textproduct: Chicago

KEY MESSAGES

- Scattered showers and thunderstorms are expected today, mainly during the afternoon and evening hours. A few storms could become severe with damaging winds and hail the main threats, but an isolated tornado cannot be ruled out.

- Dangerous swim conditions are expected at Porter County Lake Michigan beaches Saturday, then all Illinois and Indiana Lake Michigan beaches late Saturday night and Sunday.

DISCUSSION

Issued at 245 AM CDT Fri Aug 21 2026

A shortwave trough is pivoting across the northern Great Lakes early this morning and has been leading to the development of some isolated to widely scattered showers and thunderstorms in WI, southeastern MN, and eastern IA. This festering of showers is expected to continue through daybreak with a few of these showers possibly making it into portions of far northern and northwest IL. However, with the dry air mass across northern IL and northwest IN and cloud bases with these showers generally around 10000 ft any rain should struggle to reach the ground locally aside from perhaps a stray sprinkle. Therefore, expect dry conditions to prevail through the rest of the night with just some passing clouds from time to time. Though, some shallow ground fog may continue to develop in north-central IL and northwest IN where temperature-dew point depressions are low (around 1-2 degrees) and result in some locally reduced visibilities around 3-5 miles through daybreak.

The aforementioned shortwave will continue to pivot eastward across northern MI this morning which will allow the trailing shortwave in northeast ND to pivot into the Upper Midwest and then eventually northern IL later today. Ahead of this secondary wave is a cold front that will also be drifting towards our area. As these features approach, winds will become more south- southwesterly and begin to advect in the more humid (higher dew point) air in MO into northern IL and northwest IN. Couple this warm-moist advection with daytime heating and cooling mid-levels with the approaching shortwave, conditions are expected to become unstable and favorable for showers and thunderstorms by midday. Therefore, expect scattered showers and thunderstorms to begin to take shape around 12-1 PM CDT in northwest IL and gradually drift south and east across the rest of the area through mid to late evening.

In addition to the favorable thermodynamics, a jet streak on the southern periphery of the shortwave looks to generate around 30-35 kts of effective shear overhead which should allow for some storms to become better organized and become capable of severe weather. While damaging winds (up to 60-65 mph) and hail (up to half dollar size) look to be the main threats, stout westerly 850-700 mb flow will allow for some low-level turning and may result in a threat for an isolated tornado or two. With the better overlap of the low-level shear and surface instability forecast to be across north and northwestern IL (namely areas west of the Fox Valley and north of I-80), the Storm Prediction Center has upgraded this part of our area to a slight risk (level 2 of 5) for severe weather today. Though, all of our area will have the potential to see some severe weather today and thus remains under at least a marginal (level 1 of 5) risk. Furthermore, locally heavy downpours will also be possible with the showers and storms today but with their progress nature and lower PWATs the threat for flooding is low at this time.

Heading into tonight, the shortwave and cold front should push through northern IL and northwest IN around midnight and should bring an end to the shower and thunderstorm activity. Though, a few lingering showers (15-20% chance) cannot be ruled out in northwest IN tonight. Otherwise, expect dry conditions tonight with overnight lows dipping into the low to mid-60s.

While mostly dry conditions should prevail through the day on Saturday, broad upper troughing is forecast to still be over the Great Lakes region with a couple of embedded disturbances expected to be pivoting through northern IL and northwest IN which may result in a few showers or storms Saturday afternoon. Forecast soundings on Saturday do show that the mid-levels should be drying out as the aforementioned disturbances pivot overhead which should limit the coverage of any showers or storms, but have opted to maintain a slight chance (15-20%) mention in the forecast just in case any isolated activity is able to materialize. The bigger weather concern on Saturday however, will be the increasing northwest winds (gusting around 20-30 mph) which will result in building waves and an increasing threat for rip currents at area beaches. Initially, the highest waves will be in Porter County, IN on Saturday but with the breezy winds expected to linger into Sunday that the larger waves and dangerous currents will spread into the rest of northwest IN and northeast IL by Saturday night. A Beach Hazard Statement will likely be needed for this time period so make sure to keep to check the forecast before heading to the beach this weekend and remember to stay dry when waves are high!

Aside from the breezy winds, dry conditions are expected for Sunday and the early part of next week as high pressure builds into the region. However, despite the sunshine and easing winds temperatures Sunday through Tuesday look to be on the seasonably cool side with daily highs in the 70s to around 80 and overnight lows in the 50s to lower 60s. The middle and later portions of next week do look to turn active once again as another shortwave trough and associated cold front are forecast to traverse through the region and bring more chances for showers and thunderstorms. While there is decent agreement within the latest guidance suite, there is a lot of differences in timing and coverage for our area at this range. So be sure to check back as we get closer.

Yack

AVIATION /06Z TAFS THROUGH 12Z SATURDAY/

Issued at 1228 AM CDT Fri Aug 21 2026

Key Aviation Messages for the current TAF period:

- Chance for scattered showers and thunderstorms tomorrow, mainly in the afternoon/early evening

VFR conditions are expected through mid morning. Winds will be light and variable overnight before becoming southwest around daybreak and increasing to around 10 knots. There are light returns on radar near KRFD at the time this discussion was sent. While these are showers aloft, there is a layer of dry air that would prevent much of the precip reaching the surface, except for a sprinkle or two.

The next wave moves in tomorrow bringing with it chances for showers and thunderstorms. The main piece of uncertainty remains with timing, but models are consistently showing two rounds of precipitation. A late morning round that has weak instability and dry air near the surface limiting chances, and a second round in the afternoon/evening. The second round is expected to feature better instability and wind shear that could produce some strong to locally severe storms in the area. Given the uncertainty in timing a PROB30 was kept in for the Chicago terminals with only adjusted start and end times, with a TEMPO remaining at KRFD.

There is a chance for lingering clouds behind the rain to have bases around MVFR levels, but for now kept the end of the TAF VFR. Winds will slowly turn from southwest to northwest overnight toward daybreak on Saturday.

DK

LOT WATCHES/WARNINGS/ADVISORIES

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