Area Forecast Discussion
Issued by NWS Chicago, IL
Issued by NWS Chicago, IL
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459 FXUS63 KLOT 091748 AFDLOT Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 1248 PM CDT Thu May 9 2024 .KEY MESSAGES... - Showers and thunderstorms this morning, with locally heavy downpours and perhaps some isolated wind gusts. Showers become more scattered this afternoon/tonight. - Breezy and much cooler today with temperatures in the 50s and 60s. - Periods of showers and possibly some thunderstorms early next week. && .DISCUSSION... Issued at 312 AM CDT Thu May 9 2024 Through Friday: Showers and some thunderstorms were blossoming across the area early this morning, in association with increasing forcing for ascent and warm/moist advection ahead of a mid-level short wave. This wave, the old mid/upper low circulation which has been hanging out over the northern Plains over the past few days, is shearing east and will eventually be absorbed into another mid- level trough propagating southward from the upper Midwest/northern Great Lakes. Showers and thunderstorms will continue to overspread much of the area early this morning (thunderstorms more likely west and south of the Chicago area, given the northwest-southeast orientation of the MUCAPE field across the southwest half of the cwa) with some locally heavy downpours for the morning commuter rush, before becoming more scattered/showery toward midday and into the afternoon hours as the best forcing shifts off to the east. Scattered showers will linger tonight as the combined upper trough axis continues to shift slowly southeast across the area, though should become less numerous and eventually north of I-80 overnight. Weak mid- level ridging briefly develops overhead on Friday, between the departing upper trough to our southeast and another digging southeast into the upper Mississippi Valley. This will allow for mainly dry weather during the day along with diminishing winds, though a fairly extensive fair weather cumulus field should develop with cool temps aloft. With the primary surface low tracking across downstate IL/IN, low level winds will shift from east-northeast to north during the day, becoming somewhat blustery with gusts around 25 mph at times. This will draw cooler air into the region, limiting high temperatures to the 50s across northeast IL and areas along Lake Michigan, and the low-mid 60s farther west and south. Temps will dip into the 40s tonight, with low-40s especially near the IL/WI border where cloud cover should decrease after midnight. Highs on Friday will rebound slightly to the mid-60s in most areas, though a weak lake breeze along the IL shore and into northwest IN will keep readings in the low 60s in those locations. Ratzer Friday Night through Wednesday: A compact upper-level low will dig southeast over the western Great Lakes Friday night and produce a period of showers primarily across the southeast half of the forecast area mid- evening into the overnight hours. Did not include a mention of thunderstorms in the current forecast, though steepening mid- level lapse rates on the back edge of the showers could support an embedded lightning strike or two. A broad upper-level low across far northern Ontario on Sunday will pivot around southern Hudson Bay and into northern Quebec through the middle of next week. A cold front associated with the low will eventually stall nearby or potentially over the forecast area on Monday. Meanwhile, a mid-level low over the Four Corners region this weekend will drift eastward across the central Great Plains and Ohio Valley then to the mid-Atlantic. This low should interact with the stalled boundary while drawing low-level moisture back northward into the area by Monday. While confidence in the exact evolution remains medium at best, particularly whether the low slows too much and becomes detached from the broader flow pattern, consensus forecasts support periods of showers and possibly some storms Sunday night into Tuesday night (highest chances on Tuesday). Kluber && .AVIATION /18Z TAFS THROUGH 00Z SATURDAY/... Issued at 1248 PM CDT Thu May 9 2024 Key Messages: - Another period of showers expected this afternoon. - Predominately MVFR CIGs into this evening, with a 30-50% chance for a period of IFR CIGs at the Chicago area terminals late tonight. - Lake breeze expected Friday afternoon, though confidence is low with it making it to ORD late in the day. While the rain early morning rain has moved out of the area, another batch of rain is already developing eastward into northwestern IL in association with the next mid-level disturbance. Expect this area of showers to overspread the Chicago area terminals after 19-20z, with periods of showers expected through the remainder of the day before the activity ends into early this evening (00-01z). MVFR CIGS will also persist the remainder of the day, but may improve a bit as the rain onsets this afternoon. CIG trends into tonight then remains a bit unclear. While a period of improving conditions is possible for a period mid to late this evening, the potential then exists for CIGs to trend back down again overnight into very early Friday morning, especially near the lake. As such, some terminals near Lake Michigan could experience a period of IFR CIGs after 09-10z tonight through early to mid morning on Friday. Confidence is not high with this period of low CIGs, but we have opted to hint at the opportunity with a few hour tempo mention for IFR CIGs in the latest TAF iteration. Any low CIGs will quickly improve through the mid morning hours of Friday. Wind trends also look to become tricky Friday afternoon as a lake breeze develops across far northeastern IL. It remains unclear as to how far this lake breeze will be able to make it through the afternoon, which makes a deterministic wind forecast, particularly at ORD, quiet difficult. If the lake breeze remains just east of the terminal, winds at ORD would likely trend to a west-northwesterly direction Friday afternoon, while areas just east of the boundary turn easterly. At this time, I have opted to only mention an eastward wind shift at MDW, where confidence is slightly higher. KJB && .LOT WATCHES/WARNINGS/ADVISORIES... IL...None. IN...None. LM...Small Craft Advisory until 3 AM CDT Friday for the IL and IN nearshore waters. && $$ Visit us at weather.gov/chicago