Area Forecast Discussion
Issued by NWS Chicago, IL
Issued by NWS Chicago, IL
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974 FXUS63 KLOT 200629 AFDLOT Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 129 AM CDT Thu Jun 20 2024 .KEY MESSAGES... - Widely scattered thunderstorms are possible this evening mainly along and north of a Dixon to Waukegan line which could produce locally gusty winds. - Hot and muggy conditions will prevail through the end of the week, although cooler conditions are expected near Lake Michigan on Thursday and Friday. - Periodic chances for thunderstorms will continue, though most hours will remain dry. && .DISCUSSION... Issued at 252 PM CDT Wed Jun 19 2024 Through Tonight: Increasing coverage of cumulus this afternoon combined with deep mixing has helped keep heat indices mostly in check so far this afternoon with temperatures in the upper 80s to lower 90s. Expect a couple more degrees of warming yet this afternoon with peak heat indices in the 95-100 degree range. Dry conditions prevail at this hour; however, attempts at shower/storm development is already underway across portions of northwest Illinois into south central Wisconsin ahead of an approaching cold front. Expect this activity to gradually expand in coverage as it shifts southeastward into the Rockford area and along the Wisconsin-Illinois stateline. RAP mesoscale analysis highlights a narrow axis of 1000-1500 J/Kg of MLCAPE ahead of the front which intersects a region of DCAPE values to 1000 J/kg (inverted V soundings) with slow storm motions and little shear (~15kts) suggests that any storms that develop would be rather pulse-like and capable of strong downbursts resulting in locally gusty winds to 50 mph for areas mainly along and north of a Dixon to Waukegan, IL line. A very localized gust to 60 mph can`t be fully ruled out. Hit and miss isolated storm development still can`t be fully ruled out south of these areas into Chicago and south of I-80 but this potential remains low with most areas expected to remain dry. Accordingly will continue capping shower/storm chances at 20% in these areas. In the wake of the frontal passage over the lake, expect fog development over the lake overnight into Thursday morning, this could ooze inland particularly along the immediate shoreline. It is possible that it pushes further inland, per the latest RAP, but confidence in that scenario occurring is not especially high and will let the evening shift get another look at that potential. Petr Thursday through Wednesday: A frontal boundary will be meandering across the cwa on Thursday and will likely be the focus for at least isolated thunderstorms with perhaps the best coverage along/north of I-80. With northeast winds off of Lake Michigan, there may also be enough convergence for isolated thunderstorms in the Metro area as well. Overall confidence is low and current 30-40% pops north to 20% south seem reasonable for now. Some of this activity may linger into Thursday evening, but there should be a gradual decrease in coverage with most of Thursday night dry. High temps on Thursday may be back into the lower 90s for most areas, especially south of the boundary mentioned above. Lowest confidence for temps is generally from I-88 north and then across then near the lake. Temps will be coolest near Lake Michigan, perhaps only in the upper 70s/lower 80s right along the shore. Where temps do make it to the lower 90s, heat index values may be in the mid/upper 90s. Most recent trends in guidance would suggest that Friday into Friday night may end up mainly dry across the area. If any precip did develop, probably near the IL/WI state line Friday afternoon. Lowered pops to just slight chance (20%) on Friday and removed pops for Friday night. High temps on Friday are likely to be back in the lower to possibly mid 90s for most areas, except cooler near Lake Michigan with northeasterly winds. Heat index values would again be in the mid/upper 90s, away from the lake. Winds turn southwest Saturday and increase with gusts possibly into the 30 mph range, ending any cooling near the lake. High temps will reach the mid 90s for most locations but with dewpoints in the mid/upper 60s, heat index values will peak in the upper 90s to around 100. Thunderstorm chances will be increasing Saturday afternoon into Saturday evening as a cold front moves across the area. Best timing may be after sunset which may limit instability and coverage. Despite a larger