Area Forecast Discussion
Issued by NWS Chicago, IL

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000 FXUS63 KLOT 092016 AFDLOT Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 316 PM CDT Thu Jul 9 2020 .SHORT TERM... 257 PM CDT Through Friday night... The chances for some severe thunderstorms and heavy rainfall continue to be the primary concern for northern IL late this afternoon through the evening. Thunderstorm activity over our area has remained isolated to this point. However, thunderstorms have been on the uptick to our west over IA early this afternoon in advance of an approaching MCV over central IA. The MCV will track eastward into northwestern IL this evening, and over northeastern IL tonight. As it does so, it will continue to interact with an uncapped and unstable environment (1800 MLCAPE per the 18z DVN RAOB) thus leading to more numerous showers and storm development in a band or two over northern IL into this evening. The threat of severe thunderstorms will continue to be primarily focused on some isolated instances of damaging wind gusts over northern central, and perhaps parts of northeastern IL this evening. This threat should wane across the area later this evening and overnight. The other threat with these storms this evening into tonight is the threat for some periods of very heavy rainfall. Since the storms are expected to develop into north to south oriented bands, which will be nearly parallel to the steering layer flow, the potential exists for a period of training storms over northern IL this evening. This adds some concern for the potential for some areas of flash flooding as the rain rates in these storms could easily exceed 2" per hour. So, even though some of the area has been on the dry side lately, I cannot rule out this possibility especially in more urban areas. We have opted to hold off on issuing a flash flood watch, but this will need to be monitored closely for portions of northern IL this evening. Some showers and storms could linger over eastern IL and northwestern IL late tonight, but this activity should begin to shift out of the area Friday morning. Thereafter, it appears that Friday afternoon could be mainly dry across the area. Temperatures should also be cooler on Friday as some breezy northwesterly winds develop and usher in a bit cooler airmass. Expect high temperatures to be in the mid to upper 80s. KJB && .LONG TERM... 316 PM CDT Saturday through Thursday... Northwest flow expected over the weekend with some model discrepancies regarding potential northwest flow shortwave. NAM and GFS both have a more amplified shortwave (likely result of convective enhancement) with an attendant greater convective risk, though with differing timing (GFS Saturday night vs NAM Sunday). The ECMWF doesn`t really have any meaningful shortwave with the northwest flow this weekend and would support largely a dry weekend. While a more amplified solution cannot be ruled out, leaning toward the less amplified ECMWF solution. NBM pops were fairly aggressive for the weekend, so reverted back to previous shift`s lower pops. Certainly the NAM/GFS would support hanging on to these pops, and should these more amplified solutions verify, then pops would likely need to be bumped up once confidence in timing increases. Would appear the massive/intense heat dome over the central and western U.S. looks like it will briefly bulge northeastward into the area Tues/Wed time frame in advance of a northern stream shortwave trough. This should allow temps to rebound back into the 90s by Tuesday with NBM keeping 90s going the remainder of the week. Worth noting that if convection/convective debris were to linger during the afternoon, then there could be an unscheduled break in the 90 degree heat. Tightening geopotential height gradient between the strong upper ridge and northern stream trough will result in a corridor of stronger flow aloft, which could certainly support a ring of fire type pattern with attendant strong/severe convective threat on the northern flanks of the upper ridge. At this distance, hard to say where exactly this will set-up and if it will affect our CWA. Our best chances look to be in the midweek Tue-Thu time frame, which is where NBM pops were targeting the best chances. Beyond that time frame, medium range ensemble guidance continues to suggest the heart of the upper ridge will build northeast into the Midwest next weekend. If this pans out, an even more intense spell of heat would be possible at that time. - Izzi && .AVIATION... For the 18Z TAFs... Warm, unstable air mass over the area is prone to allow for isolated to scattered coverage SHRA/TSRA to develop this afternoon as they have the past couple of days. Most likely focusing mechanism is a weak lake breeze, though already seeing some storms develop just south of the WI/IL border. May need to amend to eventually include a short 1-2 hour long TEMPO for TSRA mid afternoon at ORD and MDW, depending on eventual development along the lake breeze. If coverage of storms is pretty good along the lake breeze, not out of the question that outflow could push the lake breeze westward and result in a wind shift to an easterly direction later this afternoon, but confidence is too low to include this in the TAF at this time. Potential for somewhat more organized SHRA/TSRA this evening as disturbance over IA moves eastward, with timing into immediate Chicago area terminals looking to be in the 01-04z time frame. In the wake of this activity look for a wind shift to northwest Friday morning with drier and more tranquil conditions expected Friday. - Izzi && .LOT WATCHES/WARNINGS/ADVISORIES... IL...None. IN...None. LM...None. && $$ Visit us at weather.gov/chicago Follow us on Facebook, Twitter, and YouTube at: www.facebook.com/NWSChicago www.twitter.com/NWSChicago www.youtube.com/NWSChicago

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