A severe thunderstorm produced a peak wind gust to 63 mph at Centennial Airport. Heavy rain in the Cameron Peak burn area caused localized flash flooding along Big Bear Road.
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The episode as NWS BOU wrote it up.
A severe thunderstorm produced a peak wind gust to 63 mph at Centennial Airport. Heavy rain in the Cameron Peak burn area caused localized flash flooding along Big Bear Road.
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 Denver/Boulder, CO wrote its last area forecast discussion before the first event at Aug 17, 2:06 PM MST, 2 hours 4 minutes earlier.
Area Forecast Discussion National Weather Service Denver/Boulder CO 306 PM MDT Tue Aug 17 2021 .SHORT TERM...(This evening through Wednesday) Issued at 122 PM MDT Tue Aug 17 2021 Forecast area begins to feel the effects of a pattern change in the short term period. Broad high pressure across the CONUS gives way to a digging Pacific NW trough, and light 500 mb wind turns southwest ahead of the trough and increases late tonight and Wednesday morning. Some weak QG ascent begins Wednesday afternoon, mainly in the mountains. Moisture moves in on the SW flow, with western slope seeing the greatest increases but BOU forecast area is not left out entirely. Specific humidity over the northern mountains rise from 4 g/kg to near 6 by the end of the day Wednesday, and 2.5 g/kg to around 4 g/kg on the plains, and surface dewpoints rise about 5 degrees F areawide. A weak surge down the northern foothills will turn surface wind northerly on the north I-25 corridor, then a lee side trough sets up with a return to southeast flow at the surface. Wind on the eastern plains picks up Wednesday afternoon to 15-20 mph, with region on the western edge of a strengthening low level jet over the northern plains. Good news on the smoke front, the southerly flow will push the smoke to the north and begins some improvements from mid morning onward. Showers and thunderstorms to develop over the mountains Wednesday afternoon in deeper moisture and synoptic lift and surface based instability with mid level cooling yet to come. Surface based CAPEs in the northern mountains hit 500-1000 j/kg. With SSW steering flow storms will move to NNE, and little chance for storms to move out onto the plains, not to mention subsidence with downslope component in the foothills Further east on the plains, some surface based instability but effectively capped at 600-700 mb. Temps...radiational cooling tonight, with lows similar to last night. 700 mb temps warm about a degree Wednesday vs today, although warming temps likely offset by increasing clouds. .LONG TERM...(Wednesday night through Tuesday) Issued at 122 PM MDT Tue Aug 17 2021 Upper level forcing will support a continued chance of thunderstorms across the mountains Wednesday evening. Wind, heavy rainfall and lightning will become the main hazards Wednesday evening especially over burn scar areas. Total QPF amounts will range from a trace to 0.30 inches. With favorable surface based CAPE reaching 2500 J/KG and moist mid to level conditions, scattered severe thunderstorms are likely Thursday. Supercell development is possible with favorable speed shear (40-50kts) and low level directional shear; a tornado cannot be ruled out especially for the eastern plains. High precipitation water rates reaching 1.30 inches favor heavy rainfall although speed should assist with faster thunderstorm movement. SHIP values reaching 1 and effective bulk shear reaching 35-40kts give forecast confidence to possible severe hail Thursday afternoon. Storms are likely to to continue until late evening crossing majority of the CWA above Interstate 70. Friday into next week, upper level ridging and dry conditions are likely. With westerly winds, it is likely skies will become hazy again. Afternoon high temperatures will increase each day but there is uncertainty if there will be enough haze block diurnal heating especially in the urban corridor. && .AVIATION...