Area Forecast Discussion
Issued by NWS Chicago, IL
Issued by NWS Chicago, IL
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491 FXUS63 KLOT 161743 AFDLOT Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 1243 PM CDT Fri Aug 16 2024 .KEY MESSAGES... - Another round of scattered thunderstorms this afternoon and evening, some of which could be strong to severe with the threat of damaging winds and hail. - Periods of scattered showers Saturday and Sunday afternoons. - Below average temperatures expected next week with highs in the mid-70s to around 80. && .DISCUSSION... Issued at 410 AM CDT Fri Aug 16 2024 Through Saturday: A broad upper-level low continues to spiral over northern MN this morning with several shortwave impulses pivoting across the upper Midwest and Great Lakes regions. One of these shortwaves has been pivoting overhead overnight which has allowed a complex of showers and thunderstorms to develop and progress across the southern 2/3rds of our CWA. While these storms did have some localized severe winds earlier, the gradually waning instability has turned these showers and storms into primarily a heavy rain threat as they continue into northwest and eventually northern IN through daybreak. Therefore, mostly dry conditions are expected to start our Friday with skies gradually becoming partly to mostly sunny by late morning. However, another one of the aforementioned shortwaves (currently in southern MN) is expected to pivot into southern WI this afternoon and develop more scattered thunderstorms in the process. While most guidance remains in decent agreement on storms occurring, there continues to be uncertainty on the coverage. The reason is due to the models struggling to resolve how low dew points will mix down this afternoon which will dictate how much instability will be available for storms. The general consensus seems to favor dew points dipping into the mid to upper 60s this afternoon, but there still are a couple hi- res models (namely the HRRR) which mixes dew points into the lower 60s. Regardless, most guidance does show at least 1000 to 1500 J/kg of MLCAPE developing across northern IL and northwest IN this afternoon which should be sufficient to support some scattered thunderstorms especially for areas along and north of I-80 where forcing should be maximized. Additionally, forecast soundings continue to show around 25 to 30 kts of effective shear overhead this afternoon which may allow a few storms to become strong to severe. Given that wind profiles (and resultant hodographs) are largely unidirectional the main threats with any severe thunderstorms looks to be damaging wind gusts and maybe some hail depending on degree of destabilization. However, low- level shear is forecast to increase this evening as the low-level jet redevelops which could support some supercell structures and possibly a localized tornado threat. Though the tornado threat is much more conditional on storms remaining surface based as the low- level jet ramps up after sunset. That said, the Storm Prediction Center has maintained the level 1 of 5 severe risk across the area for today which covers these threats well. Furthermore, PWATs this afternoon should also rebound into the 1.5 to 1.6 inch range as storms develop which will make the storms efficient rain makers. While some instances of localized flooding may occur if storms develop on top of already saturated areas, the progressive and isolated nature of the storms should limit this threat for most. Thunderstorm coverage is expected to diminish after 10 PM this evening as the upper wave moves east and a cold front pushes through the area. Therefore, mostly dry conditions are expected overnight with temperatures cooling into the mid to upper 60s. However, the upper low is expected to be moving into Lower Michigan during the day on Saturday. Since mid-level moisture is expected to still be in place, scattered showers are expected to develop during the afternoon as modest diurnal instability develops. While the threat for thunder generally looks low, a stray rumble or two of thunder cannot be completely ruled out just yet. Saturday Night through Thursday: The upper low will continue to gradually move east across Lower Michigan on Sunday as upper-level riding builds across the western CONUS. As this occurs, a surface high is expected to pivot into the Great Lakes Sunday night resulting in a period of drier and below average conditions through the middle of next week. However, until this high and associated dry air fully arrive scattered showers will once again be a possibility on Sunday. Though coverage should be somewhat less than on Saturday. Additionally, the entering high pressure will also tighten the pressure gradient across the area which will result in breezy northerly winds for Sunday and Monday. These breezy winds will also help to build waves at southern Lake Michigan beaches during this period which will make for hazardous swimming conditions, especially in northwest IN. So if planning to head to the beach this weekend be sure to check the forecast before hand. Winds and waves are expected to gradually subside late Monday into Tuesday as the surface high moves overhead. Therefore, tranquil weather is expected through at least the middle of next week with plentiful sunshine and high temperatures in the mid-70s to near 80 each afternoon. Yack && .AVIATION /18Z TAFS THROUGH 00Z SUNDAY/... Issued at 1242 PM CDT Fri Aug 16 2024 Aviation Forecast Concerns: - Widely scattered SHRA and possible TS early this evening. - MVFR ceilings early tomorrow morning. - Another round of widely scattered SHRA tomorrow afternoon. VFR conditions with west-southwest winds gusting to around 20 knots will develop and remain into the early evening hours before diminishing and becoming more westerly. A mid-level disturbance will move over the area from the northwest early this evening. This may produce a round of widely scattered TSRA early this evening. Low confidence in coverage leads to a VCTS mention. Slightly better coverage north of the Illinois/Wisconsin border. Guidance continues to show MVFR ceilings moving in from the northwest that will enter early tomorrow morning. Ceilings should lift to VFR by mid-morning. Another disturbance around the periphery of the upper level low over Michigan will bring more widely scattered showers to the area by tomorrow afternoon with a possibility of a thunderstorm. Kraujalis && .LOT WATCHES/WARNINGS/ADVISORIES... IL...None. IN...None. LM...None. && $$ Visit us at weather.gov/chicago