NWS Riverton WY - Kaycee 2002 Flood

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FLASH FLOOD IN SOUTHERN JOHNSON COUNTY AND
THE TOWN OF KAYCEE ON AUGUST 27, 2002

SUMMARY OF WEATHER EVENT THAT CAUSED FLOODING
Between 11:30 p.m. MDT August 26, 2002 and midnight, an intense thunderstorm developed along the eastern foothills of the Big Horn Mountains in southern Johnson County, Wyoming. Easterly low-level outflow winds from large storms over the High Plains fed moist air into the storm. Also, weak diverging winds in the upper atmosphere helped lift the moist air feeding in at the surface but kept the storm nearly stationary - for more than 6 hours!

By 1:00 a.m. on August 27th, the National Weather Service Doppler radar in Riverton, Wyoming
had estimated rainfall of 3 inches along the tributaries of the Middle Fork of the Powder River, from 10 to 14 miles southwest of Kaycee. Soon after, at 1:05 a.m., NWS Riverton staff issued a Flash Flood Warning for southern Johnson county. The rain continued through the night, with most of the rain falling between midnight and 4:00 a.m. The storm had weakened considerably by 7:00 a.m. Approximately 66 square miles were estimated to have received at least one inch of rainfall, with a peak estimate of 4.3 inches. Radar estimates of storm total precipitation at 6:00 a.m. included approximately 12 square miles of 6+ inches with a peak estimate of 7.4 inches. A nearby rain gauge measurement in excess of four inches supported the radar rainfall estimates since the rain gauge was located outside the peak rainfall area.
Doppler radar image below shows storm total precipitation. The white area south of Kaycee represents 6 inches of rain or more.

FLOODING AND DAMAGE REPORTS:

The incredible amount of precipitation from this storm in such a short period of time caused flash flooding across southern Johnson county. The typically semi-arid western environment around Kaycee receives 12 to 14 inches of precipitation annually. The first report of damage was water over Barnum Road (9 W Kaycee) at 2:30 a.m. Law enforcement reported the Middle Fork of the Powder River, which runs through Kaycee, two feet below bankfull at 5:10 a.m. and at bankfull by 6:20 a.m. By 7:15 a.m., the river was out of its banks and 4 feet of water was reported on Nolan Avenue
(Main Street) in Kaycee. A couple were rescued from a home due to water too deep to drive through, and a front-end loader moved a woman in a wheelchair to safety. At 9:25 a.m., county officials reported one home destroyed, with 12 heavily damaged. A final count provided by the Wyoming Emergency Management Agency on Wednesday, August 28^th reported flood damage to 19 trailers, 22 houses, and 12 of Kaycee�s 15 businesses. One hotel was broken in three pieces, with one piece carried 75 yards, and the other two pieces deposited a few hundred yards away on the opposite side of the Middle Fork of the Powder River. Also affected were the post office, town museum, conservation district office, and the telephone company. A damage survey conducted by the Service Hydrologist and Warning Coordination Meteorologist determined that normally tranquil Murphy Creek in southern Johnson County was at one point 300 yards wide and approximately 20 feet deep near Lone Bear Road. This creek eventually compromised the safety of the northbound Interstate 25 bridge over Murphy Creek. Northbound traffic was being diverted until a crossover is constructed for northbound travelers. Peak discharge was estimated at 13,500 cfs. Average mean discharge of the creek is 1.5 cfs. Total damage estimated range from $3 to $4 million.