Area Forecast Discussion
Issued by NWS Chicago, IL

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000 FXUS63 KLOT 261946 AFDLOT Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 246 PM CDT Tue Mar 26 2024 .KEY MESSAGES...
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- Scattered showers persist into this evening, with a chance of flurries overnight. - West-southwest winds remain blustery tonight, though gradually diminish into Wednesday. - Another period of active weather likely late week into a least early next week.
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&& .DISCUSSION...
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Issued at 246 PM CDT Tue Mar 26 2024 Through Wednesday Night: Early afternoon surface analysis displays deep 988 mb low pressure lifting north-northeast across Michigan`s upper peninsula toward Lake Superior. An occluding cold front extended south across Lake Michigan/lower Michigan into eastern Indiana, while a secondary cold front trailed into eastern Iowa. The low will continue to track off to the northeast into Ontario tonight in response to a deep mid- level short wave which will lift northward from IL/WI early this evening, while the trailing secondary cold front moves east across the forecast area. The thunderstorm and severe weather threat has moved east of the forecast area early this afternoon, as the occluded front has cleared the cwa. Ambient synoptic winds will remain blustery from the southwest with gusts reaching 35-40 mph at times within a strong low-level cold advection regime and the tight surface pressure gradient associated with the low to our north. These winds will become westerly tonight with the passage of the secondary cold front, and will remain blustery though will gradually diminish overnight into Wednesday. Temperatures have already fallen sharply into the low-mid 40s this afternoon behind the initial occluded frontal passage, and will continue to fall into the 30s behind the secondary cold front this evening. Overnight lows are expected to dip below freezing area- wide, from the mid-20s to near 30 degrees. Scattered rain showers will persist across the area through early evening, as the mid-level vort transits the area. GOES visible imagery depicts extensive low cloud cover upstream however, which is expected to linger into Wednesday morning before scattering out. Despite subsidence developing behind the short wave, a few flurries may fall from this cloud deck later this evening and overnight as the column continues to cool and low-level cold advection maintains steep low-level lapse rates. No accumulation is expected however, with the saturated cloud layer otherwise fairly shallow. Persistent dry, cold low-level advection should allow clearing skies by Wednesday afternoon. Despite the return of sunshine, temperatures look to only reach the low-mid 40s for afternoon highs (a good 5-10 degrees below our average lower 50s for late March). Winds will continue to add a chill to the air however, with westerly gusts still in the 15-20 mph range during the day. Another chilly night is then in store for the area Wednesday night, with sub-freezing overnight lows in the mid-upper 20s outside the core of the Chicago metro where readings will bottom out around 30 degrees. Ratzer Thursday through Tuesday: Broad mid-level ridging will cross the region Thursday through Friday night, bringing a period of warming conditions amid dry weather. A LLJ directed just west of the forecast area Thursday night will support increasing WAA with a stronger baroclinic zone across the forecast area. Moisture appears to be rather deficient for any notable precip chances, so have maintained dry conditions at this time. By Friday, the low-level baroclinic zone noted above will only slowly lift NE ahead of an open mid-level wave quickly moving through the departing and weakening ridge. While there remains some question on whether ample moisture will arrive in time ahead of broader forcing, the rapid transport of moisture within the LLJ combined with modest mid-level lapse rates suggests at least some elevated shower and storm chances across mainly the north half of the forecast area Friday night. Conditional upon an earlier arrival of higher moisture (e.g. Pwats over 1"), a localized heavy rain risk may exist with training storms. Beyond Saturday, guidance begins to diverge with solutions regarding the evolution of expansive ridging centered over the southeast U.S. and an upper-low attempting to cutoff across the Great Basin region. It does appear amplifying mid-level westerlies in response to a formidable trough digging SE from south-central Canada will yield a more active period of weather somewhere around or south of the region early next week. At this time, rain and thunderstorm chances appear highest Sunday night into Tuesday morning. However, the potential of any phasing of the northern trough with the upper-low to the southwest will likely continue to result in substantial model discrepancies for at least a few more days. Kluber
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&& .AVIATION /18Z TAFS THROUGH 00Z THURSDAY/... Issued at 1240 PM CDT Tue Mar 26 2024 - Period of -RADZ late afternoon/early evening with associated MVFR-near IFR VSBY/CIG reductions The earlier line of gusty showers has since moved over the lake and into northern Indiana. A few showers and patchy drizzle remain across northeast Illinois and northwest Indiana and may linger a bit beyond 18Z before clearing out. There is a separate region of rain and drizzle over the Quad Cities that will gradually push east across the area this afternoon. There remain some uncertainty as to how long the IFR VSBYS/CIGS persist with eastward extent. Opted to introduce TEMPO groups to account for this potential, beginning earliest at RFD-19Z, DPA-21Z, and ORD/MDW/GYY-22Z. Have hinted at some MVFR VSBY and low-end MVFR CIGS for the Chicago area terminals, though IFR can`t be ruled out if the drizzle is more persistent. After this period of lower CIGS, expect MVFR stratus to then persist the remainder of the overnight and linger into Wednesday morning before eventually scattering out. A few flurries can`t be ruled out during the pre-dawn hours Wednesday morning, but the potential remains too low to include in the TAFs. Winds will remain gusty through this afternoon out of the southwest with gusts up to 30kt. Winds turn more westerly as the area of rain and drizzle moves through late afternoon/early evening with gusts then easing some overnight. Petr && .MARINE...
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Issued at 246 PM CDT Tue Mar 26 2024 Southwesterly gales of 35-40 kts will persist this afternoon, before becoming westerly and gradually easing tonight. A Gale Warning remains in effect for the Illinois and Indiana nearshore waters through 7 PM this evening. A follow-up Small Craft Advisory will be needed into early Wednesday morning while winds and waves subside. Ratzer
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&& .LOT WATCHES/WARNINGS/ADVISORIES... IL...None. IN...None. LM...Gale Warning until 7 PM CDT this evening for the IL and IN nearshore waters. && $$ Visit us at weather.gov/chicago

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