372 FXUS63 KLOT 180836 CCA AFDLOT Area Forecast Discussion...CORRECTED National Weather Service Chicago/Romeoville, IL 336 AM CDT Sun Jul 18 2021 .SHORT TERM... 330 AM CDT Through Monday... Quiet conditions prevail early this morning with light northerly to near calm winds and temperatures in the 60s. Can't rule out some patchy fog along southern Ford/Iroquois counties in the next few hours but so far visibility reductions are further south. Low-mid clouds are also on the uptick early this morning across west central Indiana into east central Illinois tied to some isentropic ascent along the 300K surface that is lifting into our southern zones. This should help limit the northern extent of lower visibilities. A moist airmass will continue slosh back west into the area through the day. Although it will be competing with subsidence aloft, low to mid-level moisture will be on the increase with modest instability (SBCAPE values to 1000-1500 J/kg) possible this afternoon which would suggest that if we are able to get a few showers to develop, some isolated thunderstorms will also be possible. A mid-level vort max will be in the process of exiting the area by late morning. There is not a lot of forcing for ascent aside from diurnal heating and a subtle axis of convergence that sets up somewhere near the I-80 corridor, potentially aided by the afternoon lake breeze. Accordingly, the favored areas for any shower/thunderstorm development would be mainly along and south of I-80. Storm motions look a bit faster than what occurred yesterday into northern Indiana but some locally heavy downpours will still be possible. Expect high temperatures today in the lower to mid 80s. Lake breezes should keep temperatures cooler in the 70s downtown and other areas along the lakeshore. Expect shower/thunderstorm coverage to diminish toward sunset with the loss of diurnal heating. Skies begin to clear out overnight with low temperatures in the lower 60s, potentially dipping into the upper 50s in our typical cold spots. With continued mostly clear skies and the upper wave beginning to shift to the east on Monday, temperatures look to warm well into the 80s inland and upper 70s expected along the lakeshore thanks to another afternoon lake breeze. All in all a pleasant summer day across Chicagoland. Petr && .LONG TERM... 209 AM CDT Monday night through Saturday... Generally dry conditions are expected through much of the upcoming week, although we continue to highlight a few periods featuring some (generally low) chances for showers and thunderstorms. The main message will be one of generally warmer conditions and increasing humidity values, especially towards the end of the week when more expansive low to mid 70 degree dewpoints look to return to the region. On Tuesday we'll mainly be monitoring a fast-moving shortwave which is forecast to slice across southern Ontario and Quebec through the day. Any coherent forcing for ascent with this feature is expected to remain well-removed from northern Illinois, but we'll likely feel some effect from this feature in the form of a surging lake-enhanced cold front. We continue to note some degree of variability in this frontal passage's timing, which could be as early as mid-afternoon Tuesday, or as late as Tuesday evening and the early-overnight hours. If the former timing is correct, areas across northeast Illinois and northwest Indiana may see a fairly robust temperature drop from the upper 80s into the lower 70s as winds shift sharply out of the northeast. Either way, this backdoor front will briefly send temperatures on Wednesday back below climatological normals with mid-upper 70s across lakeside locales and upper 70s/lower 80s inland. With high pressure parked to our south leading up to this period on Tuesday afternoon and evening, Gulf trajectories will remain squarely cut off, and continue to suspect the lower 70 dewpoints being advertised by the GFS and NAM are a bit too high. With this in mind, and given lingering warm air around 5 kft yielding a fair amount of capping through the day, was ok continuing to show a dry forecast with the frontal passage for the time being. After the brief cool-down on Wednesday, temperatures should pop right back into the upper 80s on Thursday and Friday to close out the week as 850 mb temperatures warm to around +17 C. During this period we'll actually begin opening up additional moisture transport from the Gulf as low-level flow turns southwesterly. We'll commensurately begin building instability and with fast northwest flow aloft helping to guide a series of mainly low- amplitude disturbances our way, the blended offering of slight chance to chance PoPs through next weekend looked reasonable with current guidance suggesting the best chances for showers and storms may arrive on Saturday. Carlaw && .AVIATION... For the 06Z TAFs... There are no significant aviation weather concerns this forecast period although a few showers in the vicinity of the C90 TRACON are possible later this morning and afternoon. A SCT-BKN VFR cloud deck around FL050 will continue to push westward across the terminals tonight. Generally light northeasterly winds will trend north to north- northwesterly but will quickly snap back northeasterly through mid-morning. Surface convergence in the vicinity of an inverted trough axis may serve as a focus of spotty shower development by late-morning and into the afternoon hours. Latest indications are that the main corridor of moisture and instability should remain just south of MDW/GYY, but a few showers (perhaps even with a few lightning strikes) may get close to the southwestern flanks of the C90 TRACON. Otherwise, VFR conditions look to prevail through the remainder of the extended ORD/MDW TAF windows. Carlaw && .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