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Meteorology for the 1997 Flood Season the heaviest rainfall events in recorded history for the areas of Meeker and Steamboat (Doesken 1998b). One of the final major events for the season occurred from September 20 through 21 in La Plata County. 1his event was one of the last associated with the tropical moisture pattern and was caused by a late season cold front combined with the tropical moisture still lingering in the state (Kelsch 1998). 2.2 Meteorology Associated with the Events of July and August, 1997 In mid July, a ridge of high pressure in the middle and upper levels of the atmosphere began to funnel deep tropical moisture northward into the southwestern United States. Disturbances on the eastern side of the Rockies generated weak cool fronts, which became quasi-stationary and helped move low-level moisture westward toward the mountains. Storm systems across the entire state were slow moving because of the weak steering currents. For the Front Range in particular, storms moved in a slow northward direction paralleling the terrain so that storms repeated over and over again as moisture was funneled into the mountains (Forecast Systems Laboratory 1998). 1his type of general, large-scale atmospheric condition can set the stage for flash flood events in Colorado and other states in the southwest and is similar to the conditions that were present during the Big Thompson, Colorado, and Rapid City, South Dakota, flash floods. The State of Arizona also experienced flash floods in 1997 associated with this large-scale monsoonal mois~e pattern. On July 27, a high pressure system perched over southern Canada began pushing a cold front south into Colorado. At the same time, moist tropical air continued its northward movement into Colorado from the south. A surface wind from the east developed during the day on the 27th as the cold front moved southward across Colorado. 1his wind began to move the moist air from eastern Colorado and Kansas westward into the mountains (Colorado State University, Department of Atmospheric Science 1997). Evening thundershowers developed during the night of July 27 along the Front Range and elsewhere in eastern Colorado, producing heavy precipitation. On July 28, tropical moisture was present at all levels within the atmosphere across the state. The surface flow of moist air from the east continued as the cool front became quasi-stationary (Forecast Systems Laboratory 1998). 1his large-scale scenario changed very little during the day. Thunderstorms developed along the Front Range on the afternoon of the 28th. The thunderstorm cells formed a line and began moving slowly toward the east- northeast away from the foothills. As the thunderstorms moved east, they produced an outflow that enhanced the low-level upslope back toward the foothills (Forecast Systems Laboratory 1998). The regeneration of storm cells d479/chap2.doc OB/12/98 2-2