Scattered thunderstorms produced heavy rain and localized flooding during the afternoon of August 18th.
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Deaths and injuries are direct and indirect together, as the offices filed them. Damage is property and crops in the dollars of the year, not adjusted for inflation, and only where an office estimated it.
The episode as NWS LOT wrote it up.
Scattered thunderstorms produced heavy rain and localized flooding during the afternoon of August 18th.
The local office's own words before the first event, as it sent them, from the Iowa Environmental Mesonet's archive of National Weather Service text.
NWS Chicago, IL wrote its last area forecast discussion before the first event at Aug 18, 11:29 AM CST, 46 minutes earlier.
Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 1229 PM CDT Wed Aug 18 2021 .SHORT TERM... 325 AM CDT Through Thursday... High pressure is centered across much of the Great Lakes region, extending back across the plains. That places northern IL/northwest IN just on the southern periphery of the high. Dewpoint depressions are low given the elevated dewpoints in the mid 60s to near 70, and thus we have had pockets of light fog in many area. There is just a touch of wind that has limited most locations from forming thicker fog, thus keeping things in check, and in line with non- MOS based guidance. Areas void of mid and upper level clouds may still experience dense fog (aka FEP has been 1/2SM at times). Looking aloft, the big picture generally depicts most of the action is out west ahead of deeper low that may become our player later in the week; then the remnants of Tropical Depression Fred. Upper level heights are fairly high across the nation's midsection between these two features. Our weather maker is a subtle feature spinning over us in addition to the energy and driving up on a weak jet segment in southwest flow. We have seem isolated showers coming out of nowhere pop up from time to time given the moist low levels and forcing from upper vortcity max overhead. With the heating of the day, cyclonic spin, and likely aided by some lake convergence, forecast soundings from the RAP/HRRR depict a window where the cap at 700 mb may be breached to allow for a few storms this afternoon. Have generally followed previous forecasts holding fairly low coverage today given the dry 700-500 mb layer and even some subtle height rises. It will be a bit muggy again with heat indices in the upper 80s to near 90, so take some water with you today. Similar to what has occurred tonight, some isolated showers may persist/pop up at times as the cyclonic flow aloft will continue. With less high cloud the pattern will again favor fog tonight. Boundary layer moisture will remain fairly high on Thursday, certainly sufficient to continue the mugginess factor and to help build another decent late-morning and afternoon cumulus field. Warming aloft combined with an increasingly dry airmass aloft seen on water vapor across most of the Plains above should really limit precipitation chances. Enhanced surface convergence in the vicinity of the afternoon lake breeze would be the exception, but that looks to be about it. Heat indices during the afternoon should top out in the low to perhaps mid 90s, except near the lake where conditions will be a bit cooler. KMD && .LONG TERM... 330 AM CDT Thursday night through Tuesday... The main day of interest for showers and thunderstorms is on Saturday with a cold frontal approach during the afternoon and frontal passage during the evening. An overall warmer normal than pattern will persist through the middle of next work week, with occasional shower and thunderstorm chances in the early to mid work week period. Thursday night may have some potential for patchy fog development outside of Chicago, but opted to let subsequent shifts take a look at this before formal inclusion in the gridded forecast. Friday has continued to trend mainly dry due to mid-level dry air and short-wave ridging over the area. Can't completely rule out a widely isolated afternoon shower or thunderstorm over far interior areas (mainly south of I-80) where there may be a bit more instability and less capping realized. It will be a warm day in the mid to upper 80s along with dew points in the upper 60s to lower 70s for most making it fairly uncomfortable. With the guidance now in much better agreement in the timing of the cold frontal approach on Saturday and large scale forcing well removed from the region, most of if not all of Friday night could be dry as well. Saturday afternoon and possibly into the early evening continues to have the best chance for scattered thunderstorm coverage. With the stronger forcing well to the north closer to the surface low associated with the cold front, the front itself will be the main source of lift as 500 mb heights are forecast to be rising as the front moves across the area. Marginal mid-level lapse rates will somewhat limit instability though dew points into the 70s and temps into the mid to upper 80s will help compensate