809 FXUS63 KLOT 140549 AFDLOT Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 1249 AM CDT Wed Jun 14 2023 .UPDATE... Issued at 906 PM CDT Tue Jun 13 2023 Going forecast is generally on track this evening, though did make some tweaks to pops/temps based on radar and surface obs trends since late afternoon. Have seen some more intense rainfall rates and amounts (half inch to in excess of an inch) across parts of Cook county and into northwest Indiana, though rainfall will be tapering off and ending from the north over the next few hours. RAP mesoanalysis data depicts a sharp low-level thermal/moisture gradient across southern WI, northeast IL and northern and central IN early this evening. The latest (and last substantial) in a series of vort lobes was wrapping cyclonically southeastward across the area within the western periphery of an upper low centered near southern Lake Huron. This vort lobe has focused forcing along the baroclinic zone across northeast IL/northwest IN this afternoon and evening, within an axis of 1.20"+ pwats and weak instability to produce areas of light/moderate rain and even brief locally heavy rates during the late afternoon/early evening hours. The mid-level vort lobe is progged to propagate off to the southeast of the forecast area through midnight however, allowing a southeastward shift and gradual end of rain. Did raise pops a bit through mid-evening for areas south of I-80 and into much of northwest Indiana as this occurs, with the bulk of the area dry after midnight. Low clouds do look to linger into Wednesday morning however, before the upper low slowly drifts far enough to our east to allow improving conditions later in the day. Temps should settle into the mid-upper 50s across the area overnight. Other than aforementioned tweaks to pops/rain coverage and hourly temp trends this evening, going forecast has a decent handle on things, and no other changes are warranted at this time. Ratzer && .SHORT TERM... Issued at 315 PM CDT Tue Jun 13 2023 Through Wednesday night... Key Forecast Messages: * Unseasonably cool with (mainly light) rain/showers northeast 1/3 of the area coming to an end in the evening * Canadian wildfire smoke returns on Wednesday with some haze possible The very well defined and unseasonably deep closed mid-upper low centered near Lake Huron this afternoon will slowly gyrate eastward through Wednesday. A lead vorticity maxima on the western side of the expansive circulation of the closed low generated the area of primarily light rain with occasional embedded moderate rates over northeast Illinois and northwest Indiana. Another vort max dropping south along the WI shore of Lake Michigan looks to enhance/reinforce some of the rain into early this evening for generally the same area that has observed rain this afternoon, so again primarily the northeast 1/3 of the CWA. Primarily light scattered showers will extend back a bit to the west and southwest of the coherent shield of light rain. Rain/showers will then gradually come to an end the remainder of the evening, for a dry overnight for most locales, and some breaks in the clouds, especially west. Forecast lows are in the mid to upper 50s. Wednesday will be warmer and trending sunnier with time, though likely filtered a bit by the return of upper level smoke from Canadian wildfires, as indicated by RAP/HRRR smoke forecasts. In addition, some boundary layer haze may occur from portions of northwest Illinois southward to portions of east central Illinois, per the near-surface smoke progs from the RAP/HRRR. Can't rule out an isolated/stray shower Wednesday afternoon as mid to upper 50s dew points pool inland of the lake breeze. However, with little upper level support and convergence, will maintain a dry forecast officially. It will be a noticeably warmer day, especially west (locally mid 80s near/west of I-39) though still a bit below normal with eastward extent (70s to near 80), and onshore winds keeping lakeside locations primarily in the 60s for highs. Wednesday night will be tranquil with lows in the 50s to near 60. Castro && .LONG TERM... Issued at 315 PM CDT Tue Jun 13 2023 Thursday through Tuesday... By Thursday, the upper-level low that is currently bringing us our April-like weather will be over New England and the Canadian Maritimes, and another shortwave trough will scurry southeastward across Ontario Wednesday into Thursday to fill the void over the Great Lakes behind this upper low. This will help drive a cold front southward into our forecast area on Thursday, which will likely end up being "lake-charged" to some extent given the relatively cool water temperatures present across Lake Michigan (low 50s across interior portions of the lake and near 60 along the Illinois and Indiana lakeshore). The timing of this frontal passage will play a key role in dictating what Thursday's high temperatures will be, but the majority of recent guidance suggests that the frontal passage will be late enough to allow most locations in our forecast area to reach the 80s, with low 90s not entirely out of the question either for some. If this were to indeed occur, then expect temperatures to quickly come crashing down behind the lake-charged front as the post-frontal lake- influenced air mass surges inland. There is a lower-end chance that some spotty showers and perhaps a storm or two could develop along the front as it progresses inland, but most, if not all, shower and storm activity should remain to our east where the core of the forcing support aloft associated with the shortwave will be. Cooler and drier conditions can be expected on Friday in the wake of the frontal passage, along with primarily cloud-free skies. Though, with flow through much of the column carrying a northerly component behind the front/shortwave, we'll need to keep an eye on whether a renewed incursion of Canadian wildfire smoke will result in yet another period of milky/hazy skies heading into the weekend. Our weather for the weekend and into early next week will primarily depend on a shortwave that is progged to eject out of the Rockies into the central Plains on Friday and to what extent ridging/high pressure over the Midwest/Great Lakes can quash its eastward progression. 12Z deterministic and ensemble guidance was in fairly good agreement that most or all of our forecast area should remain dry now on Saturday, and the NBM PoPs have responded to that trend, though they still came in with some slight chance and low end chance probabilities due to some of the NBM's time-lagged members. Have kept these for now, but suspect that most or all mentionable PoPs on Saturday will need to be removed one way or another if these recent forecast trends continue to hold in forthcoming guidance. Precipitation chances for Sunday into Monday are still less certain with the latest deterministic runs of the ECMWF and GFS still keeping us dry in spite of some continued support for precipitation during this time frame in the 12Z EPS and GEFS. Left the NBM's chance PoPs for Sunday and Monday untouched for now given the existing spread in guidance, but suspect that they are probably overdone. Ogorek && .AVIATION... For the 06Z TAFs... * MVFR cigs tonight through mid morning for ORD/MDW/GYY and at times DPA (some dips to IFR possible) * Light NW winds overnight, may lean light NNE at ORD/MDW at times - lake breeze turns winds east in the afternoon While the rain has ended across the area, low stratus remains overhead as it wraps around the slow to exit upper low. Currently have MVFR cigs overnight through mid morning for the Chicago area terminals. Some lake adjacent observations in Wisconsin and in Indiana show IFR and even LIFR stratus is out there in spots. It is possible some of this tries to drift inland into Illinois though confidence in this occurring is too low to add in the going TAFs. Observational trends will be closely monitored. Think perhaps the best chance of seeing IFR would be closer to the lake (GYY). Ceilings improve mid to late morning as the edge of the stratus slowly shifts to the east. Winds remain light overnight favoring a NW to NNW direction, especially further inland. ORD/MDW may lean NNE at times toward daybreak though speeds should remain generally under 10 kt. A lake breeze will push inland this afternoon (timing unchanged with this update) turning winds more consistently easterly. Winds then turn light and variable overnight eventually favoring a southwest direction toward daybreak. Petr && .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