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From: To: Date sent: Subject: "Markus Ritsch" <HORSETOOTH/M:LR> m1r Thu, 26 Mar 1998 15:05:52 MST-0700 Met Discussion Matt, Here is my attempt to describe in laymans term:; L:1e statewide meteorological picture leading to the 1997 flooding. I wrote this draft with info from your FSL article and Nolan's precip study. I appreciate you taking a few minutes to read through this sure I am consistent with your knowlege of what happened. free to make any corrections you see fit, remember, I am a hydrologist. to make Please feel Thanks, I owe you one. Markus The summer of 1997, particularly the months of July and August, wi~l be remembered for the extreme precipitation experienced in eastern Colorado and along the Front Range. A brief dE:;scription of the large-scale meteorological conditions present in the state during the end of July and early August is provided from natE:;rial prepared primarily by Nolan Doesken and Matthew Kelsch. The year began in typical fashion with much of the state .:)f Co1orado experiencing dry weather from mid June into mid July (Doesken, 1997). In mid July, a ridge of pressure in the middle and upper levels of the atmosphere began to funnel deep tropical moisture northward into the southwestern U.S. Disturbances on the eastern side of th2 Rockies generated weak cool fronts which become quasi-stationary and helped move low-level moisture westward toward the mountains. Slow moving wind currents kept the storm cells moving in a northerly d,ireccion away from their point of inception as new storm c€:;lls developed from the moist air corning into the mountains from the west (Kelsch, 1997). This general large~scale atmospheric condition can set the stage for flash flood events in Colorado and are similar to the concH tions tJ"lat were present in the Big Thompson and Rapid City flash floods. On Jely 27, a high pressure system perched over southern Canada oeqan pushing a cold front south into Colorado. At the same time, moist tropical air continued its northward movement into Colorado from the south. An easterly surface wind developed duri.ng the day on the 27th as the cold front moved southward acrosS Colorado. This E~asterly wind began to move the moist air from eastern Colorado and Kansas westward into the mountains (Doesken, 1997). Evening thundershowe:cs developed during the night of July 27 along the Front Range which p:coduced heavy amounts of precipitation. On July 28, tropical moisture was present at all levels within the atmosphere across the state. The easterly surface flow o~ moist air continued as the cool front became quasi-stationary (Kelsch, 1997) This large-scale scenario changed very little during the day. Thunderstorms developed along the Front Range in the afternoon of the 28th. The thunderstorm cells formed a line anc. began moving slowly toward the east-northeast away from the foothills. As thEe thunders torms moved east, they produced a soutt.easterly outflow which enhanced the low-level upslope back toward the foothi.lls (Kelsch, 1997). The regeneration of storm cells due to the southeasterly outflow was one process working to generate the qc.asi-stationary sto:::-m complex. In addition, southwest-northeast banc.s of convective enhancement appeared to join forces with the 1mV' level southQasterly Markus Ritsch -- 1 TUE, 21 Apr 1998 lO:4~:29