14%) POPs across our northern counties to tease the shower/storm potential. Obviously this is something we will continue to monitor overnight, but anyone with outdoor plans (especially those west of the Fox Valley) should keep a close eye on the forecast just in case this changes.

Regardless of what happens with the storms, Sunday will be a warm and muggy day with highs in the mid to upper 80s inland with mid-70s near the lake. Winds will also be breezy out of the east-southeast with gusts around 20 mph.

All updated forecast products will be published shortly.

Yack

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 /00Z TAFS THROUGH 06Z MONDAY/

Issued at 633 PM CDT Sat Jun 27 2026

Forecast concerns for the terminals...

- Chance for patchy 5-6 SM fog at GYY with better fog coverage likely south of the Kankakee River Valley.

- Breezy east-southeast winds Sunday afternoon with 17-20 kt gusts.

Generally quiet weather is expected for the TAF period as high pressure will begin to build into the area on Sunday. In the meantime, some lingering mid-level moisture will maintain some 4000-5000 ft BKN to OVC skies through tonight before clouds scatter out Sunday afternoon. While there is a chance for some MVFR clouds to develop and lift out of central IL tonight, confidence on their coverage with northward extent remains low. Thus have opted to tease this potential with a SCT030 mention at in the TAFs. In addition to the clouds, there remains a chance for some patchy fog to develop tonight as the humid air mass to our south progresses northward. With the expected cloud cover limiting radiational cooling suspect that the better fog coverage will remain south of the Kankakee River Valley tonight. However, have opted to maintain a 6 SM BR mention at GYY where cooler air off the lake may aid in some patchy fog development late tonight into early Sunday morning.

Otherwise, expect light east-northeast winds tonight to increase and become east-southeast on Sunday. A few 17-20 kt gusts are also expected for Sunday afternoon before winds taper to around 7-9 kts Sunday evening.

Lastly, any earlier concerns for some spotty showers to develop along the elevated instability axis late tonight into early Sunday morning continue to wane. Therefore, we expect dry conditions to prevail through Sunday. However, there is a new concern for the cluster of storms that is developing in the Dakotas to drift into portions of northwest and northern IL Sunday afternoon if it holds together. While the steering flow should take this cluster into central WI, there is a play that if the cluster is still in southern MN Sunday morning that the veering steering flow may allow some of these showers/storms to drift into northwest and northern IL. Right now confidence is very low (<10% chance) that any showers/storms even make it into northwest IL, but this will be something to keep a lazy eye on just in case.

Yack

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.