Flash Flood — Natrona, Wyoming

2009-07-03 · near Mountain View, Natrona, Wyoming

Event narrative

A vicious thunderstorm struck the city of Casper between about 1715 and 1800MST at the start of the July 4th holiday weekend. Extreme rainfall rates falling on the urbanized landscape produced flash flooding throughout Casper, with the most extensive inundation centered around the intersection of Poplar and Collins streets.

Nearly one inch of rain fell in 21 minutes at the Natrona County International Airport northwest of the city. A precipitation gauge along the North Platte River in Casper reported 1.79 inches of rain in 30 minutes. Final tallies around town showed rainfall from around one inch up to a high of about 2.25 inches occurred within about a 35 minute time frame.

Several streets, including Poplar and McKinley were reported to be rivers carrying rocks and other debris toward downtown Casper. Reports indicated one to three feet of water was present on some city streets. The flash flooding floated several cars and sent storm drains shooting wildly into the air. One storm drain cover injured an individual as the surging water displaced it. Emergency management estimated anywhere from 800 to 1000 structures, mainly homes, were impacted by the flooding.

In addition to hundreds of impacted homes, many well known locations also sustained damage, including the Nicolaysen Art Museum, Wyoming Medical Center, Central Wyoming Fairgrounds, Three Crowns Golf Course, and several city school buildings. Major damage also occurred at Casper College where five buildings, including the gymnasium and theater, were significantly impacted. Retaining wall bricks and landscape gravel were washed from a hillside at the college and cascaded to neighboring residences and streets below. The Hall of Justice on North David Street also sustained significant damage when water poured through a door into an underground garage. The building's elevator shafts and the ceilings in several sheriff's offices were also damaged.

Wider weather episode

Intense rainfall accompanied a strong thunderstorm which gained strength just west of Casper before blasting through town. The thunderstorm formed in a uncharacteristically moist airmass where precipitable water values approached 150% of normal. Longtime residents reported this to be the worst flash flooding they had seen in the city.

View location on OpenStreetMap → (42.8355, -106.3637)

Source: NOAA Storm Events Database, event_id 177920. Narrative written by the NWS forecast office that issued the report.