Deep monsoon moisture over Arizona and an approaching trough of low pressure from the Pacific Northwest helped produce local flash flooding and large hail. Thunderstorms reformed during the evening over Yavapai County which continued flash flooding in smaller streams and rapid rises on The Verde River (a main stem river).
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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 FGZ wrote it up.
Deep monsoon moisture over Arizona and an approaching trough of low pressure from the Pacific Northwest helped produce local flash flooding and large hail. Thunderstorms reformed during the evening over Yavapai County which continued flash flooding in smaller streams and rapid rises on The Verde River (a main stem river).
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 Flagstaff, AZ wrote its last area forecast discussion before the first event at Aug 18, 2:55 AM MST, 1 hour 5 minutes earlier.
Area Forecast Discussion National Weather Service Flagstaff AZ 255 AM MST Wed Aug 18 2021 .SYNOPSIS...Scattered showers and thunderstorms develop this morning south of I-40, then becoming more widespread during the afternoon and evening across northern Arizona. Flash flooding will be possible in areas of heavy rainfall. Drier conditions are expected Thursday through the weekend. Cooler temperatures are also expected through the rest of the week. && .DISCUSSION...A few isolated showers have begun to develop over the lower elevations south of the Mogollon Rim. This activity is expected to continue to increase in coverage through this morning. Heavy rainfall is expected to lead to a few localized areas of flash flooding this morning over Gila County. Showers and thunderstorms are expected to increase in coverage this afternoon and into the evening, especially for areas along and north of the Mogollon Rim. With increased moisture in place once again and many areas saturated from heavy rainfall yesterday, an enhanced threat of flash flooding remains over portions of central and northwester Arizona. The Flash Flood Watch has been extended as a result to include areas along the Mogollon Rim northwestward into the Coconino and Kaibab Plateau where HREF guidance continues to indicate the greatest potential for heavy rainfall. While flash flooding will be the primary threat this afternoon, a few strong to severe storms are also possible with the potential to produce large hail and damaging winds. Longwave trough continues to move into the Intermountain West today, with the center of the trough passing over Utah during the day Thursday. Breezy and cooler conditions are expected as a result today and again on Thursday. As the trough passes to the north, the monsoonal moisture in place gets swept away with dryer conditions expected in its wake Thursday and into the weekend. Shower and thunderstorm chances do remain over mainly in the high terrain of far northern Arizona near the Utah border Thursday and Friday. Dry conditions remain over much of the areas through the weekend. Though temperatures begin to warm slightly each day as heights rise from a ridge gradually building in from the east. Monsoonal moisture does begin to creep back into Arizona, especially over the eastern portions of the state, towards the first half of next week as the ridge builds. However, confidence in any widespread coverage of showers and thunderstorms is low at this point. && .AVIATION...For the 12Z package...MVFR ceilings this morning near and along the Mogollon Rim should dissipate after 14-15Z. Showers are expected to develop across central Arizona and spread north through the morning hours. More widespread showers and storms will develop area wide after 18Z. Isolated IFR in heavier storms with hail and gusty winds also possible. Outside of storm activity, mainly VFR conditions and southwest winds around 10-15 kts with gusts around 25 kts. Aviation discussion not updated for TAF amendments. && .FIRE WEATHER...Scattered to numerous showers and thunderstorms are expected again today. Large hail, localized flash flooding, and very gusty outflow winds will be the main hazards. Winds will also become breezy out of the southwest today with afternoon gusts around 30 mph. Much drier conditions are forecast Thursday with a continued chance for storms along the far northern zones. Friday through Sunday...Drier weather is expected through the weekend, with below normal temperatures and breezy southwest winds. && .FGZ WATCHES/WARNINGS/ADVISORIES... Flash Flood Watch through this evening FOR AZZ004>007-009-012- 015-016-018-038. && PUBLIC...Humphreys AVIATION...MCT FIRE WEATHER...MCT For Northern Arizona weather information visit weather.gov/flagstaff
The Cedar City radar (KICX), 138 km away, was scanning. The volume nearest the first event began at 10:59 UTC.
11 flash flood, 1 hail. Local standard time, as filed.
The Paria River gauge at Lees Ferry showed a 4 foot rise between 6:00 AM and 7:00 AM MST. The Paria River drainage originates in Utah and flows into the Colorado River near Lees Ferry.
The East Verde River near Childs (EVDA3) had a 2.9 foot rise between 0800 and 0845 MST.
The Tonto Creek stream gauge above Gun Creek (TNRA3) measured a 3.1 foot rise between 0845 and 1000 AM MST.
The water was flowing 4.85 feet deep (normally dry) in Munds Canyon under Interstate 17 and about 25 feet wide. The estimated flow was 900 CFS.
Pictures on social media show at least 10 inches of water flowing in Dinnebito Wash near Hardrock. The water had been flowing for at least 2 hours. Road going into Hardrock were not passible due to the water flowing over the roadways.
Granite Creek in Prescott (GCRA3) rose from 7.17 to 10.22 feet in 45 minutes.
Granite Creek (GRWA3) below Watson Lake rose 2.2 feet to 3.97 feet in one hour and 44 minutes. Water crested at 7:20 PM MST.
Golf ball sized hail (1.75 inches diameter) was reported near the entrance to the North Rim of The Grand Canyon National Park.
Emergency Management reported a 4 foot rise on Black Canyon Creek at 8:08 PM MST. This creek is downsteam from parts of the Tiger Fire (2021).
The Verde River near Clarkdale rose over 17 feet in 58 minutes (VDCA3). The water crested at 19.49 feet at 942 PM MST.
The Wet Beaver Creek gauge (BCMA3) near Montezuma Castle National Monument rose from 1.0 to 3.03 feet in 65 minutes. It crested at 11:13 PM MST.
The Verde River near Camp Verde rose just over 10 feet in 4 hours (VAVA3). The water crested at 14.23 at 9:30 AM MST.
After NWS Directive 10-1605, the rules offices file Storm Data by.
Every storm in Arizona in 2021Every state in 2021This day in weather historyWarnings in Arizona, 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: 983550, 983551, 983552, 983553, 983558, 983555, 983560, 983554, 983709, 983706, 983703, 983707.