Area Forecast Discussion
Issued by NWS Chicago, IL
Issued by NWS Chicago, IL
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195 FXUS63 KLOT 021953 AFDLOT Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 253 PM CDT Thu May 2 2024 .KEY MESSAGES... - Scattered showers and thunderstorms will increase in coverage this afternoon, and continue through the overnight hours. - A few storms may be severe this afternoon with hail up to 1 inch in diameter, 60 mph winds, and soaking downpours. The severe threat should wane after sunset. - Periodic chances for showers and thunderstorms will continue from this weekend onward, with some potential for severe weather on Tuesday. && .DISCUSSION... Issued at 253 PM CDT Thu May 2 2024 Through Friday night: Showers and thunderstorms are off to an early start along the I-55 corridor, seemingly due to the interaction with an eastward-moving mid-level gravity wave and the northward-moving warm front. Additional showers and storms are expected to develop throughout the afternoon and especially after sunset as the right entrance region of an upper-level jet streak moves over the Great Lakes. As we already saw with the first batch of showers and thunderstorms, a few may pulse to severe levels with 1" hail and 60 mph "splat" downbursts through about midnight thanks to MLCAPE 1000-1250 J/kg and meager deep layer shear. With that said, widespread severe weather is not expected today, and the threat should wane with the loss of heating after sunset (though a few non-severe thunderstorms may continue through the overnight hours). Showers will likely continue through daybreak Friday as a cold front sweeps across the area. Chances for rain will then end from west to east Friday morning behind the front as drier air works into the region. Clearing skies and highs in the lower 70s are expected tomorrow afternoon, though stout northerly winds off Lake Michigan will keep lakeshore locations some 10 to 15 degrees colder, particularly in northwestern Indiana. As a surface high pressure system slides overhead tomorrow night, slacking winds and clear skies will allow for efficient radiational cooling. Overnight lows should dip into the upper 40s with locally cooler readings in typical cold spots. If drier guidance verifies (allowing for even more efficient radiational cooling), some lower 40 degree readings cannot be ruled out. Borchardt Saturday through Thursday: Primary forecast concerns are the potential for severe thunderstorms Tuesday and possibly isolated strong/severe thunderstorms Saturday afternoon/evening. The models are now showing a compact system moving across the local area Saturday afternoon/evening with a chance for thunderstorms. The ensembles are in reasonable agreement for this potential and there is some instability. There will be the potential for a few strong/severe storms during peak heating in the afternoon/early evening. Blended pops are generally in the low chance range but these may need to be increased if these trends continue. Sunday looks mainly dry and there is quite a bit of uncertainty for precip chances Sunday night into Monday morning with the Canadian/ECMWF fairly dry with only the GFS showing precipitation. After the wave on Saturday afternoon/evening and as the pattern shifts into early next week, this time period may end up being dry but made no changes to the blended pops which are likely too high, in the chance range, for Sunday night into Monday. By Monday afternoon into Monday night, convection is expected to develop well west of the area and possibly reach northwest IL by daybreak Tuesday and if it does, most likely in a decaying phase. With temps likely to reach 80 and dewpoints in the 60s on Tuesday, this will provide plenty of instability for additional thunderstorms to form Tuesday afternoon and evening and some of these thunderstorms may become severe. Precipitable water values look to reach into the 1.5 to 1.75 inch range, which will allow for heavy rain. The pattern stays unsettled into the end of next week with periodic chances for showers and thunderstorms, though there will also be several dry periods. cms && .AVIATION /18Z TAFS THROUGH 00Z SATURDAY/... Issued at 1237 PM CDT Thu May 2 2024 Key Messages: - Southerly wind shift expected at the terminals by 20z this afternoon. - Isolated to widely scattered late afternoon and early evening thunderstorms around the terminals after 21z. - A period of more widespread showers (and a few storms) expected mid to late this evening through early Friday morning. - West-southwesterly winds shift northerly by daybreak Friday, then northeastward through the day. A couple periods of active weather are expected later today and tonight as a storm system tracks north-northeastward from IA into the Upper Great Lakes. In the near term, a southerly wind shift is expected in the 19 to 21z timeframe as a surface warm front lifts northward across the terminals. Once this occurs, a warm and moist airmass will allow for the development of some isolated to widely scattered thunderstorms later this afternoon. While it remains a bit unclear if these storms will move over a given terminal late this afternoon, there is moderate confidence that a few will be around the area. For this reason, we continue to advertise this potential in a tempo group in the 22 to 24z timeframe. More widespread showers (with some embedded storms) are expected to develop/shift eastward across the terminals tonight as a cold front approaches. The best timing for the Chicago terminals looks to be in the 04 to 09z timeframe, and more in the 03 to 08z period at KRFD. A period of MVFR CIGs and VSBYs may also accompany some of the rain overnight. MVFR CIGs may linger for a few hours following the end of the rain late tonight, but conditions should improve back to VFR through the morning following the cold front. Winds will shift north-northwesterly late tonight following the cold frontal passage. Directions will then settle into a northeasterly direction during the day Friday. KJB && .LOT WATCHES/WARNINGS/ADVISORIES... IL...None. IN...None. LM...None. && $$ Visit us at weather.gov/chicago