481 FXUS63 KLOT 060148 CCA AFDLOT Area Forecast Discussion...CORRECTED National Weather Service Chicago/Romeoville, IL 848 PM CDT Sun Sep 5 2021 .UPDATE... 845 PM CDT There are a few showers along and ahead of the cold front in southeast Wisconsin along with a an embedded thunderstorm or two. These are focused in a narrow axis of limited instability where there is a subtle increase in moisture. This axis should continue to shift over southern Lake Michigan and into lower Michigan state. Closer to home, 0z forecast soundings depict a moisture starved and capped atmosphere, indicating that in spite of the sharp shortwave across the western Great Lakes, we do still envision a largely dry frontal passage tonight. KMD && .SHORT TERM... 142 PM CDT Through Monday night... The main focus through Monday night will be the passage of a cool front tonight. Gusty afternoon winds will ease this evening. The cool front across southern Minnesota and central Wisconsin this afternoon will reach the IL/WI border by midnight then continue south across the remainder of the forecast area overnight. Winds will turn northerly with the passage of the front. Should see a modest increase in clouds with the front and perhaps a sprinkle or isolated shower but am thinking those chances are pretty minimal so will carry a dry forecast. Overnight lows will fall into the mid to upper 50s most areas and around 60 in the Chicago metro. High pressure will move across the area Monday resulting in a pleasant day with highs in the lower 80s for most areas. The exception will be areas within a few miles of Lake Michigan which will be held into the mid to upper 70s thanks to a lake breeze. Winds will turn southerly Monday night allowing for warn advection to commence. This will lead to milder overnight lows in the lower 60s. MDB && .LONG TERM... 222 PM CDT Tuesday through Sunday... The period will begin on the active and warm side on Tuesday, with gusty winds and a potential for strong to severe thunderstorms during the day in response to a strong storm system moving across the Upper Great Lakes. Thereafter, a period of quieter and cooler weather is expected for much of the remainder of the week. An upper level impulse, and an associated upper level speed max, currently over the northeastern Pacific into the Pacific Northwest is expected to be our main weather maker on Tuesday. Forecast guidance remains in agreement that this feature will dig southeastward into the Upper Midwest on Tuesday thanks to a strong 115 kt upper jet shifting around the northeastern periphery of a building western CONUS upper ridge. The strong dynamics likely to accompany this system will support a sub 995 mb low either near Lake Superior or into southern Ontario Tuesday afternoon. While this will be well north of our latitude, a regional wind response is likely with such a storm system. The net result will be a rather windy day (southwesterly wind gusts of 35+ mph) across our area on Tuesday due to rapidly falling surface pressures occurring ahead of the frontal trough. Warmer temperatures into the middle to upper 80s are also likely on Tuesday ahead of the approaching cold front. This front will move southeastward over the area Tuesday afternoon. The main concern then shifts to how much shower and thunderstorm activity is able to develop along this front during the day. The best Gulf Moisture is likely to be cut off from our area due to a lingering surface ridge of high pressure over the Gulf Coast. However, guidance suggests that a corridor of lower to middle 60s dewpoints may emerge ahead of the front Tuesday afternoon, potentially supporting MLCAPE values of 1500 J/KG. Capping may be a problem initially on Tuesday, but the thought is that as mid- height falls (upper level cooling) begin to overspread the area during the afternoon that capping will weaken sufficiently to allow for at least some scattered storm development across the area. Should any storms develop, shear profiles will be favorable to support storm organization and possible severe weather from damaging wind gusts during the afternoon and early evening on Tuesday. The threat of storms will come to an end from northwest to southeast early in the evening as the cold frontal passage occurs. Our forecast area will remain in northwest flow aloft from Mid to late week. This will result in cooler and drier conditions across the area. Wednesday may still be somewhat breezy as the deep surface low pulls away to the northeast of the Great Lakes, with cooler temps in the 70s expected to last through the end of the week. Mainly dry weather is expected, though some diurnal showers can't be completely ruled out if a more vigorous short wave ripples through the northwest flow aloft at some point. Ensemble guidance does indicate a weakening of the northern periphery of the western upper ridge heading into next Saturday, with some evidence of a somewhat more amplified short wave moving through the upper Midwest. This may result in another rain chance for the area Saturday along with some moderation in temps, though confidence continues to be low enough at this distance to maintain NBM blended dry pops. KJB/Ratzer && .AVIATION... For the 00Z TAFs... Concerns: - Wind directions on Monday behind tonight's cold front - Timing the lake breeze A cold front across WI will shift across the airspace later tonight in the 5z-8z time frame. Winds will shift to the northwest behind the front. Precipitation is almost non existent along the front, thus expect a dry frontal passage. Weak high pressure across the region in its wake will result in generally light winds, though model guidance is mixed as to whether we hold a northwest or flop to a light northeast before the lake breeze. Either way speeds should be under 6-7 kt. A lake breeze will pass through ORD/MDW in the afternoon. Current TAFs have this pegged around 19z, though there are several short term guidance sources that bring it through a bit later. We have held the current timing given the light overall wind field in place tomorrow under the surface high, but timing is currently medium confidence. Expect winds to briefly tag 9-10 kt with and behind the lake breeze. KMD && .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