Area Forecast Discussion
Issued by NWS Chicago, IL
Issued by NWS Chicago, IL
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948 FXUS63 KLOT 200046 AFDLOT Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 746 PM CDT Wed Jun 19 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 /00Z TAFS THROUGH 06Z FRIDAY/... Issued at 746 PM CDT Wed Jun 19 2024 - ISOLD SHRA in VC of Chicago terminals, with locally variable/gusty winds, will fade by around sunset - Wind shift to light northwest, then light northeast expected tonight - Northeast winds expected to continue Thursday - SCT TSRA expected, mainly west of ORD/MDW Thursday afternoon ISOLD SHRA in VC of the Chicago terminals should dissipate by around sunset, but in the meantime small scale weak microbursts will likely lead to splats and variable and gusty winds near the showers. Cold front will move across the terminals mid-late evening with winds becoming light northwesterly for a time, before likely flopping around to light northeasterly late this evening or during the overnight. There is some uncertainty on timing of shift to light northeasterly, it is possible that could be after 06z even. Once winds settle in from light northeasterly direction, could see some fog development overnight. Most likely scenario would be some MVFR VSBY in light fog during the pre-dawn hours Thursday. If skies clear out and winds go lighter, than some patches of thicker fog and lower VSBY would be possible, especially DPA/RFD/GYY. Weak frontal boundary will be stalled out in the area Thursday afternoon. Lake should contort the boundary around the south end of Lake Michigan and may push the more favored area for afternoon SCTD TSRA south and west of ORD/MDW/GYY. Can`t rule out SCTD TSRA developing west of ORD/MDW and then being pushed east closer to the terminals into the cool side of the weak front. Confidence is low, so opted to keep TSRA out of ORD, MDW, and GYY for now, but will be something to keep a close eye on. - Izzi && .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 Thursday night for INZ001- INZ002-INZ010-INZ011. Air Quality Alert from 1 AM Thursday to 1 AM EDT Friday for INZ019. LM...None. && $$ Visit us at weather.gov/chicago