Area Forecast Discussion
Issued by NWS Chicago, IL
Issued by NWS Chicago, IL
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823 FXUS63 KLOT 090555 AFDLOT Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 1255 AM CDT Thu May 9 2024 .KEY MESSAGES... - Showers and perhaps a few thunderstorms are expected through mid-morning. - Cooler weather arrives today, with periodic showers Friday night into the weekend. && .DISCUSSION... Issued at 246 PM CDT Wed May 8 2024 Through Thursday night: Today has been awfully pleasant for us so far with sunny skies and temperatures having reached the middle 70s to near 80 degrees area- wide. This is not the story across the rest of the Midwest, however. Early this afternoon, we find two centers of surface low pressure in the region: one subtle little system in northwest IA and a broad, more prominent low marching across the Ozark Plateau. The former is producing nothing more than light showers out west while the latter is responsible for a swath of severe storms extending from western MO into south-central Kentucky. These two systems will phase into one over the next several hours before moving into central IL this evening. This will bring showers and likely a handful of storms to the area this evening through the better part of tomorrow. We should begin to see isolated to widely scattered showers move into the SW CWA early-mid evening and progress across the rest of the area. With only a little bit of elevated instability squeezing into the CWA, thunderstorms look rather unlikely with this first little push any farther north than say the US Rt 24 corridor. More widespread activity will work in not long after midnight along the northern flank of the low as it passes through central IL. It looks like the overnight period will be our best chance at seeing embedded thunderstorms when several hundred joules of elevated CAPE look to build over parts of the area. The greatest thunder potential will exist in our western and southern CWA where we`ll find the most appreciable instability with notably steeper low-mid level lapse rates. The environment becomes less and less favorable approaching the lake. While CAPE values certainly won`t stand out tonight, most of it will be confined within the hail growth zone and we will have a respectable amount of mid level shear to work with, roughly 25-30 kt at 3-6 km. This means it`s not out of the question to see some small hail develop within any relatively stronger elevated cores. Some dry air in the low levels and a ~30 kt LLJ could also promote some gusty winds with any thunderstorm or heavier shower tonight. The instability will wane to the east early tomorrow pretty much driving the thunder potential out of here through the morning. Scattered showers are expected to continue through much of the afternoon before high pressure builds in and clears conditions up during the evening. A tight pressure gradient on the backside of the system will bring breezy winds to the area during the day. Between the cloudy, rainy conditions and cold advection, tomorrow will be noticeably cooler with highs forecast near 60 south of I-80 and stuck in the 50s farther north. Doom Friday through Wednesday: Shortwave ridging will produce stout mid-level warming through the day Friday, though low-level moisture will remain sufficient for a decent coverage of shallow cumulus during the afternoon. On the heels of the ridge, a compact upper-level low will dig southeast over the western Great Lakes Friday night into Saturday. Earlier guidance had advertised a diurnally favorable passage of the low directly over our area midday Saturday, but has since supported a faster solution with the low passing to the northeast around daybreak. Additionally, more favorable thermodynamic profiles with anomalously cold mid-level temperatures will remain to the northeast. Have therefore continued to cap PoPs in the chance range with the expectation that shower coverage will be no greater than scattered. Northwest flow aloft will persist Sunday into early next week. Another shortwave trough is progged to cross the area sometime late Sunday afternoon through Sunday night. Low-level moisture availability looks marginal at best for any widespread precipitation as a sprawling surface ridge remains anchored in the Deep South, but an earlier arrival of the trough could support isolated to scattered storms in our forecast area during the late afternoon and evening. Beyond Sunday night, substantial differences in guidance regarding the evolution of a building ridge across the eastern seaboard and a central CONUS trough support leaving the NBM`s slight chance and chance PoPs untouched from Monday through Wednesday. Kluber/Ogorek && .AVIATION /06Z TAFS THROUGH 12Z FRIDAY/... Issued at 1255 AM CDT Thu May 9 2024 Key Aviation Messages: - Area of -SHRA through mid-morning, with a low (10%) chance for an embedded TS through around 11/12Z. - Isolated to scattered -SHRA and MVFR ceilings through this evening. An area of SHRA with some TS across eastern Iowa early this morning is expected to drift E or ESE across northern Illinois through mid-morning. Ongoing TS should begin to favor a general SE trajectory along a NW to SE warm front toward central Illinois. However, a couple widely isolated TS cannot be ruled out into the Chicago metro roughly 09-12Z given remaining steep lapse rates between 10-20kft. If the downward TS trend is slower than expected, VCTS to the SW may be required for MDW/GYY. Beyond mid-morning, isolated to scattered -SHRA with MVFR ceilings are expected into this evening before precip coverage diminishes and ceilings begin to SCT by late tonight. Otherwise, NE winds will persist through today. Speed will diminish under 10 knots tonight while direction begins to settle between 350- 020, but should favor just east of north. Kluber && .LOT WATCHES/WARNINGS/ADVISORIES... IL...None. IN...None. LM...Small Craft Advisory from noon today to 3 AM CDT Friday for the IL and IN nearshore waters. && $$ Visit us at weather.gov/chicago