


Area Forecast Discussion
Issued by NWS Chicago, IL
Issued by NWS Chicago, IL
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328 FXUS63 KLOT 091136 AFDLOT Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 636 AM CDT Wed Jul 9 2025 .KEY MESSAGES... - Dangerous swimming conditions will develop at Lake Michigan beaches Wednesday afternoon into early Thursday, especially east of Gary, Indiana. - There is a low chance (20 to 30%) for isolated thunderstorms today, primarily near the lakeshore. - Chances for showers and thunderstorms return Friday and Saturday with the best chance (50-70%) on Saturday. Strong/ severe storms and heavy rain/localized flooding are possible. && .UPDATE... Issued at 545 AM CDT Wed Jul 9 2025 Did end up having to hoist a Dense Fog Advisory for much of northwestern Illinois this morning as visibilities uniformly tanked to 1/4 to 1/2 of a mile. The expectation is for fog to gradually erode in the next few hours as the sun angle increases. The Dense Fog Advisory is in effect through 9 AM CDT. The leading edge of the backdoor/lake enhanced front is quite evident in satellite data, and is currently just north of Sheboygan, Wisconsin. Based on the current speed, the front should reach Waukegan by 11 AM and Chicago by 1 PM. There is quite the expansive stratus bank behind the front, so clouds appear poised to fill quickly across the area this afternoon and evening. One final note - would not be surprised to have to add central Cook and Lake (IL) counties to the Beach Hazards Statement pending how quickly winds and waves pickup behind the front. Will let the incoming day shift take a close look at upstream observations to inform any adjustments to the product during the scheduled morning update time. Borchardt && .DISCUSSION... Issued at 222 AM CDT Wed Jul 9 2025 Through Thursday: A broad upper-level trough is evident across the Great Lakes this morning in water vapor imagery atop a very weak/baggy surface low pressure system centered over Lake Michigan. An embedded shortwave within the broad trough is forcing an expansive area of upper-level clouds from Lower Michgian though Southern Illinois, with the back edge more or less oriented along I-57 at press time. Meanwhile, a surface high pressure system is building southward toward Lake Superior along the backside/inflection point of the upper-level trough. With nearly calm winds, clearing skies, and pockets of wet soil from showers and storms yesterday, areas of fog are developing early this morning. So far, the highest coverage of fog has been near and west of I-39 (where clearing occurred earliest this morning). Going forward, would expect fog to continue gradually expanding and become locally dense with visibility less than 1/2 of a mile. For now, will plan to handle the locally dense fog with a Special Weather Statement. However, will continue to evaluate trends to determine if a targeted Dense Fog Advisory will be needed instead. After sunrise, any remaining fog should erode. Attention then turns toward a lake-enhanced front due to race down and inland from Lake Michigan from late morning through early afternoon. Based on surface observations across the Upper Peninsula of Michigan, the backdoor front appears to be approaching the northern tip of Lake Michigan at press time. High res guidance appears ubiquitous in showing the front zooming south throughout the morning and reach northeastern IL in the 10 am to noon window, and northwestern Indiana in the 11am to 1 pm window. The front will then spread inland through the remainder of the afternoon. Thanks to the seasonably warm lake water temperatures in the upper 60s to lower 70s, the marine airmass won`t be particularly cool. With that said, highs will range from the mid 80s well inland to the mid 70s lakeside this afternoon. Of greater interest will be in increase in wave heights behind the front, with an attendant threat for rip currents particularly this afternoon through Thursday morning. Experimental WW3 model data run using winds exclusively from the NAMNest (which typically performs well in these types of regimes) shows waves hitting 3 to 4 feet by this evening along beaches from Gary to Michigan City, Indiana. Considering conditions will be rather pleasant this morning, would like to get the message out for rapidly changing conditions this afternoon. So, will go ahead and issue a Beach Hazards Statement for Lake and Porter county beaches in northwestern Indiana (and hold at a "Moderate" swim risk for northeastern Illinois beaches). As mentioned in previous forecast discussions, will have to watch for the isolated to perhaps scattered coverage of showers and storms along the front from late morning through early afternoon. Owing to mid-level subsidence in the wake of the departing upper-level trough, capping is