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Summary

A low pressure system over Texas pushed west and interacted with deep monsoonal moisture already over the Southwest from July 22-25, 2021. This led to a few days of widespread thunderstorms with very heavy rainfall rates. By the weekend, soils were saturated across the area enhancing the already high flash flood threat. Some rivers in central and northern Arizona crested near known record values. Numerous roads were washed out or impassable due to flooding. The Millet Swale Levee experienced a slow break and resulted in minor flooding in Taylor, Arizona. Structural flooding was reported near Show Low, in St. Johns and the village of Moenkopi. This was one of the most widespread impactful flooding events during a monsoon season in recent memory.

 
Timeline
 
11 AM Wed July 21 - 11 PM Fri July 23
Radar Loop July 21 - 23Radar loop showing continuous bands of strong thunderstorms.
11 PM Fri July 23 - 11 PM Sun July 25
Radar loop July 23 - 25Radar loop showing more continuous bands of strong thunderstorms.
Precip Loop 21 - 23Radar estimated one hourly precipitation showing continuous bands of moderate to heavy rainfall.
Precip loop 23 - 25Radar estimated one hourly precipitation showing more continuous bands of moderate to heavy rainfall.

 

 
  • Thursday, July 22, 2021
    • Thunderstorms formed over the region Thursday afternoon
    • Storms continued into Thursday night, primarily in east-central Arizona
    • Coverage of flooding expanded late Thursday night
  • Friday, July 23, 2021
    • Heavy rainfell with widespread rain rates of 1-2" and locally up to 3" per hour
    • Flooding impacted many communities and travel on area roadways
    • Large thunderstorm complexes persisted throughout the overnight hours
  • Saturday, July 24, 2021
    • Even more widespread coverage of flooding
    • A third day of large thunderstorm complexes moved from east to west
  • Sunday, July 25, 2021
    • Most thunderstorms were limited to areas of the Mogollon Rim, Little Colorado River valley, and across Coconino and Yavapai counties
    • Millet Swale Levee failed
    • Localized flash flooding continued
Impacts
  • Multiple fatalities were reported across northern Arizona due to flash flooding. 
  • Flooding of numerous paved roads which led to closures and delays.
    • US 191 in multiple locations near St. Johns
    • SR 260 near Pinedale
  • Numerous roads were washed out and impassable on Navajo Nation, including Navajo Route 2 north of Winslow from prolonged minor flooding of the Little Colorado River. 
  • Businesses and homes were flooded in St. Johns, near Show Low, in the village of Moenkopi and other area communities.
  • Millet Swale Levee failed and led to minor flooding in the Taylor area. 
  • Sewer spill occurred in Carroll Canyon Wash near Sedona due to rain water inundating the El Camino Lift Station.
Unpaved road flooding

Flooding on a road in Cowsprings, Arizona on the Navajo Nation. Similar scenes were very common across the Navajo Nation and northern Arizona for any unpaved roads. Image courtesy of Sheena Lee from Facebook

Moenkopi village flooding

Flooding occurred in Moenkopi Wash and into the village. The photo was taken around 11 AM MST on Sunday, July 25, 2021 overlooking the village by Travis Honanie.

LCR near Joseph City

Little Colorado River in Joseph City in the morning of July 24, 2021. Image courtesy of Vicki Miller Sutton from Facebook

Flooded low water crossing

Rio De Flag crossing on Herold Ranch Road on Sunday, July 25th, 2021. Image courtesy of Betsey McKellar from Facebook

Hydrology
Agua Fria Hydrograph

Water levels on the Agua Fria River experienced multiple peak flows on July 23rd and 24th, 2021 due to very heavy rainfall upstream near Dewey-Humboldt and Prescott, Arizona. Shown is a hydrograph for the Agua Fria River at Humboldt with river stage (in feet) on the left and river flow (cubic feet per second) on the right.

LCR at Winslow Hydrograph

Water levels on the Little Colorado River near Winslow began to rise around 5 AM MST on Saturday July 24 and peaked just under Minor Flood Stage about twenty four hours later. Flood waters dropped below Action stage (when roads can become washed out) by Sunday evening. Shown is a hydrograph for the LCR at Winslow with river stage (in feet) on the left and river flow (cubic feet per second) on the right. 

Oak Creek at Cornville Hydrograph

Water levels on Oak Creek experienced multiple peak flows on July 23rd, 24th and 25th, 2021 due to very heavy rainfall north of Sedona. Minor flooding occurred twice. Shown is a hydrograph for Oak Creek at Cornville with river stage (in feet) on the left and river flow (cubic feet per second) on the right.

Moenkopi Wash at Tuba City Hydrograph

Water levels in Moenkopi Wash rose nearly thirteen feet to peak at 29.8' (from a base flow just under 16') as very heavy and persistent rainfall on Black Mesa flowed downstream. Homes and low water crossings in the village of Moenkopi and some near Tuba City were inundated from  the flooding. Shown is a hydrograph for Moenkopi Wash just south of Tuba City with river stage (in feet) on the left and river flow (cubic feet per second) on the right.

Daily observed precipitation

Twenty-four hourly rainfall across northern Arizona from July 21-25, 2021. The most widespread heaviest precipitation occurred on July 23 and 24, 2021.

Percent Normal rainfall

Month of July 2021 percent of normal precipitation for northern Arizona. The very high values, especially over the Navajo Nation, mostly fell between July 22-25, 2021.

Soil moisture after rainfall

The wet month of July (see Percent of Normal map) left soils in central and northern Arizona very saturated. The saturated soils (upward of the 98th percentile in much of the region) created significant concerns for flash flooding anywhere even after the event ended.

Drought monitor comparison before and after precipitation

The widespread heavy rainfall over northern Arizona helped to decrease the severity of drought conditions over the state. The drought monitor for Arizona issued on July 27, 2021 is shown left, while greater coverage of Extreme to Exceptional drought conditions can be seen right on the map issued July 20, 2021.

Meteorology

Deep monsoon moisture had been in place across northern Arizona for several days leading to afternoon thunderstorm activity. An easterly wave over Texas pushed west and interacted with the deep monsoonal moisture providing an unusual amount of dynamics for this time of year. This led to a few days of widespread thunderstorms with very heavy rainfall rates. Convection persisted into the overnight hours on most days leading to an almost around the clock flood threat for much of northern Arizona. By the weekend, soils were saturated across the area enhancing the already high flash flood threat.

GEFS Mean 500 mb Heights and Wind VectorsA very anomalous easterly wave undercut high pressure over the northern CONUS region to bring a dynamic setup for three days over northern Arizona. The mid-level easterly flow created a west-southwestward storm motion. Shown are GEFS mean 500 mb heights from 5 AM July 21 - 11 PM July 25, 2021 with wind vectors highlighting that easterly flow.

 

NAM Analysis Precipitable Water in IchesNAM12 Analysis precipitable water from 5 AM July 21 - 11 PM July 25, 2021 is shown for Arizona. Precipitable water values in this period were much higher than what is typical during any normal monsoon day, especially in northern Arizona. This created the very moist atmosphere needed for three days of daytime and nighttime riverine, areal and flash flooding across the state.

 

Points of Contact

 

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
  • Navajo County Public Works: at 928-524-4100
  • Apache County Public Works: at 928-337-4364 
  • Navajo Nation Division of Community Development: at 928-871-7182