Area Forecast Discussion
Issued by NWS Chicago, IL
Issued by NWS Chicago, IL
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746 FXUS63 KLOT 141110 AFDLOT Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 610 AM CDT Wed Aug 14 2024 .KEY MESSAGES... - Period of showers and some t-storms likely late tonight into Thursday morning. - After a likely break in rain most of Thursday afternoon, another round of showers and t-storms is possible Thursday evening, with some threat for severe weather. - Highly conditional chance for strong thunderstorms toward northwest Indiana and east-central Illinois Friday afternoon. && .DISCUSSION... Issued at 257 AM CDT Wed Aug 14 2024 Through Thursday Night: Despite increasing cloudiness this afternoon, today should be a warm day, though with comfortable humidity. A lake breeze will provide for some modest cooling near the lake. Guidance has trended slower with onset of rain tonight and it now looks like most of the area should squeak out a warm and dry evening. Still a strong signal in guidance that an MCS or two will develop this evening across IA into southern MN, though likely developing farther west than guidance suggested last night. This activity should gradually spread eastward during tonight, with some showers and perhaps a couple of thunderstorms spreading into our western CWA during the early part of the overnight hours, then possibly getting as far east as the Chicago area by sunrise. By late tonight, the low level jet is progged to veer and begin weakening, resulting in weaker moisture convergence. In addition, as the convection moves east of the Mississippi River it will encounter an increasingly stable air mass. This should result in a weakening MCS as it moves across the CWA late tonight into Thursday morning with chances of thunder dropping off substantially over eastern IL and northwest IN. Still appears as though Thursday afternoon will largely be devoid of convection as convectively enhanced shortwave moves east of the area and warm air advection weakens. Surface low will track northeastward into western Wisconsin Thursday afternoon into the early evening which should drag the synoptic warm front northeast across our CWA with a much more humid air mass overspreading the area. Meanwhile, westerly mid level flow should advect a modified elevated mixed layer and steeper lapse rates east into the CWA during the afternoon and evening. How warm temperatures get Thursday will hinge on how much skies clear out. Best chance of clearing appears to be western CWA where temps should be able to climb into the 80s. Confidence in how warm temps get farther east into the CWA decreases, though with guidance trending less impressive with decaying morning MCS, perhaps temps could overperform. Combination of steepening mid level lapse rates, substantial increase in boundary layer dewpoints, and at least some heating should result in moderate to strong instability developing by late afternoon. Forcing looks nebulous at best during the afternoon, so unless some unpredictable at this distance outflow boundary kicks something off, the chance of convection in our CWA during the afternoon looks low. Thursday evening guidance is in good agreement in digging a developing/deepening closed mid-upper level low east-southeast into the upper Great Lakes. DCVA in advance of this system will result in strengthening synoptic ascent Thursday evening across the area. Convergence along the weak approaching cold front is very unimpressive, so it isn`t terribly clear where, when, and even if convection will redevelop Thursday evening. However, if it does, strengthening kinematic fields associated with the tightening geopotential height gradient with the upper low should result in increasingly favorable deep layer shear profiles. So if/where we get sufficient boundary layer destabilization and if/where convective initiation occurs, the forecast shear profiles would support a threat of severe thunderstorms and even supercells. The synoptic forcing looks to arrive mostly after dark in our area, but with such high dewpoints, boundary layer based convective inhibition should be rather slow to increase tomorrow evening. All that being said, confidence is low, but there certainly is a conditional threat of severe weather in our CWA Thursday evening and potentially even into the early overnight hours. Any showers and storms that do occur, should end late tomorrow night as weak cold front and trough axis shift east of our CWA. - Izzi Friday through Tuesday: The mid-level low responsible for the active period of weather on Thursday will slow over northern Wisconsin by Friday morning. An associated surface front is expected to cross the forecast area Friday morning into the afternoon. Meanwhile, guidance is in decent agreement that a embedded trough axis will swing across southern Wisconsin and into northern Illinois/Indiana Friday afternoon and evening. While the kinematic profile and modest mid-level lapse rates look favorable for strong storms, dry-air advection behind the front should greatly diminish thunderstorm coverage and intensity. However, any slowing of the front will result in increasing strong to severe storm chances across the southeast CWA Friday afternoon. The upper-level low will drift eastward toward Lake Huron through Saturday night. Forecast guidance has gradually trended drier with wrap-around precip on Saturday, especially with substantial capping developing above low-level CAA. While some diurnally-driven isolated to scattered shallow showers cannot be ruled out Saturday afternoon and evening, thunderstorm chances look to be extremely low. Beyond Saturday, latest trends continue to suggest that Tropical Cyclone Ernesto will remain far enough east that any blocking or Fujiwhara effects with the upper-level low should also remain well east of the forecast area. This favors a generally quiet and seasonable period Sunday into Monday as a weak ridge shifts across the western Great Lakes. Overall ensemble guidance does suggest that the impacts of the tropics will result in a highly amplified pattern and loose omega block across North America toward the middle of next week. While this could place us in a typically favored area for active weather on the eastern edge of the ridge, moisture return looks to be questionable while the better kinematic environment remains displaced well north into southern Canada. Kluber && .AVIATION /12Z TAFS THROUGH 18Z THURSDAY/... Issued at 609 AM CDT Wed Aug 14 2024 VFR conditions are expected through tonight. SE winds around 5 knots will persist through this morning as a surface ridge continues to drift northeast of the area. Expect a lake breeze with E winds 10 to 15 knots to cross ORD/MDW early this afternoon before settling back to SE under 10 knots this evening. A line of decaying SHRA with SE gusts around 20 knots should affect the terminals late tonight and well into the morning Thursday. Included a PROB30 TS at ORD/MDW from 12-15Z, though coverage may be isolated at best as the precip will be moving into a substantially less favorable environment with eastward extent. Kluber && .LOT WATCHES/WARNINGS/ADVISORIES... IL...Dense Fog Advisory until 8 AM CDT this morning for ILZ005- ILZ006. IN...None. LM...None. && $$ Visit us at weather.gov/chicago