Area Forecast Discussion
Issued by NWS Chicago, IL
Issued by NWS Chicago, IL
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428 FXUS63 KLOT 222009 AFDLOT Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 309 PM CDT Sat Jun 22 2024 .KEY MESSAGES... - Potential for strong to severe storms with damaging winds, torrential downpours and localized flooding this afternoon near the Wisconsin state line and area-wide this evening. - Breaks in heat and humidity are expected Sunday and Monday, and again Wednesday through the end of the workweek. - Tuesday may feature dangerous heat and/or severe weather in the general region. && .DISCUSSION... Issued at 309 PM CDT Sat Jun 22 2024 Through Tonight: Forecast attention this afternoon continues to focus on the expected development of severe thunderstorms to our west-northwest across northeastern sections of IA. Satellite imagery continues to depict breaks in the cloud cover across this area within the vicinity of a west-to-east oriented stationary frontal boundary. Heating of a very high theta-e airmass along and south of this boundary is resulting in rapid destabilization. This was noted in the recent 18z RAOB from DVN. Continued heating of this airmass over the next hour or two will likely continue to erode what little cap still exists, thus allowing for the initiation of thunderstorm development. As noted in previous discussions, the kinematic environment across the area will be favorable for the development of organized severe convection, with supercells likely becoming the initial mode across northeastern IA late this afternoon. While a few tornadoes and instances of hail will be possible with this supercellular activity for a couple hours late this afternoon, amalgamating outflows are expected to transition the predominant storm mode into east-southeastward moving organized linear clusters across northern IL this evening. Rather poor mid-level lapse rates with eastward extent over northern IL in combination with a very high PWAT airmass (up to 2.25") suggests the threat of hail will remain rather low outside of any well developed supercell structures. Aside from the damaging wind threat, the near record high PWATs will also support instances of extremely heavy rainfall with the storms, with rain rates of 1 to 2"+ per hour likely. For this reason, even a brief period of training storms could result in some localized 2 to 3"+ amounts before the storms sag southward through the evening. While we still cannot rule out the need for a short fused flash flood watch for western parts of the area, we will continue to run with the current ESF (hydrological outlook) for areas generally along and north of I-80 in IL. The heavy rain and severe weather threat will gradually end from northwest to southeast later this evening into the overnight as the clusters of storms sag southeast out of the area. KJB Sunday through Saturday: Forecast focus for the extended is on two time periods, Tuesday and Friday. Before then, lingering showers will be possible Sunday morning as the cold front departs the area and isolated showers will be possible Sunday afternoon into Sunday evening, but confidence is very low and maintained a dry forecast for now. Cooler and slightly drier air will spread across the area on Sunday. Lowered high temps to the lower/mid 80s but these are still above some of the cooler guidance. Winds will turn northerly over the lake, which will keep temps cooler near the Lake Michigan shore. Low temps by early Monday morning likely to be in the lower/mid 60s for much of the area. Dry and mostly sunny for Monday as highs likely to be back into the mid to possibly upper 80s. A lake breeze will turn the lake shore areas cooler for the afternoon. Much of Monday night will likely be dry but by early Tuesday morning, there is considerable disagreement/spread among the models and their ensembles. There is one solution of morning convection either over or near the cwa which continues through midday Tuesday, as it moves south. The other solution keeps the local area dry through early/mid afternoon Tuesday and then as a cold front approaches, convection develops along/ahead of the front and continues into Tuesday evening. While this time period has consistently been shown in the models for convection for the local area, either solution seems plausible and there is little to support one over the other at this time. Strong to severe storms along with heavy rain would be possible in either. When precipitation develops, assuming it does, will have a big impact on temps. Morning convection will keep temps cooler and while they`ll still likely recover into the afternoon, unlikely to get past lower 90s. If Tuesday morning is dry, southwest winds will likely tap into the stronger winds aloft, with gusts into the 30 mph range, pushing highs into the mid 90s, perhaps a few locations reaching upper 90s. The strong winds and deeper mixing will likely keep dewpoints in the upper 60s/near 70. This would result in heat indices in the upper 90s to around 100. There is the potential for thunderstorms to linger or continue into Wednesday morning, though if they do, it would likely be south of I-80 and departing through mid morning. Dry conditions are then expected through at least Thursday evening. Cooler and less humid conditions are expected Wednesday and especially on Thursday, along with cooler conditions near Lake Michigan with northeast winds. Friday has been another time period the models have been showing the potential for thunderstorms, again with little confidence for timing. Another cold front will moving across the area during this time period. Similar to Tuesday, there is the potential for both morning and afternoon/evening. cms && .AVIATION /18Z TAFS THROUGH 00Z MONDAY/... Issued at 1232 PM CDT Sat Jun 22 2024 Key Aviation Impacts for the current TAF period: - Southwest winds gusting up to 30 knots through this evening - A line of thunderstorms is expected to move over the airspace tonight through early Sunday morning. Downpours and strong wind gusts are the main impacts - MVFR cigs are expected to move in with the squall line and continue to lower behind the line, with the slight chance for IFR cigs. Other than some VFR showers around KRFD at the time this discussion was published, main impacts through 00Z will be from strong southwesterly wind gusts. Observations have been consistently between 25 to 30 knots, which is expected to continue through the afternoon; however the occasional gust just greater than 30 knots cannot be completely ruled out though not expected to be prevailing. A squall line is still expected to drop southeastward over Northern Illinois this evening. Timing has wavered slightly but it still looks like it should arrive around KRFD just before 01Z, then by 02Z to the Chicago terminals. Warm temperatures and sufficient moisture should make highly efficient rain rates resulting in downpours and reduced vis and MVFR cigs as the line passes over. Strong northwesterly winds are expected along the front with gusts 25-35 knots. Winds become westerly behind the line and diminish briefly before gusts up to 20 knots return. Higher confidence in MVFR cigs remaining through mid Sunday morning, but there remains a weak signal on the potential for IFR cigs. There was not enough confidence to add to the TAF currently, but left the SCT sub-1000 ft group in TAF. Sunday should be VFR with northwesterly winds around 15 knots, potentially with the occasional gust up to 20 knots. DK && .CLIMATE... Issued at 335 AM CDT Sat Jun 22 2024 Here are the daily high and warmest low temperature records for Chicago and Rockford for today: Chicago ------------------------- Day: 6/22 ------------------------- Record High: 97 Record Warm Low: 76 Rockford ------------------------- Day: 6/22 ------------------------- Record High: 97 Record Warm Low: 73 NWS Chicago && .LOT WATCHES/WARNINGS/ADVISORIES... IL...None. IN...None. LM...Small Craft Advisory until 1 AM CDT Sunday for the IL and IN nearshore waters. && $$ Visit us at weather.gov/chicago