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.

UPDATE

Issued at 1023 AM CDT Fri Aug 21 2026

Moving through the mid-morning, a mid level vort is sliding east across northern IL along a subtle shortwave bringing a blossoming cluster of showers this morning, just now moving into Chicago from the west as of 10AM. To the northwest, a less subtle shortwave is cutting across the upper Midwest coincident with a surface cold front that's now pushing into northeast IA and western WI. And finally, a mature MCS is diving SSEward out across central WI. This system is riding along a N-S oriented instability gradient that's just east of the Mississippi and is scooting east with time. Latest guidance has had a terrible handle on this complex suggesting that it should have weakened a lot more by now than it has. As of this writing, this system is still very mature and only recently has IR satellite began exhibiting warming cloud tops. It appears likely now that this MCS, while it may continue to gradually weaken, should hold together nicely as it crosses the IL state line near the start of the afternoon. By then, the instability gradient is expected over north-central IL which places the main area of concern for storms in the early-mid afternoon window roughly east of I-39. Strong to marginally severe wind gusts may be possible with this first round.

The more appreciable severe threat will be with the cold frontal passage late this afternoon and evening, although this initial MCS does provide a good deal of uncertainty regarding the pre-frontal environment. Our western CWA should have a much easier time recovering for the late afternoon than areas east with less coverage of precip anticipated earlier in the day and stronger theta-e advection behind this initial wave. Latest CAM guidance largely agrees on a reservoir of 2-3,000 Joules of MLCAPE building in our western CWA later today and less into NE IL and NW IN owing to drier low level air. However, again, none of these models have a good handle on this MCS or this scattered activity currently blossoming across northern IL, so it's very tough to say exactly what the thermo environment will look like by late afternoon/evening. Wind shear will be pretty marginal at 25 to 30 kt of effective shear, most of that found veering through the low levels which would support a low- end tornado threat where the low levels can destabilize sufficiently. Even when discounting the initial MCS, latest CAMs want to maintain at least some MLCIN over the eastern CWA through the afternoon which will only be reinforced by diurnal cooling when the front arrives in the evening. For that reason, this evening's marginal tornado threat will be focused west of the Fox Valley and north of I-80 where a better low level thermo environment is anticipated. We'll also have to watch for any lingering outflow boundaries from the earlier MCS to act as a focus for convection or low level rotation. This is also where the severe wind and hail threat will be maximized given the greater instability and generally more support expected from the left exit region of a 700 mb jet max along the base of the shortwave. Severe hail and damaging winds are on the table area- wide but it's not clear the eastward extent of deep convection this evening.

Pockets of very heavy rainfall are expected this afternoon and evening as well. The storms look progressive enough in nature to avoid many widespread flooding concerns, but scattered pockets of 1- 2" is a strongly favored signal among guidance, which would likely lead to localized flooding impacts, mainly in low- lying and other flood prone areas. The severe threat looks to exit the area to the southeast shortly after midnight. Forecast products have been updated to reflect latest thinking.

Doom

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 /12Z TAFS THROUGH 18Z SATURDAY/

Issued at 641 AM CDT Fri Aug 21 2026

Forecast concerns for the terminals...

- Period of scattered showers and thunderstorms this afternoon and evening ahead of a cold front.

- A few storms could become severe with damaging winds and hail the main threats.

A shortwave trough continues to pivot over the northern Great Lakes this morning which continues to develop some isolated showers across eastern IA and southern WI. So far these showers have been struggling to reach the ground given the high cloud bases (around 7000-10000 ft) and the dry sub-cloud layer and that trend should continue through mid-morning. However, there is a non-zero (~10%) chance that an isolated sprinkle makes it to the terminals, especially RFD. Otherwise, expect SCT to BKN VFR skies this morning with southwest winds around 5-10 kts.

Heading into this afternoon, a secondary shortwave (currently over eastern ND) and its associated cold front will begin to pivot into the area. By this time, better moisture and instability should be in place which is expected to result in the development of scattered showers and thunderstorms through the afternoon and evening hours. The generally favored time window for storms at the terminals will be between 20z and 02z, but with coverage of storms being more uncertain have opted to maintain the PROB30 groups at the Chicago area terminals for now. At RFD the TEMPO group timing was adjusted to reflect a slightly early storm development, but given that storms may fester longer than the four hour TEMPO allows have opted to introduce a VCTS here through 03z for that potential. Regardless of storm coverage, any storms today will have the potential to become severe with damaging winds (up to 60 mph) and large hail (up to half dollar size) the main threats. Though an isolated tornado cannot be ruled out, particularly west of the Fox River Valley.

Shower and storm chances should diminish after 03z this evening as the cold front makes its way across the area. As a result mostly dry conditions are expected overnight with VFR conditions. While there is a signal that some lower ceilings (around 2000-3000 ft) could develop after midnight, confidence on their coverage remains low. Winds will also become northwesterly behind the front with speeds gradually increasing through the night into the 10-12 kt range by Saturday morning.

Yack

LOT WATCHES/WARNINGS/ADVISORIES

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