A surge of tropical moisture, combined with a an upper level low centered over an area near state borders of Arizona, Nevada and California is producing moderate rainfall over|central and northern Arizona.
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The episode as NWS FGZ wrote it up.
A surge of tropical moisture, combined with a an upper level low centered over an area near state borders of Arizona, Nevada and California is producing moderate rainfall over|central and northern Arizona.
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 Sep 1, 3:39 AM MST, 4 hours 21 minutes earlier.
Area Forecast Discussion National Weather Service Flagstaff AZ 339 AM MST Wed Sep 1 2021 .SYNOPSIS...Showers and thunderstorms will continue through today. Some storms will bring heavy rainfall and may lead to flash flooding. Thunderstorm coverage will decrease Thursday with much more limited coverage expected this weekend. Cool temperatures through Friday will start to increase this weekend. && .DISCUSSION...A surge of tropical moisture, combined with a an upper level low centered over an area near state borders of Arizona, Nevada and California is producing moderate rainfall over central and northern Arizona very early this morning. Through mid-morning...Bands of moderate rain showers will propagate north-northeastward through the early to mid-morning hours today. Observed and estimated rainfall rates are around 0.25-0.5 inches per hour in the heavier area, elsewhere expect rates more towards 0.1-0.3 inches per hour. Isolated thunderstorms have been embedded in these bands and will continue mainly in northeastern Arizona. These storms could produce locally higher rainfall rates, and resulting storm total precipitation in the slower cells, mainly in eastern Coconino County and eastward towards the New Mexico border. Due to the persistent threat of flash flooding, the Flash Flood Watch for all of central and northern Arizona persists through late tonight. Today...Flow aloft will begin to turn shift from southerly to southwesterly as the upper low enters western Arizona later today. While the deep moist air mass will then largely exit the state, chances for showers and thunderstorms persist into this afternoon. These will be possible from instability aloft and wrap- around flow interacting with residual moisture for scattered thunderstorms today in northwestern and north-central Arizona. Storm motion is expected to be relatively fast today and towards a general east-northeastward direction. The trough axis moves into eastern Arizona this evening and bring a quick end to rain chances from west to east across the forecast area as much drier westerly flow infiltrates the state. Tonight...Very low level moisture and colder forecast temperatures for tonight have increased confidence in a low level cloud deck setting up over parts of northern Arizona tonight. From that low cloud deck there is potential for limited visibility in areas of fog. Be aware of this potential if you are traveling on the roads from late this evening through early Thursday morning. Thursday...Enough mid-level moisture could persist into Thursday for isolated to scattered thunderstorms Thursday afternoon. The westerly flow from today is progged to rapidly decrease through the day on Thursday so some uncertainty exists in the chances of precipitation. If the faster westerly steering flow lasts into the afternoon hours then the greatest chances for storms would be east of high terrain areas. Otherwise, storms will be largely dependent upon outflows from the first storms to form over the high terrain. In wither scenario though, the highest chances for any notable rainfall and possible storm impacts will be over the high terrain on Thursday afternoon. Temperatures will remain cool, but start a very slow warming trend. Friday through Sunday...Thunderstorm chances have decreased again for Friday and Saturday with the morning forecast as confidence grows in a building area of high pressure over the Southwest starting Friday. Any moisture will be largely limited to far eastern Arizona so the greatest storm chances are for the White Mountains and eastern Mogollon Rim these days. As the high strengthens and sets up north of the region though, moisture could then return to Arizona and expand thunderstorm coverage from Sunday onward. Overall, a low-grade monsoon pattern is expected Friday and Saturday with the potential for expanding coverage Sunday onward. With the weakened monsoonal pattern this weekend under high pressure, temperatures could warm to where heat impacts are felt in lower valleys and within the Grand Canyon. While not certain, it is better to begin any heat safety precautions that might be needed for outdoors activities this weekend in lower elevations. Monday and Beyond...The low grade monsoonal pattern with isolated thunderstorm coverage is expected to continue into early next week. && .AVIATION...For the 12Z package...MVFR ceilings with local IFR in areas of heavy rain should clear to mainly MVFR/VFR through today as coverage of showers/storms decreases into this evening. Gusty winds up to 30 kts are possible near storms, otherwise expect 10-15 kts or less. Patchy fog is possible at high terrain terminals after 06z Thursday. Aviation discussion not updated for TAF amendments. && .FIRE WEATHER...Showers and thunderstorms will continue through today, along with cool temperatures. The main threat today will be the potential for flash flooding, especially on area fire scars. Isolated to scattered thunderstorms are expected Thursday, mainly over higher terrain. Friday through Sunday...Isolated to scattered storms are possible each afternoon with the greatest coverage in eastern Arizona high terrain areas. Daytime temperatures will warm through the weekend. && .FGZ WATCHES/WARNINGS/ADVISORIES...Flash Flood Watch through late tonight FOR AZZ004>018-037>040. && PUBLIC...NL AVIATION...NL FIRE WEATHER...NL For Northern Arizona weather information visit weather.gov/flagstaff
The Flagstaff radar (KFSX), 107 km away, was scanning. The volume nearest the first event began at 15:01 UTC.
7 flash flood. Local standard time, as filed.
Carrizo Creek near Carrizo (CRZA3) rose from 1 to 3.7 feet in less than an hour. The water dropped back below 2 feet about 3 hours later.
Heavy rain flooded a wash which created a waterfall over the rim and into Glen Canyon just below/south of Glen Canyon Dam.
The East Verde River near Childs (Gauge EVDA3) rose from 10 to 12.4 feet in less than 30 minutes at 930 AM on September 1. It dropped below 12 feet about 3 hours later. At 830 PM, it rose to 13.8 feet. The water dropped back to around 11.6 feet by 900 AM on September 2.
Water was flowing over a 40 foot section of Highway 87 at Mile Post 343 near Coyote Wash.
The stream gauge on Moenkopi Wash near Tuba City rose from 17.6 feet to 20.8 feet in less than half an hour at 430 PM on 1 September. The gauge went down to 18.4 feet then rose again to 23.7 feet in less than half an hour at 900 PM on 1 September. The water rose a third time to 22 feet in less than half a hour at 3:00 AM on 2 September. The water receded below 18 feet by 800 AM on 2 September.
Heavy rain over the Little Colorado River Basin and tributaries created a surge of water at the river gauge (LCCA3) on the Little Colorado River near Cameron. The water rose from 1.8 feet to 5.43 feet on the gauge in less than 15 minutes at 1245 AM on 2 September. The water was still above 4.7 feet at 945 AM.
Highway 191 was closed due to flooding over the highway at Mile Post 491.8 between Round Rock and Rock Point (by Whale Rock) at 1135 PM. A social media video post showed a dangerous swollen wash along Highway 191.
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, September 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: 984862, 984867, 984877, 984871, 986255, 986254, 984873.