scale feature, guidance still showing a range of possibilities from the potential for heavy rain to only scattered coverage. Blended pops are in the likely range (70%) and this seems fine from this distance, though duration is likely to only be a few hours at any one location. Precip also looks to be progressive with perhaps some lingering showers Sunday morning with most of Sunday likely ending up dry. Dry weather is then expected through at least Tuesday morning. By that time the models show another potential chance for thunderstorms sometime from Tuesday afternoon into Wednesday morning and while confidence is low, this has been a time period the models and their ensembles have been highlighting for potential thunderstorms. The frontal boundary Saturday night/early Sunday morning will bring slightly cooler and slightly drier air, but given peak sun angle and mainly sunny skies, high temps Sunday and Monday may still reach the upper 80s for most locations. cms && .AVIATION /06Z TAFS THROUGH 12Z FRIDAY/... Issued at 129 AM CDT Thu Jun 20 2024 Key Messages: - Potential for low clouds and mist/fog early this morning and again early tomorrow (Friday) morning. - Scattered showers and isolated thunderstorms expected this afternoon. Fog is expected to develop along the Lake Michigan shoreline overnight and may then creep inland and affect ORD, MDW, and/or GYY -- most likely as low stratus, though some visibility reductions can`t be ruled out from this either. Confidence in MVFR or lower conditions prevailing from this is currently low, so have continued to carry prevailing VFR conditions in the TAFs for now and will continue to monitor how things evolve. In addition, with the light winds, mostly clear skies, and moist boundary layer, patches of radiation fog may also develop inland and produce visibility reductions at DPA, GYY, and/or RFD before dissipating after sunrise. Slightly better large-scale forcing for ascent today will likely lead to a greater coverage of showers and storms this afternoon compared to yesterday. In particular, a frontal boundary that is expected to stall out in the area today may serve as a focus for convective development near the terminals. Thunderstorm coverage may still only be isolated, though, due to a mid-level warm nose inhibiting vertical cumulus growth, and the hit-or-miss nature of these "pulse"-like storms lends low confidence to any one TAF site being impacted by a storm as well, so have continued to keep any TSRA mentions confined to PROB30 groups for now. Needless to say, any deeper convection that develops this afternoon will likely produce sub-VFR visibilities and gusty winds. Shower and storm coverage will then dwindle this evening with the loss of diurnal instability. Similar to early this morning, the overnight into early Friday morning time period should also be characterized by light winds, mostly clear skies, and a moist boundary layer in the wake of this afternoon`s and evening`s showers. Thus, it is possible that patches of fog may develop near DPA, GYY, and/or RFD once again and that marine-induced fog and/or low clouds may also ooze inland and affect ORD, MDW, and/or GYY once again. Confidence in sub-VFR conditions being observed early tomorrow morning is currently low, though think that whatever happens this morning could be a harbinger for what might happen tomorrow morning. Ogorek && .CLIMATE... Issued at 347 AM CDT Wed Jun 19 2024 With a low temperature of 78 degrees, Chicago broke its warmest low temperature record for June 18th yesterday, while Rockford tied its daily warmest low temperature record of 74. A few additional temperature records (namely today`s warmest low temperature records and Saturday`s high and warmest low temperature records for Chicago and/or Rockford) may be threatened over the next few days. Here are the daily high and warmest low temperature records for Chicago and Rockford for each day through Saturday, June 22nd: Chicago ---------------------------------- Day: 6/19 6/20 6/21 6/22 ---------------------------------- Record High: 102 104 101 97 Record Warm Low: 78 78 74 76 Rockford ---------------------------------- Day: 6/19 6/20 6/21 6/22 ---------------------------------- Record High: 99 101 100 97 Record Warm Low: 76 73 71 73 Ogorek && .LOT WATCHES/WARNINGS/ADVISORIES... IL...None. IN...Air Quality Alert until midnight CDT /1 AM EDT/ tonight for INZ001-INZ002-INZ010-INZ011-INZ019. LM...None. && $$ Visit us at weather.gov/chicago