Flash Flood Case Study

Navajo Lake State Park

August 2003

Here is an example of an unwarned flash flood with significant human impact that occurred at a long range (about 125 miles) from the Albuquerque radar in a portion of the New Mexico back country hydrologically favored for flash flooding.

Rugged Canyons and Arroyos Surround Navajo Lake

Photo of Navajo Lake on a sunny day. Rocky canyon walls act as the shoreline.

The steep and rocky terrain makes this a hydrologically favored area for flash flooding; especially in normally dry arroyos and washes, where gravity forces flood waters to drain.

 

Navajo Lake is Located in Northwest New Mexico on the Colorado Border

Map of northwest New Mexico and Navajo Lake, which is located on the boarder of Rio Arriba and San Juan Counties and also crosses the Colorado and New Mexico border
C = Campsite in Cottonwood Canyon
  • Eight people took a boat to an undeveloped camp site in an isolated canyon (Cottonwood Canyon)
  • Pitched tent in a wash (low spot) next to Navajo Lake
  • Camping spot exceeded the range of both cell phones and NOAA Weather Radio
  • Afternoon storms soaked the ground
  • Another storm crossed Cottonwood Canyon around 915 pm; flood waters quickly drained through the wash and swept a tent into Navajo Lake with a 7-year-old boy inside
  • Boy rescued, but seriously hurt
  • Flooding did not impact any of the park’s developed camp sites

 

Radar Images of Evening Storms

Taken around 915 pm, this series of composite reflectivity images depicted a couple of ordinary summer thunderstorms gradually decreasing in intensity as they passed from northeast to southwest across Navajo Lake. Also depicted are cloud-to-ground lightning strikes, mostly as small white dashes and a few white plus signs.
903 PM
915 PM
926 PM
C = Campsite in Cottonwood Canyon
D = Navajo Lake Dam
  • Images from Albuquerque’s radar, located 125 miles away
  • Showers earlier in the day wetted the ground in advance of these storms
  • Forecasters did not issue warnings or advisories for any of the storms depicted here
  • These were not the strongest storms on this particular day. Forecasters issued arroyo and small stream flood advisories in other parts of NM for stronger storms in populated areas.

 

Satellite Estimated Precipitable Water

The 1201 pm geostationary satellite precipitable water estimate indicated late-day thunderstorms would be capable of producing around 25 cm (nearly 1 inch) of rain.

 

Radar Precipitation Tally

Three-hour precipitation estimates from the Albuquerque radar, and other data, indicated the late afternoon storms developed in an environment of greater instability and produced significantly more rain near the Cottonwood Canyon drainage basin than the evening storms. Although the evening storms probably produced less rain into the canyon, they may have produced greater runoff into the wash due to the already saturated ground.
 
*Approximate location
4-7 PM
7-10 PM
  • Radar estimated up to 2.25 inches of rain near the Cottonwood Canyon drainage basin during the late afternoon; however, the presence of hail and other radar limitations may have resulted in overestimation of the upper bound of the precipitation estimate by a factor of two. Thus, the actual afternoon rainfall into the Cottonwood Canyon drainage basin may have peaked closer to 1.13 inches, which matched the satellite precipitable water estimate fairly closely.
  • Radar estimated less than one quarter inch of rain from the evening storms near Cottonwood Canyon, and other radar data indicated the evening storms didn't build to the height of the afternoon storms. Since the lowest radar beam crossed Cottonwood Canyon at around 13,050 feet above the ground, the radar may have underestimated the evening precipitation by overshooting a significant portion of the shallower storms, which may not have contained hail because of the lower instability.
  • The rain gage at the dam measured 0.36 inches , which matched the sum of these radar estimates fairly closely. There were no other rain gages in the area.

 

Case Study Conclusions
  • After taking radar limitations into account, the satellite and radar data presented here appeared reasonably consistent and matched the Navajo Lake Dam rain gage measurement
  • If these storms were closer to the radar, they probably would have been depicted stronger with higher precipitation estimates
  • Even if forecasters had issued an Arroyo and Small Stream Flood Advisory or Flash Flood Warning for Navajo Lake, NOAA Weather Radio signals fade before reaching Cottonwood Canyon. Since cell phones didn't work at the camp site, pager notifications may also have failed.
  • Ordinary thunderstorms develop frequently during summer, they tend to flood washes and arroyos, and forecasters can not detect all of these floods
  • Everyone, including campers, should take necessary precautions in case of flash floods. Never camp in low spots, drainages or other flash flood prone areas.

Next section: Prepare and React Appropriately