A weak area of low pressure from southern California through southern Nevada helped draw monsoon moisture northward across Arizona. Scattered showers and storms developed over the high terrain.
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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.
A weak area of low pressure from southern California through southern Nevada helped draw monsoon moisture northward across Arizona. Scattered showers and storms developed over the high terrain. A few of the storms produced rain rates of nearly 2 inches an hour.
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 31, 2:36 PM MST, 2 hours 24 minutes earlier.
Area Forecast Discussion National Weather Service Flagstaff AZ 236 PM MST Tue Aug 31 2021 .SYNOPSIS...Showers and thunderstorms will increase this evening becoming more widespread tonight through Wednesday. Some storms will bring heavy rainfall and may lead to flash flooding. Temperatures are expected to cool to below normal by the middle and end of the week. Thunderstorm coverage will decrease by Thursday with a more typical monsoon pattern returning through the weekend. && .DISCUSSION...A weak area of low pressure will interact with substantial monsoon moisture to bring rain and thunderstorms across the area through Wednesday. Some of these storms will produce heavy rainfall that may lead to flash flooding and Flash Flood Watches are in effect today through Wednesday night. A weak area of low pressure stretches from southern California through southern Nevada this afternoon and has helped to draw increased monsoon moisture northward across Arizona. Scattered showers and storms have developed over the high terrain so far this afternoon. A few of these storms have already produced rain rates of nearly 2" in an hour, indicating the sufficiently moist air mass overhead. Increasing moisture along with the approaching trough will result in increasing coverage of showers and storms tonight, especially through the overnight hours into Wednesday morning. Embedded areas of heavy rain will likely lead to some 1-2" per hour rainfall rates. In addition, some areas may end up seeing multiple rounds of rainfall through midday Wednesday. Flash Flood Watches remain in effect for all of northern Arizona. The main update today was to start the watch in northeastern Arizona earlier tonight (now starting at 8 PM MST / 9 PM Navajo Nation time) as the latest guidance suggests this area seeing an earlier onset of precipitation. A few strong storms could produce hail and strong winds, but instability looks to be pretty marginal keeping flash flooding the primary threat for this event. As the trough slowly tracks through northwestern Arizona on Wednesday, showers and storms will continue. The best coverage looks to be early in the day before a gradually drying westerly flow begins to move in. Storm coverage should begin to decrease by late Wednesday afternoon, though scattered storms that develop will still have the potential to produce heavy rainfall amounts. Flash Flood Watches remain in effect through Wednesday night. Temperatures will be significantly cooler through Wednesday with the shower/storm activity and additional cloud cover. By Thursday afternoon, a return to a more typical monsoon pattern is expected as moisture gradually decreases across the area. Expect isolated to scattered afternoon storms each day through the weekend. Forecast ensembles point toward a building ridge of high pressure over the west this weekend and early next week which will lead to warming temperatures, much closer to early September averages. && .AVIATION...For the 00Z package...Scattered showers and storms this afternoon will increase in coverage tonight. Expect some areas of MVFR ceilings to develop with local IFR possible in areas of heavy rain. Expect more widespread rain and thunderstorms tonight through midday Wednesday before activity becomes more scattered again. Gusty winds near 40 kts near storms, otherwise generally 10-15 kts or less. Aviation discussion not updated for TAF amendments. && .FIRE WEATHER...Showers and thunderstorms will increase in coverage tonight, continuing through Wednesday. The main threat during this time will be the potential for flash flooding, especially on area burn scars. Rain rates of 1-2 inches per hour are likely. Expect cooler temperatures and light winds. A more typical monsoon pattern returns Thursday. Friday through Sunday...A return to more typical monsoon conditions is forecast with isolated to scattered storms each afternoon. High temperatures will warm closer to normal. && .FGZ WATCHES/WARNINGS/ADVISORIES...Flash Flood Watch through late Wednesday night FOR AZZ004>008-015>018-037-038. Flash Flood Watch from 8 PM MST this evening through late Wednesday night FOR AZZ009>014-039-040. && PUBLIC...MCT AVIATION...MCT FIRE WEATHER...MCT For Northern Arizona weather information visit weather.gov/flagstaff
Special Weather Statement
National Weather Service Flagstaff AZ
1248 PM MST Tue Aug 31 2021
AZZ015-312000-
Western Mogollon Rim-
1248 PM MST Tue Aug 31 2021
...A strong thunderstorm will impact portions of Coconino County
including west Flagstaff and Fort Valley through 100 PM MST...
At 1247 PM MST, Doppler radar was tracking a strong thunderstorm over
Downtown Flagstaff, or near Flagstaff, moving north at 5 mph.
HAZARD...Winds in excess of 30 mph and half inch hail.
SOURCE...Radar indicated.
IMPACT...Gusty winds could knock down tree limbs and blow around
unsecured objects. Minor damage to outdoor objects is
possible.
Locations impacted include...
West Flagstaff and Fort Valley.
This includes the following highways...
Route 66 between mile markers 192 and 196.
Highway 180 between mile markers 217 and 223.
PRECAUTIONARY/PREPAREDNESS ACTIONS...
If outdoors, consider seeking shelter inside a building.
Torrential rainfall is also occurring with this storm and may lead to
localized flooding. Do not drive your vehicle through flooded
roadways.
&&
LAT...LON 3518 11165 3519 11176 3528 11172 3525 11161
TIME...MOT...LOC 1947Z 187DEG 6KT 3522 11166
MAX HAIL SIZE...0.50 IN
MAX WIND GUST...30 MPH
TPSThe Flagstaff radar (KFSX), 67 km away, was scanning. The volume nearest the first event began at 00:02 UTC.
1 flash flood. Local standard time, as filed.
The Verde River near Camp Verde rose almost two feet with a crest of 4.39 feet at 6:00 PM MST (VAVA3).
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: 983713.