Area Forecast Discussion
Issued by NWS Chicago, IL

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000 FXUS63 KLOT 011539 AAA AFDLOT Area Forecast Discussion...UPDATED National Weather Service Chicago/Romeoville, IL 1039 AM CDT Sat Oct 1 2016 .UPDATE... 1039 AM CDT The going afternoon and evening forecast is in very good shape. Satellite imagery indicates a pronounced area of jet-enhanced lift across northeast Illinois on the west-northwest side of the low center. This area of lift is forecast to weaken the next couple hours and have already seen the concentrated area of moderate to heavy rain weaken as well. Broad ascent will continue though associated with the closed low, and do think scattered showers and some pockets of drizzle will be seen into the afternoon. Going forecast for temperatures and winds is on track. MTF && .SHORT TERM... 307 AM CDT Through Tonight... The main focus for today is the timing of showers and thunderstorms with the next disturbance expected to impact portions of the area. Water vapor imagery this morning shows the center circulation of the upper low now centered over far southeastern Indiana. This continues to allow small scale perturbations to rotate westward along its northern periphery, namely over the lower Great Lakes. One such disturbance is currently driving a complex of showers, with some embedded thunderstorms over portions of northeastern Indiana. Believe it or not, these storms will be headed for portions of northeastern Illinois and northwestern Indiana this morning as they shift westward within the deep broad cyclonic flow over the region. Current timing of these showers and storms puts them in and around the Chicago area by around 14Z (9am) this morning. As a result, I have boosted POPs into the 70 to 80 percent range across the eastern CWA mid to late this morning. Although it is not a good diurnal time of day for thunderstorms this time of year, the northwestward influx of cold mid-level temperatures (500MB temps -18 to -20C) associated with the northward moving upper low, will support the presence of steep enough lapse rates for charge separation over the area. Therefore, I have also mentioned isolated thunderstorms in the forecast with these showers this morning. Additional showers and storms will remain a good bet (at least on a scattered basis) over the eastern half of the area into the early to mid afternoon hours as the main PV anomaly retrogrades northwestward towards the area. Forecast guidance continues to indicate that this will act to drive an inverted surface trough northwestward towards southern portions of the Chicago area by early afternoon. As a result, this will continue to act as a focus for showers and some storms through mid afternoon, especially over my southern and eastern CWA. Any lingering showers or storms should come to an end over the area by early this evening. However, there will remain a small chance for a few showers later Saturday night as another area of showers tries to drop southward over portions of northeastern Illinois. KJB && .LONG TERM... 326 AM CDT Sunday through Friday... The upper low that has been hanging out around the region for the past several days, and really outstayed its welcome, is finally expected to gradually shift towards the eastern Great Lakes by Sunday evening, then over New England on Monday. Attention will then begin to focus on another upper level trough, which is expected to dig over the western CONUS late Sunday and into Monday. The good news is that it appears that we will be in for some dry and warmer weather into midweek as the low-level flow turns southerly across the central CONUS ahead of this western CONUS trough. However, this system will shift gradually shift eastward over the Plains, with surface low pressure likely to shift over the Upper Midwest and into Ontario by mid to late week. This still looks to drive a decent cold front eastward over the area, sometime around Thursday. Showers and some storms will be possible with the frontal passage. Cooler weather is likely in the wake of this front for Friday into Saturday. KJB && .AVIATION... For the 12Z TAFs... The current cluster of showers and isolated thunderstorms extending through Northern Indiana into lower Michigan will continue westward this morning. Based on the track of the surface low the area of thunderstorms may reach GYY but then drift south of the Chicagoland terminals. Therefore have not carried TS at ORD/MDW at this point. But the main batch of showers in lower Michigan will also make steady westward progress as a leading shortwave moves through. The inverted trough axis will linger in the area this afternoon as additional energy swings east around the upper low. This will maintain at least scattered showers and possibly even a storm closer to the lake through the afternoon period. Feel the best bet for IFR conditions would be during the morning when the first batch of rain showers arrive. Either way, expect current MVFR to fill in more across the area with some mvfr vsbys as well. With the rain IFR is possible with occasional 2-4sm vsby. Current feeling for ORD/MDW that cigs will stay at 700-800 or higher. Low confidence exists for lower IFR conditions. The low will approach southern Lake Michigan, and this will result in lighter winds today, light and somewhat variable under the low in NW Indy, and more of a NNE direction across northeast and north central IL. Winds will slacken and eventually shift to a NNW area wide tonight as the low moves overhead then drifts back northeastward toward Lake Huron tonight into Sunday. Some lower cigs again appear likely with vis reductions possible too. KMD && .MARINE... 306 AM CDT An inverted low pressure trough will expand over the lake this morning as the center of low pressure moves to the southern tip of Lake Michigan. This will maintain an east-northeast wind today. Waves at the Wilmette buoy have settled in the 3.5- 4 ft range or so, and therefore will carry 3-5 occ 7 ft in the NSH and let the small craft Advisory go with the wind field weakening. Expect waves will hold in this range at least through the morning for the IL nearshore. The low will eject to the northeast toward Lake Huron tonight into Sunday which will finally allow high pressure to begin moving east, and will also let winds shift to more NNW. This will aid in decreasing the waves tonight into Sunday. The high will build across the northern and western Great Lakes Sunday night, and then it will shift to New England through the middle of the week. The next low will head north of the lakes mid to late week. KMD && .LOT WATCHES/WARNINGS/ADVISORIES... IL...NONE. IN...NONE. LM...NONE. && $$ VISIT US AT HTTP://WEATHER.GOV/CHICAGO (ALL LOWERCASE) FOLLOW US ON FACEBOOK...TWITTER...AND YOUTUBE AT: WWW.FACEBOOK.COM/NWSCHICAGO WWW.TWITTER.COM/NWSCHICAGO WWW.YOUTUBE.COM/NWSCHICAGO

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