Area Forecast Discussion
Issued by NWS Chicago, IL

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000 FXUS63 KLOT 251922 AFDLOT Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 222 PM CDT Mon Sep 25 2017 .SHORT TERM... 222 PM CDT Through Tuesday... Looking at hot but quiet conditions through this afternoon. Cumulus clouds have sprouted across the region, but not expecting any precip given 30 degree dewpoint depressions at the surface. A lake breeze is pushing inland lowering 90 degree temps into the upper 70s to low 80s. Low temps tonight will be in the mid 60s, while Chicago will be around 70. Winds become southwest ahead of a front Tuesday morning, and cloud cover increases. Kept high temps in the upper 80s, but cloud cover will likely limit warming. 90s are more likely along and east of I-55 where clouds will take longer to arrive. I have low confidence in measurable precip Tuesday as the front moves through the region. The main upper level wave will separate itself from the front as the wave shifts northwest over Minnesota. Forcing along the front is forecast to weaken as the front moves through. When I also factor in the recent dry spell, I think showers and storms will struggle to form. The best chance for showers and storms will be around Rockford Tuesday afternoon, but isolated showers and storms may occur from a Waukegan to Amboy, IL line. Severe storms are not expected as CAPE will be less than 500 J/kg and shear will be around 30 kt. JEE && .LONG TERM... 222 PM CDT Tuesday night through Monday... A return to more seasonably average temps is expected mid-week, before a brief push of colder air arrives Friday and lingers into the start of the weekend. Other than fairly minimal chances of showers with the two cold fronts (Tuesday night and early Friday morning), our dry spell looks to persist through the coming week. However, a few lake effect showers may bring some spotty rain mainly to parts of northwest Indiana with the cooler weather at the end of the week. Guidance remains in good agreement in moving a cold front east across the forecast area Tuesday evening/overnight. While the presence of a boundary moving into our unseasonably warm air mass would suggest the threat of showers/thunderstorms, the mid-level short wave associated with low pressure moving through the northern Great Lakes is well to our north, with the stronger and better focused forcing for large scale ascent. In addition, while surface dew points pool in the mid-60`s ahead of the front, southwesterly winds aloft maintain deeper moisture transport well north of the cwa. Forecast soundings indicate relatively weak lapse rates above the immediate boundary layer as well, yielding weak instability with a bit of weak capping as well. Earlier sunset times and loss of this diurnal instability will also be working against significant convective shower/thunderstorm coverage, and model QPF is light and decreasing with eastward progress of the front as well. All of this points to maintaining only low/slight chances pops along the front for Tuesday evening/night. Cooler air spreads in behind the front on Wednesday and Thursday, as high pressure ridge spreads east/southeast across the area. Low level thermal fields support daytime temps in the low-mid 70`s Wednesday with decreasing cloud cover on breezy north-northwest winds. Mainly clear skies and light winds in the vicinity of the surface ridge axis will likely allow for fairly cool min temps across parts of the forecast area dipping into the low-mid 40`s by sunrise Thursday. NAM depicts dew point temps in the mid-upper 30`s across northwest/north central IL within the surface ridge axis early Thursday, suggesting some cooler spots may be possible there. Thursday should see temps similar to Wednesday. With the cooler air over warm lake waters, lake effect stratocu is likely downwind of Lake Michigan into northwest Indiana, through shallow convective cloud depth and dry air may not support precip with this first push of cooler air. Medium range guidance solutions continue to indicate another digging mid-level short wave will amplify the trough across the Great Lakes region Thursday night, with a reinforcing push of colder air arriving behind an associated cold front which arrives very late Thursday night/early Friday morning. Forecast soundings depict fairly dry thermodynamic profiles with rain/shower potential looking fairly minimal with the front, and little/no QPF generated. While guidance has trended a little more progressive/less closed off with amplifying mid-level trough, H8 temps still drop into the +3 to +4C range across the region. This sets the stage for daytime temps only in the 60`s Fri-Sat (and perhaps only low-mid 60`s in many spots Saturday). Colder air aloft should support lake effect cloud cover into northwest Indiana and perhaps some showers, as lake surface to H8 delta-T increases to 15/16C, and convective cloud depth is significantly deeper. Flow turns more northeasterly Saturday as surface high builds in from the northwest, which may bring some lake clouds into northeast IL. With amplified but progressive upper pattern indicated, moderation in temps occurs back to around average for early October for Sunday and Monday. Return flow becomes more established across the Plains and Mississippi late in the period, which along with moderation in temps would potentially bring the next chance for more organized rain Monday, or more likely Tuesday. Ratzer && .AVIATION... For the 18Z TAFs... Light winds and VFR conditions are being reported across the area on the backside of departing high pressure. VFR CU will continue to form this afternoon. A lake breeze is pushing inland, and looks like it will reach ORD and MDW btwn 21Z and 22Z. I have medium- high confidence in when the lake breeze will turn winds easterly at ORD and MDW. At least the initial push of wind should be around 10 kt. Winds become southwest ahead of a front Tuesday morning. VFR clouds increase, but I have low confidence in precip occurring. The main wave of upper level energy will lift northwest over Minnesota and southern Ontario. Meanwhile energy along the front is forecast to weaken as it moves through the region. Throw in the dry spell, and I think showers will struggle to form. However, if any isolated showers or storms form, they will be near RFD after 18Z. I doubt that any of the eastern sites will receive precip. JEE && .MARINE... 222 PM CDT The western edge of a high pressure ridge will extend over the western Great Lakes while a front will extend from northwest WI through western IA. Southerly winds will become northwest to north 15-25 kt behind the cold front Tuesday night. A small craft advisory may be needed for the IN nearshore waters. Another strong push of northerly winds is expected Friday morning behind another cold front. While low end gales will be possible, northerly winds will be at least 25-30 kt through Friday evening, and a small craft may be needed. High pressure will spread over the lake Saturday. Winds will diminish and become southernly by Sunday morning. JEE && .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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