752 FXUS63 KLOT 162322 AFDLOT Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 622 PM CDT Mon Oct 16 2023 .SHORT TERM... Issued at 242 PM CDT Mon Oct 16 2023 Through Tuesday night... A broad upper trough continues to reside across the eastern CONUS with a couple of vort lobes passing over northern Illinois. These vorts have allowed for some very light lake effect showers/drizzle to develop this afternoon near the IL-IN line. While lake effect parameters continue to look modest, the showers are expected to linger into the evening before gradually coming to an end around midnight. However, as winds become westerly this evening the convergence axis will get shoved eastward which in turn will force the greater shower coverage into northwest Indiana. Regardless, it does appear that at least some lake effect cloud cover will persist through the night and therefore hold temperatures in the low to mid- 40s for most. Though a few spots in our west may see some upper 30s where cloud cover is starting to erode. Mid-level ridging is expected to build into the area on Tuesday which should aid in eroding the majority of any lingering cloud cover. However, forecast soundings are showing a rather stout moist layer trapped below the subsidence inversion which may allow low- level clouds to persist through Tuesday afternoon, especially near the lake. Given the uncertainty as to how quickly, if at all, the clouds will erode I have decided to go with a slower clearing in the forecast and also lowered Tuesday's high temperatures a couple of degrees into the upper 50s to around 60. But if clouds do erode quicker then temperatures may over perform and be solidly in the lower 60s. Never the less, dry conditions are expected for Tuesday and Tuesday night. Yack && .LONG TERM... Issued at 248 PM CDT Mon Oct 16 2023 Wednesday through Monday... A mid-level wave nearing Vancouver Island this afternoon will consolidate into a compact low in the lee of the northern Rockies mid-week before crossing Lake Michigan on Thursday. Broad upper- level diffluence above increasing mid-level moisture will support bands of elevated showers crossing the CWA late Wendesday afternoon and evening. While a decent amount of dry air below 10-15kft cloud bases will limit some initial precip, 600-300 hPa lapse rates around 7C/km do support deeper showers and perhaps lightning that will allow showers to reach the ground. The mid/upper-level low will then yield scattered showers across the CWA on Thursday amid blustery conditions. Shower coverage and intensity remain in question as a feed of mid-level dry air will begin to shrink saturated cloud depth through the day. Scattered light showers may persist well into Thursday night, with marginally favorable thermo profiles for lake effect rain showers continuing into northwest Indiana on Friday. A seasonably strong ridge centered over the Four Corners region will then produce strong NW mid-level flow across the mid- Mississippi River Valley this weekend as Thursday's low amplifies across the northeast CONUS. A wide range of solutions exists with the location and intensity of a pair of trailing mid-level waves Friday night through Sunday, which is not entirely surprising given the shear instability within strong mid-level flow this far out. So while the forecast is generally dry with seasonably cool conditions during this window, one or two periods of showers do remain a distinct possibility. Kluber && .AVIATION... For the 00Z TAFs... The main aviation weather concerns are: * Increasing potential for BR/FG tonight, particularly at DPA and RFD. Some potential at ORD, MDW, and GYY as well. * Somewhat uncertain cig trends on Tuesday. MVFR cigs possible in the morning and early-afternoon. The back edge of an expansive stratus deck continues to make steady eastward progress early this evening. Expectation is for cigs to gradually scatter at DPA, and then at ORD and MDW through the mid and late evening hours. Persistent onshore flow may help hold MVFR conditions at GYY overnight. With the clearing skies and slackening winds, the potential for BR/FG appears to be increasing, particularly at outlying TAF sites (DPA and RFD) tonight and have introduced a mention of BR with the 00z TAFs. Will continue to closely monitor guidance and observation trends, and offer up AMDs to show more pessimistic conditions should the need arise, including at ORD, MDW, and GYY. Cig trends on Tuesday are a little uncertain, with it possible that high-MVFR cigs develop with mixing, but get trapped under a lingering subsidence inversion. Confidence in MVFR is too low for a mention in the TAFs at this time, however. Otherwise, winds will trend westerly and then southwesterly through Tuesday before eventually becoming southerly or southeasterly Tuesday evening and overnight. Carlaw && .LOT WATCHES/WARNINGS/ADVISORIES... IL...None. IN...None. LM...Small Craft Advisory...IL nearshore waters until 10 PM Monday. Small Craft Advisory...IN nearshore waters until 4 AM Tuesday. && $$ 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