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Summary

Strong thunderstorms trained over the Musuem Fire Scar with a second cluster of storms near Fort Valley on August 17, 2021. Very slow storm motion combined with high moisture content in the air produced extremely high rainfall rates. Over the Museum Fire Scar, 2-3.5 inches of rain was measured in one to two hours. Just to the northwest near Fort Valley, another rainfall maxima was observed with widespread 3-4 inch amounts. Both of these events combined to bring major flooding impacts to both the Museum Fire Scar and the Rio de Flag. This event prompted the first Flash Flood Emergency issued by the National Weather Service Office in Flagstaff.

 

Timeline and Impacts

 

Radar loop from 11:09 AM - 2:05 PM August 17, 2021 showing very strong thunderstorms training over both the Museum Fire Scar and near Fort Valley. The white outline in the loop just north of Flagstaff is the Museum Fire Scar.

Radar estimates of well over 3-4" near Fort Valley and just north of Hwy 180 led to flooding along the Rio de Flag through much of town. Additional areas of well over 2-3.5" across the Museum Fire Scar led to additional flooding downstream of the fire into Sunnyside and the neighborhoods in northeast Flagstaff. The colored image here is radar estimated rainfall with the white numbers indicating actual rainfall measurements from rain gauges.

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• August 17, 2021
  • 11:15 - 11:45 AM - First thunderstorms formed in Doney Park, Flagstaff, and Fort Valley
  • 12:15 - 1:15 PM - Multiple storms producing heavy rainfall trained over the Museum Fire Scar
  • 12:25 - 1:40 PM - Fort Valley and just northeast of Highway 180 were impacted by several rounds of very heavy rainfall
  • 12:45 PM - 3:00 PM - Flash flooding due to the rain over the fire scar ongoing in the Mount Elden Estates, Grand View and Sunnyside neighborhoods
  • Late afternoon - Last rain fell over the scar with flooding decreasing in coverage through the evening
• Impacts
  • Several sections of the Flagstaff Urban Trail Systems were closed due to the flooding
  • Killip Elementary School was closed the following day due to flood damage
  • Children were held in schools half an hour past the normal release time due to the flooding
  • Flooding from the Rio de Flag closed Highway 180 for a few hours near Fort Valley
  • Significant damage to property and several homes along the Rio de Flag near Hwy 180
  • At least 24 homes sustained flood damage in the Mt. Elden Estates and Sunnyside areas
  • Numerous streets flooded in north Flagstaff neighborhoods with multiple roads closed by local officials. The most impacted areas were along and north of Route 66
• Products Issued by the National Weather Service in Flagstaff
  • 12:15 PM - Initial Flash Flood Warning for the Museum Fire Scar was issued
  • 12:41 PM - Flash Flood Warning was upgraded to a Flash Flood Emergency indicating the potential for catastrophic damage
  • 1:06 PM - A second Flash Flood Warning was issued for west Flagstaff up Highway 180 through Fort Valley
  • 3:38 PM - Flash Flood Emergency was allowed to expire as the flood threat has lessened in the Museum Fire Scar area
  • 6:12 PM - Flow began to gradually recede across portions of the Rio de Flag so the Flash Flood Warning for west Flagstaff and Fort Valley was allowed to expire

Dangerous flash flooding was once again observed on Grandview and Linda Vista from the heavy rain over the Museum Fire Scar. Image courtesy of ABC15 Arizona

Very heavy rain near Fort Valley caused flooding along the Rio de Flag especially along Highway 180 and in west Flagstaff. Image courtesy of Mark Stubblefield.

Flash flooding occurred in Paradise Wash as a result of the heavy rain over the Museum Fire Scar. Image courtesy of the Arizona Daily Sun

Killip Elementary School had flood waters make their way into the hallways. School was cancelled on Wednesday August 18th due to the flood damage. Image courtesy of the Arizona Daily Sun

Flooding impacts were not limited to north of Route 66 in Flagstaff. Butler Avenue was closed for a period of time near downtown due to flood waters. Image courtesy of the Arizona Daily Sun

Flood waters moved down the Rio de Flag into west Flagstaff, at times threatening to flow in residents backyards. Image courtesy of Mark Stubblefield.

The Rio de Flag stream gauge at Peak View rose over 6 feet in a short period of time during this flood event.

Significant flood debris was observed in the hardest hit areas. Image courtesy of the Flagstaff City Government

 

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Meteorology

Deep monsoonal moisture was in place across northern Arizona on August 17th, 2021. Very weak steering flow aided in slow storm movement and promoted training of thunderstorms over the same location. Daytime heating led to the development of thunderstorms around the Flagstaff area late in the morning. These thunderstorms expanded in coverage and covered an area from Fort Valley to north Flagstaff to Doney Park. Radar estimates and automated gauges indicated rainfall rates reached 2-4 inches per hour at times. The heavy rainfall rates lasted between 1-2 hours before lightening. The rainfall totals well exceeded the "high-impact" thresholds for the Museum Fire Scar which prompted the upgrade to a Flash Flood Emergency. A separate area of heavy rainfall near Fort Valley brought over 6 foot rises on the Rio de Flag through Flagstaff. Water levels quickly receded through the evening hours after rainfall ended.

For more information on impacts and general inquiries about this event, please reference the contacts below:

• National Weather Service:
  • Brian Klimowski, Meteorologist-in-Charge at brian.klimowski@noaa.gov
  • Tony Merriman, Warning Coordination Meteorologist at tony.merriman@noaa.gov
• Coconino County Public Works: at 928-679-8300