expected to develop around 600mb by early afternoon. as a result, what should be a chunky cumulus field along the front may struggle to sprout more than an isolated storm or two. All things considered, will maintain the inherited 20 to 30% chances for showers and storms through the evening hours, with the highest values along the front. The southward build of the Great Lakes high pressure system will set the stage for a quiet night. A few patches of fog may develop toward daybreak Thursday, mainly south of I-80 (south of the remnant lake breeze/front). Overnight lows are expected to fall into the low to mid 60s. On Thursday, the upper-level pattern will be undergoing a transition as a series of upper-level shortwaves propagate from the central US toward the Great Lakes, south of a deeper trough approaching from the Pacific Northwest. There is a growing signal for one such shortwave to approach northern IL in the late afternoon, though will have to think that the instability gradient positioned well to our west would guide any upstream convection safely around the perimeter of our area. Nevertheless, this will be a trend to monitor. Thursday otherwise looks like a nice day with highs in the low to mid 80s and partly cloudy skies. Borchardt Thursday Night through Tuesday: Primary forecast remains thunderstorm chances Friday through Saturday. There has been little change to the overall pattern/trend for late week with an upper trough moving across the northern Plains and upper midwest with a wave ejecting out of the central Plains with an eventual surface low developing somewhere nearby or over the Lakes region. There seems to be a bit better consensus that much of Friday and Friday evening could be dry for the local area with better chances northwest of the area, with perhaps some decaying activity late Friday night/Saturday morning across northwest IL. Then on Saturday when the the main front moves through and a possible surface low is shifting east of the area, this would be the best chance of thunderstorms. Blended pops for Friday evening and Friday night are now high chance, low likely and these seem too high based on the latest trends but made no changes. Pops have increased to high chance and likely on Saturday. Quite a bit of ensemble support for the Saturday precip timing, especially from the ECMWF. With timing looking a bit better for Saturday and perhaps early to mid afternoon with the heating of the day, some stronger storms are possible along with heavy rain and possible localized flooding as precipitable water values will be around 2 inches ahead of the front. Any thunderstorms on Friday and Friday night would also have the potential to be strong and produce heavy rain. High pressure appears to move far enough into the area Saturday night and Sunday to end the precipitation chances with Monday also possibly being dry. High temps look to warm back into the upper 80s/lower 90s Monday and Tuesday. Then by midweek, current trends would suggest another larger trough/cold front moving across the region. cms && .AVIATION /12Z TAFS THROUGH 18Z THURSDAY/... Issued at 636 AM CDT Wed Jul 9 2025 Forecast concerns include... Lifr/Vlifr vis/cigs with fog this morning. Wind shift to northeast this afternoon. Chance of showers this afternoon. Possible fog early Thursday morning. Fog and low cigs are widespread across much of northwest and central IL and will steadily lift/dissipate over the next few hours. Few/sct ifr level clouds are still possible for ORD/MDW but confidence has decreased for ifr cigs. However, as a cold front moves south across the area late this morning into early this afternoon, there is a potential for ifr cigs, though the best chance for these lower cigs may be closer to Lake Michigan. Light southwesterly winds will turn northwesterly this morning, possibly northerly ahead of the front and then will shift to the northeast as the front moves through. There may be some gusts into the 15-20kt for a short time with and just behind the front. Northeasterly winds will slowly diminish into this evening and may become light northerly overnight. There is a chance of showers this afternoon, likely confined to the front as it moves inland. There may also be an isolated thunderstorm, but overall coverage looks low and prob shower mention still looks on track. There is another chance for fog early Thursday morning, mainly west and south of the Chicago terminals and included tempo mention at RFD for this possibility. cms && .LOT WATCHES/WARNINGS/ADVISORIES... IL...Dense Fog Advisory until 9 AM CDT this morning for ILZ003- ILZ004-ILZ005-ILZ008-ILZ010-ILZ011-ILZ012-ILZ019-ILZ020- ILZ021-ILZ032. IN...Beach Hazards Statement from 1 PM CDT this afternoon through Thursday morning for INZ001-INZ002. LM...None. && $$ Visit us at weather.gov/chicago