(For the 18Z TAFS through 18Z Wednesday afternoon) Issued at 107 PM MDT Tue Aug 17 2021 South wind today, with plentiful smoke obscuring slant visibility on approach. Surface based visibility P6SM and continued use of FU with unlimited visibility to convey presence of smoke. Radar showing showers developing over the foothills, and there is a possibility that showers or thunderstorms could move off the foothills onto the adjacent plains. Best chances on Palmer Divide and KAPA, but still small enough left any mention out of TAF. Any showers could also create gusty winds mainly at KDEN and KAPA, but confidence low and timing difficult to to pinpoint and did not include in TAF either. Mainly south drainage wind overnight. Wednesday morning in 15-16z time range surge of north wind, then shift around to east by mid day. && .HYDROLOGY... Issued at 122 PM MDT Tue Aug 17 2021 Burn area flood threat rises on Wednesday afternoon as showers and thunderstorms develop. Storms will be moving at 30 mph, and residence time over burn areas will be short, however there is the potential for multiple storms to move over the same area. Greatest threat will be in Grand County burn areas and into western Cameron Peak, closer to the deeper moisture. The threat of flash flooding will continue into Thursday night due to an upper level tough moving through the area. && .BOU WATCHES/WARNINGS/ADVISORIES... None. && SHORT TERM...Hanson LONG TERM...AD/Entrekin AVIATION...Hanson HYDROLOGY...AD/EJD/Hanson
Hazardous Weather Outlook National Weather Service Denver/Boulder CO 916 AM MDT Tue Aug 17 2021 COZ030>051-181530- Jackson County Below 9000 Feet- West Jackson and West Grand Counties Above 9000 Feet- Grand and Summit Counties Below 9000 Feet- South and East Jackson/Larimer/North and Northeast Grand/ Northwest Boulder Counties Above 9000 Feet- South and Southeast Grand/West Central and Southwest Boulder/ Gilpin/Clear Creek/Summit/North and West Park Counties Above 9000 Feet-Larimer and Boulder Counties Between 6000 and 9000 Feet- Jefferson and West Douglas Counties Above 6000 Feet/Gilpin/Clear Creek/Northeast Park Counties Below 9000 Feet- Central and Southeast Park County- Larimer County Below 6000 Feet/Northwest Weld County- Boulder And Jefferson Counties Below 6000 Feet/West Broomfield County- North Douglas County Below 6000 Feet/Denver/West Adams and Arapahoe Counties/East Broomfield County- Elbert/Central and East Douglas Counties Above 6000 Feet- Northeast Weld County-Central and South Weld County-Morgan County- Central and East Adams and Arapahoe Counties- North and Northeast Elbert County Below 6000 Feet/North Lincoln County- Southeast Elbert County Below 6000 Feet/South Lincoln County- Logan County-Washington County-Sedgwick County-Phillips County- 916 AM MDT Tue Aug 17 2021 This hazardous weather outlook is for northeast and north central Colorado. .DAY ONE...Today and Tonight Scattered thunderstorms are expected this afternoon over the mountains. There is a slight chance of a late afternoon storm drifting east off the foothills. Storms will produce localized light to moderate rain and wind gusts to 40 mph. .DAYS TWO THROUGH SEVEN...Wednesday through Monday Isolated to scattered thunderstorm chances increase Wednesday afternoon as incoming moisture will begin in the mountains spreading east into the foothills and adjacent plains. Heavy rainfall and potential flash flooding is possible over the burn scar areas. Thursday, there is a higher chance of more widespread precipitation across the region as the upper level disturbance moves through Colorado. Isolated severe thunderstorms are possible Thursday afternoon mainly on the plains. Friday through the weekend becomes a more settled weather pattern with dry conditions and afternoon high temperatures beginning to increase above normal. .SPOTTER INFORMATION STATEMENT... Spotter activation will not be needed today or tonight.
The Cheyenne radar (KCYS), 88 km away, was scanning. The volume nearest the first event began at 23:10 UTC.
1 flash flood, 1 thunderstorm wind. Local standard time, as filed.
Localized flash flooding damaged Big Bear Road and some areas upstream.
After NWS Directive 10-1605, the rules offices file Storm Data by.
Every storm in Colorado in 2021Every state in 2021This day in weather historyWarnings in Colorado, 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: 981821, 981034.