and would expect minimal capping to be eroded with peak heating. The weak lapse rate and marginal deep layer shear will generally limit the risk for any organized convection, though localized stronger wind gusts from taller cores are not uncommon in these setups. Otherwise lightning and heavy downpours will be the main threats. The front will clear the area during the evening and end the precip threat, followed by a less humid but still warm Sunday in the mid 80s away from the minor lake breeze cooling. In the early to mid work week period, a fairly potent mid-level ridge is forecast to be centered from the mid South to the southern Plains, which would place us on the potentially convectively active northeastern periphery. We're showing some low PoPs each period Monday and onward and do envision there being some MCS activity somewhere over or near the region if the forecast mid- level height pattern holds. Outside of possible convection, it will be very warm for late August with uncomfortable humidity levels. Castro && .AVIATION... For the 18Z TAFs... The main aviation weather concerns through the period focus on the coverage of showers and thunderstorms this afternoon, and also the threat of some fog late tonight. A very warm humid airmass is allowing widely scattered showers and thunderstorms to develop across eastern IL and northwestern IN. This activity will continue to be scattered through the afternoon near the terminals as a weak mid/upper level disturbance moves overhead. Storm motions are slow (10mph or less), so areas that get under one of these could see a period of heavy down pours and potentially some locally strong wind gusts. Expect this activity to fade with sunset this evening. Expecting a mainly clear sky tonight coming off afternoon dew points forecast to be higher than yesterday. With winds turning calm overnight, this should set the stage for a higher coverage of BR and FG, some of which could be dense. For this reason, no changes were made with the indicated temporary IFR VSBY at RFD, DPA, and GYY and MVFR at ORD and MDW. Lower VSBYs are certainly possible. The BR/FG will erode quickly Thursday morning. KJB/Castro && .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
Hazardous Weather Outlook National Weather Service Chicago/Romeoville IL 1024 AM CDT Wed Aug 18 2021 ILZ003>006-008-010>013-019>021-023-032-033-039-103>108-INZ001-002- 010-011-019-191530- Winnebago-Boone-McHenry-Lake IL-Ogle-Lee-De Kalb-Kane-DuPage- La Salle-Kendall-Grundy-Kankakee-Livingston-Iroquois-Ford- Northern Cook-Central Cook-Southern Cook-Northern Will- Southern Will-Eastern Will-Lake IN-Porter-Newton-Jasper-Benton- 1024 AM CDT Wed Aug 18 2021 /1124 AM EDT Wed Aug 18 2021/ This Hazardous Weather Outlook is for portions of North Central Illinois...Northeast Illinois and Northwest Indiana. .DAY ONE...Today and Tonight. Weather hazards expected... Limited Thunderstorm Risk. Limited Fog Risk. DISCUSSION... Isolated to widely scattered thunderstorms are possible this afternoon. Locally heavy rainfall would be the highest hazard outside of lightning. Fog will then be possible across the area late tonight. .DAYS TWO THROUGH SEVEN...Thursday through Tuesday. Thursday... Limited Thunderstorm Risk. Limited AM Fog Risk. Friday and Saturday... Limited Thunderstorm Risk. Monday and Tuesday... Limited Thunderstorm Risk. Limited Excessive Heat Risk. .SPOTTER INFORMATION STATEMENT... Spotter activation will not be necessary through tonight. GENERAL STORM MOTION OF THE DAY: Moving toward the east northeast at 5 to 10 mph LMZ740>745-191530- Winthrop Harbor to Wilmette Harbor IL- Wilmette Harbor to Northerly Island IL- Northerly Island to Calumet Harbor IL- Calumet Harbor IL to Gary IN-Gary to Burns Harbor IN- Burns Harbor to Michigan City IN- 1024 AM CDT Wed Aug 18 2021 This Hazardous Weather Outlook is for portions of the Illinois nearshore waters of Lake Michigan and the Indiana nearshore waters of Lake Michigan. .DAY ONE...Today and Tonight. Weather hazards expected... Limited Thunderstorm Risk. .DAYS TWO THROUGH SEVEN...Thursday through Tuesday. Thursday through Saturday... Limited Thunderstorm Risk. Monday and Tuesday... Limited Thunderstorm Risk. KJB Visit us at http://weather.gov/chicago Follow us on Facebook, Twitter, and Youtube at: www.facebook.com/nwschicago www.twitter.com/nwschicago www.youtube.com/nwschicago
The national radar mosaic for the moment the first event began.
1 flood. Local standard time, as filed.
A photo shared on social media showed several inches of standing water on Route 6 at the intersection with Hollywood Road.
After NWS Directive 10-1605, the rules offices file Storm Data by.
Every storm in Illinois in 2021Every state in 2021This day in weather historyWarnings in Illinois, August 2021
NOAA National Centers for Environmental Information, Storm Events Database, compiled from the National Weather Service’s monthly Storm Data. Complete through 2026-06; NCEI adds each month about 75 days after it ends and revises past years a few times a year. Rebuilt 2026-09-24. Each event’s entry at NCEI: 972781.