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Area Forecast Discussion
Issued by NWS Chicago, IL
Issued by NWS Chicago, IL
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104 FXUS63 KLOT 141038 AFDLOT Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 538 AM CDT Sun Jul 14 2024 .KEY MESSAGES... - Additional clusters of storms are expected today, with continued threats for severe weather and flash flooding. - Today and tomorrow will be hot and humid with peak heat index values above 100 degrees. Tomorrow (Monday) looks to be the hottest day of the pair, and will likely need a Heat Advisory. - Additional rounds of thunderstorms are expected Monday night through Tuesday with continued threats for severe weather and flash flooding. - The pattern finally breaks Tuesday night with a return of tranquil conditions Wednesday onward. && .MESOSCALE... Issued at 538 AM CDT Sun Jul 14 2024 A mesoscale convective system continues to roll down the Mississippi River across far northeastern Iowa and southwestern Wisconsin along the eastern fringe of an EML plume. Over the past few hours, lightning production and reflectivity cores have been on a downward trend, likely as a result of less instability across southeastern Wisconsin and within the effective inflow layer of the system in the wake of an earlier- morning/overnight MCS. However, recent 1-min GOES water vapor imagery depicts a broad region of ACCAS lifting into the southwestern flank of the MCS as recent KLOT radar imagery depicts blossoming "speckled" radar returns west of the Mississippi river. Both features suggest the nose of the 30 to 35kt southwesterly low-level jet (sampled nicely by the KDVN VWP) is now supporting a renewed feed of instability into the MCS, which may allow for regenerating convection along the southwestern flank of the MCS over the next 1 to 2 hours. With the current speed and evolution of the MCS, such regenerating convection may develop in close proximity to where 2 to 7 inches of rain fell last night across far northern Illinois, potentially reinvigorating a threat for flash flooding. A Flood Watch remains in effect generally along and north of Interstate 88 until 1 PM. Borchardt && .DISCUSSION... Issued at 322 AM CDT Sun Jul 14 2024 Through Monday: On the heels of a robust cluster of overnight thunderstorms, yet another cluster of storms is surging down the Mississippi River along the edge of the EML plume. While the next cluster of storms is moving into a relative minima of instability, a steady feed of instability via a 925/850mb low-level jet should allow it to reach at least the Wisconsin state line and likely into the northern-most row or two of counties of northern Illinois around or after daybreak. Given the areas of heavy rainfall that fell overnight, including significant flash flooding that continues around Rockford at press time, opted to issue a Flood Watch for counties along and north of I-88 through 1 PM. Behind the morning round of storms, a period of dry weather is anticipated into the early afternoon. Attention turns firmly toward yet another round of thunderstorms this evening. Regional radar imagery shows a derecho racing across southeastern South Dakota. The latest radar trends indicate that the gust front is now well ahead of the reflectivity, which is not surprising given the system is well south of the upper-level jet and source of deep-layer shear displaced to the north. As a result, the derecho should begin to weaken as it surges into northwestern Iowa later this morning. With that said, there appears to be a highly organized MCV within the core of the system, which will approach northwestern Illinois by mid- afternoon and ignite the next round of storms. With low- to mid- level shear augmented in the vicinity of the circulation, any storm may be severe with damaging winds, damaging hail, and even a tornado. In addition, flash flooding may occur as well especially if thunderstorms favor far northern Illinois once again. With the threat for two rounds of storms today, confidence in the temperature and heat index forecast is quite low. In scenarios with minimal convective coverage and a footprint of outflow, highs should climb into the lower 90s. When combined with dew points in the low to mid 70s, afternoon heat indices would top off in the 100-105F range. I gave serious consideration to issuing a Heat Advisory, but simply didn`t have high enough confidence in widespread 105F+ readings today. Now, it does appear that there will be a break in the waves of thunderstorms during the day on Monday, which gives higher confidence in heat- headline criteria being archived. Indeed, highs in the low to mid 90s and dew points in the mid to upper 70s will support peak heat indices of 105 to 110 areawide. Additional rounds of storms are expected Monday night through Tuesday, which will include the threats for severe weather and flash flooding. Borchardt Tuesday night through Sunday... Following the cold front clearing our southern CWA Tuesday night into Wednesday morning, mean troughing will set up over northeastern North America while powerhouse 500 mb ridging rebuilds over the western US. This should bring an extended period of dry, cooler and less humid conditions Wednesday PM onward through next weekend. Castro && .AVIATION /06Z TAFS THROUGH 12Z MONDAY/... Issued at 1218 AM CDT Sun Jul 14 2024 The back edge of thunderstorms will steadily work southeast from RFD to DPA/ORD/MDW/ORD and then GYY over the next 2-3 hours giving way to a dry period. Trends in winds overnight are an item of low confidence as a residual cold pool, though a general southwesterly direction is expected by daybreak. Attention then turns to the next convective system developing over eastern Minnesota, which is poised to dive southeastward into Wisconsin this morning. At this point, the expectation is for the next complex to miss the terminals to the north, but felt introducing PROB30 groups at all but RFD from about 15-17Z would be prudent just to be safe. Notwithstanding the threat for the Minnesota complex to impact the terminals tomorrow morning, most daylight hours are expected to be dry tomorrow. Yet another convective episode may occur during the evening, for which the inherited TEMPO groups were maintained. Borchardt && .LOT WATCHES/WARNINGS/ADVISORIES... IL...Flood Watch until 1 PM CDT this afternoon for ILZ003-ILZ004- ILZ005-ILZ006-ILZ008-ILZ010-ILZ011-ILZ012-ILZ013-ILZ103- ILZ104-ILZ105. IN...None. LM...None. && $$ Visit us at weather.gov/chicago