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<br />2.0 Meteorology for the 1997 Flood Season <br /> <br />The summer of 1997, particularly the months of July and August will be remembered for the <br />extreme precipitation experienced in Colorado and along the Front Range. The State of Colorado <br />among other states in southwestern U.S. experienced an unusually strong influx of warm, moist air <br />from the south that contributed to the extreme precipitation. Rainfall rates across the entire statc in <br />1997 were very high due to the influx of this moist air mass, A brief description of the large-scale <br />meteorological conditions present in the state during the summer as well as during the end of July <br />and early August are provided from materi~1 prepared primarily by Nolan Doesken and Matthew <br />Kelsch <br /> <br />General Meteorology of the 1997 Flood SeaSon <br /> <br />The year began with atmospheric conditions that were typical for the state in May and early June. <br />Early June did exhibit some atmospheric instability that manifested itself in a large precipitation <br />event in northeastern Colorado on June 6-7 as well as a Tornado in Boulder on June 6. The events in <br />early June were not associated with the influx of southern tropical moisture. <br /> <br />Following tile occural1ces in early June, most of the state of Colorado experienced dry weather from <br />mid June into mid July (Doesken, 1997). Beginning in mid July, the state experienced an abrupt <br />change in atmospheric conditions, namely the unusually strong influx of warm, moist air from the <br />south. This influx of tropical moisture contillUed throughout the summer and slowly trailed off by <br />the month of September. <br /> <br />One of the final major events in the state o(cured on September 20-21 in Lei Platta County. This <br />event was probably one of the last associated with the tropical moisture pattern and was due to a <br />combination of a typical late season cold frollt and the tropical moisture still lingering in the state <br />(Kelsch, ] 998, verbal communication). <br /> <br />Meteorology Associated with the Events of filly and August, 1997 <br /> <br />In mid July, a ridge of high pressure in the middle and upper levels of the atmosphere began to <br />funnel deep tropical moisture northward into the southwestern U.S. Disturbances on the eastern side <br />of the Rockies generated weak cool fronts which become quasi-stationary and helped move low-level <br />moisture westward toward the mountains. Storm systems across the entire state were slow moving <br />due to the wry weak steering currents. For the front range in particular storms moved in a slow <br />northward direction parralling the terrain so that storms repeated over and over again as moisture <br />was funnelled into the mountains (Kelsch, 1997). This general large-scale atmospheric condition can <br />set the stage for flash flood events in Colorado and other states in the southwest and are similar to the <br />conditions that were present in the Big Thotflpson, Colorado and Rapid City, South Dakota flash <br />floods. The state of Arizona also experienced flash floods in 1997 as~;ociated with the large-scale <br />monsoonal moisture pattern. <br /> <br />On July 27, a high pressure system perched over southern Canada began pushing a cold front south <br />into Colorado (Figure 2). At the same time, moist tropical air continued its northward movement <br />into Colorado from the south. An easterly surfacc wind developed during the day on the 27th as the <br />cold front moved southward across Colorado. This easterly wind began to move the moist air from <br />eastern Colorado and Kansas westward into the mountains (D':Jesken, 1997). Evening <br />