771 FXUS63 KLOT 101141 AFDLOT Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 641 AM CDT Thu Jul 10 2025 .KEY MESSAGES... - Patchy dense fog will continue mainly south of Interstate 80 through mid-morning. - Dangerous swimming conditions will continue at Lake and Porter county Indiana Lake Michigan beaches through mid-morning. - Several waves of showers and storms are possible from this afternoon through Saturday across the area. Some storms may be severe with damaging winds and flash flooding. - Provided low coverage of thunderstorms, Friday may be hot and humid with heat indices near 100 degrees. && .UPDATE... Issued at 426 AM CDT Thu Jul 10 2025 Dense fog continues to thicken across areas along and south of the Kankakee River Valley with visibilities ranging from 1 mile to less than 1/4 of a mile. Based on current and expected trends, opted to expand the Dense Fog Advisory to Benton, Jasper, and Newton counties in northwestern Indiana. Will also issue an SPS for areas north of the Dense Fog Advisory for patches of dense fog. Borchardt && .DISCUSSION... Issued at 250 AM CDT Thu Jul 10 2025 Through Friday: Through Mid-Morning: A baggy surface pressure gradient has once again allowed for patchy dense fog to develop early this morning. This time around, the fog is focused along and south of the Kankakee River Valley where mid-level clouds are less prevalent. A Dense Fog Advisory is in effect for Livingston, Ford, Iroquois, and Kankakee Counties until 9 AM this morning. As fog evolves, counties may be added to the Dense Fog Advisory through the next few hours. Meanwhile, waves continue to gradually subside along Lake Michigan beaches. Will maintain the inherited Beach Hazards statement for Lake and Porter counties in Indiana through 10Z given the northerly flow trajectory. Though, will cancel the statement for Illinois locations given lower waves. Borchardt Mid-Morning Onward: The main forecast concern from mid-morning onward is the threat for rounds of thunderstorms lasting through at least Friday. The overarching pattern will be defined by weakly-defined upper- level shortwaves parading along a diffuse baroclinic zone draped across the Lower Great Lakes. Such patterns can be unforgiving to forecast for basically all variables (temperature, cloud cover, QPF, etc) with dependencies on and cascading consequences from episodic thunderstorm clusters parading within the baroclinic zone. While confidence is high that our general region and indeed parts of our local area will see episodes of storms over the coming days, confidence is notably lower on pinpointing the exact locations and favored time windows. Early morning radar and water vapor imagery shows broken lines of thunderstorms from western Minnesota southwestward through southeastern Nebraska. An MCV is becoming quite apparent behind one line segment moving along the Iowa/Minnesota border, with recent radar imagery from KFSD indicating nearly 50 to 60kt of flow at 10kft on the western side of the circulation (i.e., impressively strong). As the morning progresses, the expectation is for thunderstorms to continue developing across central and eventually eastern Iowa along eastward-suring outflows and along the southeastern influence of the eastward-drifting MCV. As consolidated outflow crosses the Mississippi River by late morning, thunderstorms may continue to fester (if not reintensify given the warming boundary layer), and reach the I-39 corridor by early afternoon. Should this occur, DCAPE of 850-1000 J/kg and PWATs of 1.5" may support torrential downpours and gusty winds in the strongest storms. With instability decreasing with eastward extent, coverage and intensity of storms may tend to wane as they approach Lake Michigan this afternoon. Will carry mid-range (30 to 40%) chances for storms from mid-morning to mid-afternoon for this potential, focused mainly along and north of I-80. Another round of showers and storms may develop late this afternoon through the evening hours across northwestern Illinois as the aforementioned MCV moves toward southwestern Wisconsin. Now, the intensity of the storms during the second round appears dependent on (1) earlier shower and storm coverage being at a minimum to preserve instability; and (2) the strength of the MCV maintaining integrity to provide enhanced veering 850 to 500mb flow across eastern Iowa and northwestern Illinois. If both dependencies are realized, have quite a bit of concern that there could be a focused area with a threat for damaging winds and even tornadoes northwest of the I-39 and I-80 corridors back into northeastern Iowa and southwestern Wisconsin. The 00Z and 06Z NAM3 runs are probably overly aggressive in decaying early afternoon storms and beefing up the strength of the MCV, but does provide a glimpse at a reasonable worst case scenario with a string of supercells moving into northwestern IL this evening. The more favored scenario appears to be for just another round of gusty and torrential-rain producing thunderstorms this evening across northwestern IL. Of course, trends will need to be monitored closely as the MCV approaches. Tonight, the MCV should track across southern Wisconsin providing a focus for showers and storms through the overnight hours. Coverage of showers and storms may end up highest just north of the state line, though feel anyone along and north of I-80 will be within striking