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LESSONS of RECOVERY' A REViEW of TIlE 1 997 COlORAdo Flood DisASTER Pawnee Creek Storm I In historical context, the large rain amounts produced by the summer storms of 1997 are not unprecedented for the Front Range and Eastern Plains areas of the state, which are subject to convective thunderstorm activity when monsoon moisture or other weather patterns supply sufficient humidity to the atmosphere (Grigg, et ai, 1998). By comparison, the thunderstorm that caused the disastrous flash flood in Big Thompson Canyon in 1976 produced between 12 and 14 inches of rainfall at the western end of the canyon in less than five hours. On July 27, 1997, wet tropical air streaming northward from Mexico supplied the moisture and an approaching cold front from the west provided the trigger for setting off heavy thunderstorms as the two moist air masses met over the state. IMPACTED COMMUNITIES Fort Collins July 28, 1997 6.5 hours 10.2 inches Larimer Fort Collins Schaefer Draw July 29-30, 1997 15 hours 10.5 inches Mor9an Weldona Pawnee Creek July 29-30, 1997 9 hours 14 inches Logan Atwood/Sterling . . . ' '. . . ' . . , Fort Collins Storm The Fort Collins storm produced the heaviest rains ever documented to have fallen over an urbanized area of Colorado (Grigg, et ai, 1998). The ten inches of rain that fell in parts of Fort Collins between 5:30 p.m. and 11 :00 p,m. on July 28 set rainfall records for largest one-day, three-hour, and six-hour precipitation totals at the CSU gage. Soils in the area were already saturated due to heavy rainfall (four to six inches) from the previous evening. Runoff from the Fort Collins storm flooded several drainages, most notably . along Spring Creek where the five fatalities occurred and where 120 mobile homes were destroyed and hundreds of other homes were damaged, Schaefer Draw Storm Schaefer Draw in Morgan County is a tributary of the South Platte River and encompasses approximately 14 square miles of drainage area. The community of Weldona is situated at the lower (southern) end of the basin, approximately 0.5 miles from the river. During the evening of July 29-30, 1997, a storm centered about six miles southeast of Weldona produced as much as ten inches of rainfall, generating heavy runoff that flooded homes and businesses in Weldona. According to the National Weather Service, up to 10.5 inches of rain fell in a 15-hour period near Weldona. Much of the precipitation total probably fell during a six- hour period from 7:00 p.m. on July 29 until 1 :00 a.m, on July 30, Flood problems in Weldona were magnified as a result of breaches in two irrigation ditches above the town. COlORAdo OFfiCE of EMERGENCY MANAGEMENT 2