Area Forecast Discussion
Issued by NWS Chicago, IL
Issued by NWS Chicago, IL
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333 FXUS63 KLOT 210532 AFDLOT Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 1232 AM CDT Sat Sep 21 2024 .KEY MESSAGES... - Isolated to widely scattered showers and thunderstorms possible through early evening, mainly along and east of I-57 - Unseasonable warmth will continue through Saturday, with one last 90 degree day of 2024 possible - Periods of showers and some thunderstorms late Saturday night possibly continuing into Monday && .DISCUSSION... Issued at 325 PM CDT Fri Sep 20 2024 Through Saturday: Isolated to widely scattered showers and thunderstorms will continually try to develop through early evening ahead of a cold front for areas along and east of I-57. While forecast soundings continue to show around 30 kts of effective shear, the modest instability looks like it will limit the intensity of any storms that develop so the threat for severe weather appears to be decreasing. Therefore, expect any storms to remain sub- severe with the main threat being lightning and perhaps a localized wind gust of 30-40 mph. Showers and storms will taper by 7 PM this evening as the aforementioned cold front exits into north-central IN. As a result, rain-free conditions are expected overnight as much drier air moves in. Though, there is the potential for some patchy fog to develop late tonight into Saturday morning due to the combination of strong radiational cooling and light winds. While dew points are forecast to diminish overnight, it looks as if some residual moisture will linger near the aforementioned front in northwest IN and adjacent areas of eastern IL. Since forecast soundings show the moisture being rather shallow confidence is lower on coverage and intensity of fog especially with westward extent. Nevertheless, felt the potential was sufficient to warrant the introduction of a formal patchy fog mention to the forecast mainly in northwest IN and far eastern IL. Any fog that does develop tonight will erode by mid-morning on Saturday. So expect another partly sunny and unseasonably warm afternoon with highs once again forecast to top out in the upper 80s to around 90 degrees. The exception, however; will be for areas along the northern IL lakeshore which will see highs in the upper 70s to lower 80s due to onshore winds and a lake breeze. Heading into Saturday night, another weather system is expected to develop across the central Plains as a broad upper trough ejects out of the southwest CONUS. This weather system will then begin to move into northern IL (and eventually northwest IN) late Saturday night into Sunday resulting in another period of showers and thunderstorms with beneficial rainfall for our ongoing drought. Yack Saturday Night through Friday: Late Saturday into Sunday a closed mid-upper low will shift east- northeastward from the Desert Southwest out across the central Plains. As this occurs, a lead impulse emanating from this low is expected to foster shower and thunderstorm development across much of the Corn Belt late Saturday into Saturday night as it ejects out across the Mid-Missouri Valley in tandem with a southeastward shifting cold front. We are likely to see this activity shifting/developing into our local area Saturday night into Sunday. Favorable deep moisture pooling along this frontal boundary should favor some beneficial rainfall for the area, though possibly enough to be an overall drought breaker. The threat for showers and thunderstorms will continue Sunday night into Monday, though the highest chances by Monday look to be sagging south of I-80 with the surface frontal boundary. Following the departure of this early week system, mean long wave upper troughing and northerly lower-level winds across the Great Lakes will foster a much cooler, more seasonable type pattern across our area. Accordingly, expect daily high temperatures to primarily be in the middle 70s following early morning lows in the 50s. KJB && .AVIATION /06Z TAFS THROUGH 12Z SUNDAY/... Issued at 1233 AM CDT Sat Sep 21 2024 - Dense fog over northwest IN, including GYY, until just after sunrise - Lake breeze expected to result in wind shift to easterly (ORD/MDW) and northeasterly (GYY) this afternoon - SCTD SHRA chances will increase this evening for RFD and late evening into the overnight at immediate Chicago area terminals Dense fog bank over northwest Indiana is expected to persist and likely expand westward into far eastern Illinois until quickly burning off just after sunrise. Suspect urban heat island warmth will keep MDW warm/dry enough to limit threat of fog, but will need to monitor for a brief period of fog around sunrise. Otherwise, VFR conditions today, before some SCTD SHRA begin affecting northern Illinois tonight. Initially, highest chances are west in the RFD area this evening, but spreading east into the immediate Chicago area terminals as well by late this evening into the overnight. Light/variable winds should become light southerly this morning, but lake breeze is likely to develop and move inland resulting in a wind shift to easterly at MDW and ORD later this afternoon. - Izzi && .LOT WATCHES/WARNINGS/ADVISORIES... IL...None. IN...Dense Fog Advisory until 9 AM CDT /10 AM EDT/ this morning for INZ001-INZ002-INZ010-INZ011-INZ019. LM...None. && $$ Visit us at weather.gov/chicago