distance. With a southwesterly low-level jet expected to develop in the mid-Mississippi River Valley overnight, will have to watch for any signs of backbuilding to provide a threat for flash flooding somewhere in general region. With that said, the orientation of the low- level jet and approach of another shortwave may instead focus a separate area of storms firing across Iowa, disrupting moisture transport into the Lower Great Lakes. All things considered, will advertise mid-range chance PoPs (30-50%) for showers and storms during the overnight hours, highest near the Wisconsin state line. On Friday, subsidence in the wake of the departing MCV should promote a period of dry weather at least through the first half of the day. Confidence in a dry forecast lowers substantially by the afternoon as another shortwave (or several) track northeastward toward the Lower Great Lakes. It`s easy to envision scenarios where the afternoon is completely dry or completely stormy depending on the arrival time of the waves. With PWATs nearing 1.75" and hints of enhanced midlevel flow provided by each shortwave, any storm in the general region Friday could be severe with damaging winds and torrential downpours. For now, will carry low-end (20-30%) chances for storms through the entire day keeping in mind they can essentially vanish or double as we get closer. Note that if storms are at a minimum Friday afternoon, the combination of heat and humidity will make it feel like 100 degrees. Borchardt Friday Night through Wednesday: Thunderstorms may be ongoing Friday evening, especially across the northwest cwa but overall confidence is low for timing as each potential wave of thunderstorms is dependent on the previous. If thunderstorms are not ongoing or develop Friday evening, that may lead to a better chance of thunderstorms late Friday night into early Saturday morning. The main cold front will then move across the region Saturday afternoon and this still appears to be the best chance of thunderstorms for the entire area. Precipitable water values are generally in the 1.5 inch range Friday night, increasing to around 2 inches Saturday, allowing for heavy rain and localized flooding. Dewpoints may pool in the mid 70s ahead of the front on Saturday, allowing for even more available moisture. Thunderstorm evolution Friday night into Saturday morning will determine where storms may form on Saturday, but current trends would suggest that the best chance of strong/severe would be along/east of I-55, likely moving east of the area by early Saturday evening. Sunday is still looking dry for most of the area, but the GFS has been showing a wave lifting through central IL the past few days and as a result, there are now blended low chance pops through I-80 or so and maintained those for now. Monday is looking dry but by Tuesday, the airmass is again quite moist and given how uncertain the pattern has become, low chance pops seem reasonable. Another stronger cold front is expected to move across the region midweek, likely in the Wednesday time period with a chance of thunderstorms, possibly strong/severe along with heavy rain/localized flooding. Quite a bit of ensemble support for at least a few dry days, near normal temps and lower humidity for late next week. cms && .AVIATION /12Z TAFS THROUGH 18Z FRIDAY/... Issued at 641 AM CDT Thu Jul 10 2025 Primary forecast concern and challenge is thunderstorm chances through the period. In the short term, areas of dense fog south and southwest of the terminals will steadily lift/dissipate over the next few hours. Light and variable winds will become northeasterly for the Chicago terminals this morning and then a lake breeze will allow winds to become easterly, to perhaps 10kts for ORD/MDW/GYY. Further inland, winds will turn south/southeast this morning and then likely turn to south/southwest this afternoon. Light southeast winds are expected tonight, becoming southwest Friday morning. Thunderstorm chances are quite uncertain for the Chicago terminals through Friday morning. Scattered thunderstorms are expected across northwest IL this afternoon into this evening though still some uncertainty for timing and location and thus, maintained a longer duration prob thunder at RFD, that will need to be refined as trends emerge. Some of this activity may make it to the Chicago terminals this evening, but all current trends would suggest it will be weakening before or as it arrives. For now, have only included prob shower mention for the the Chicago terminals. How this convection evolves will likely determine how much, if any, thunderstorms develop overnight. There is some guidance showing thunderstorms developing near the IL/WI state line overnight which could also be a bit further north or a bit further south, near or at the terminals. Confidence is too low to include any mention with this forecast but later forecasts may need to include thunder mention for the Chicago terminals. Any thunderstorms that do form will have the potential to produce torrential rain with brief ifr/lifr cigs/vis. cms && .LOT WATCHES/WARNINGS/ADVISORIES... IL...Dense Fog Advisory until 9 AM CDT this morning for ILZ023- ILZ032-ILZ033-ILZ039. IN...Dense Fog Advisory until 9 AM CDT /10 AM EDT/ this morning for INZ010-INZ011-INZ019. Beach Hazards Statement until 10 AM CDT this morning for INZ001-INZ002. LM...None. && $$ Visit us at weather.gov/chicago