Area Forecast Discussion
Issued by NWS Chicago, IL
Issued by NWS Chicago, IL
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588 FXUS63 KLOT 131827 AFDLOT Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 127 PM CDT Thu Jun 13 2024 .KEY MESSAGES... - Several rounds of severe thunderstorms are possible are likely this afternoon and evening, especially along and south of I-80. - Heat and humidity will return next week, as well as chances for showers and thunderstorms. && .DISCUSSION... Issued at 127 PM CDT Thu Jun 13 2024 Through Friday night: Recent satellite imagery depicts an axis of agitated cumulus clouds extending from waukegan to Davenport within a broad zone of low-level confluence across the Lower Great Lakes. Lingering capping has thus far kept a lid on early attempts for sustained convection. However, continued surface heating (temperatures rising toward 90F), low-level moistening/pooling via southwesterly flow and evapotranspiration (dew points rising toward 70F), and mid-level cooling/moistening via evaporation of convective turrets into the base of the cap should eventually win the battle and lead to explosive thunderstorm development within the next few hours. Extrapolating the position of the axis of agitated cumulus clouds, such explosive development is poised to occur between the I-88 and I-80 corridor sometime between 2 and 4 PM. With MLCAPE >2500 J/kg (including some 1000 J/kg in the hail growth layer) and convective-layer shear >45kt (largely focused from 2-6 km), sustained thunderstorms will be poised to become supercells with a threat for destructive hail locally greater than 2" in diameter through the afternoon. With time, clustering of cells may lead to an increasing damaging wind threat including localized gusts as high as 75 mph. Coverage of thunderstorms this afternoon remains an item of lower than average confidence, with anything from just an isolated cell or two to literally an entire line of supercells plausible. Meanwhile, a second area of explosive thunderstorm development is expected in southern Iowa this afternoon near an area of focused low-level convergence along a cold front (e.g. the Level 3/5 threat area in the SPC outlook). Upscale growth into one or more clusters appears plausible as they move east-southeastward into western and central Illinois this evening. If coverage of thunderstorms this afternoon is relatively sparse, incoming convection from Iowa may provide a "round 2" of sorts along and south of I-80 where the receiver of MLCAPE >2500 J/kg remains. Similar to this afternoon, any storm this evening will carry a threat for destructive hail >2" and, if clusters can become established, winds >75 mph. Moreover, if a west-to-east axis of thunderstorms were to materialize within a mesoscale zone, PWATS >2" would certainly support efficient downpours with a threat for localized flash flooding. The WPC threat level 2/5 area for flash flooding highlights the area well. In all, "round 2" may occur from 7 PM to as late as 3 AM. Tomorrow will be decidedly quiet with seasonable temperatures (highs in the low to mid 80s) and a northwesterly breeze. A lake breeze should surge inland during the afternoon leading to cooling temperatures along the lakeshore. Borchardt Saturday and beyond... Coming Soon... NWS Chicago && .AVIATION /18Z TAFS THROUGH 00Z SATURDAY/... Issued at 1235 PM CDT Thu Jun 13 2024 Forecast concerns for the terminals... - Scattered to widespread thunderstorms this afternoon and evening, some storms could be severe - Winds become northerly behind a cold front this evening - Lake breeze to move through the Chicago terminals Friday afternoon Breezy west-southwest winds have developed at the terminals, especially the Chicago ones, this morning as the atmosphere has mixed into a low-level jet overhead. While this jet is gradually diminishing, gusts in the lower 20 kt range will likely persist through early afternoon. At the same time, an upper disturbance and associated cold front will continue to move towards northern IL from WI which will aid in developing scattered to widespread thunderstorms this afternoon and evening. While confidence is high that storms will develop, the confidence on exact timing and coverage over the terminals, especially at RFD, is lower. Therefore, I have maintained the TEMPOs and their respective timings which seem reasonable based on latest guidance trends. However, I did extend the VCTS mentions at MDW and GYY an hour later to account for the potential for the front and associated storms to stall near I-80 this evening. Additionally, there is also the threat that any storms this afternoon and evening could become severe with the threats of large hail (possibly in excess of 2 inches), damaging winds (possibly in excess of 60 kts), and locally heavy rain which will result in reduced visibilities. Storms will gradually come to an end from north to south late this evening as the front moves through. As a result winds will become northerly (possibly varying between 350 and 010) with speeds in the 6 to 8 kt range overnight and persist into Friday morning. Though a lake breeze is expected to develop early Friday afternoon and push through the Chicago terminals between 18z and 20z. Otherwise, expect mainly VFR conditions outside of storms with gradually scattering clouds overnight into Friday. Yack && .LOT WATCHES/WARNINGS/ADVISORIES... IL...None. IN...None. LM...None. && $$ Visit us at weather.gov/chicago