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<br />Each of these storms shared a very warm, moist atmosphere with PWI values of 1.25 inches or <br />more, surface dew points of 60F or greater and stronger sub-cloud layer than cloud layer winds <br />(inverse wind shear). In each case, the sub-cloud layer winds were easterly in direction, 25 knots <br />or stronger and directed off the plains into the higher terrain of the foothills. The combination of <br />the strong low level winds and inverse wind shear created an ideal situation for the storm rainfall <br />mechanism to become "locked" over the terrain of a particular basin. The depth of the warm layer <br />of air greater than OC in the storm updraft averaged 2.8 km or almost twice the 1.5km associated <br />with efficient Front Range thunderstorms. The combination of the moist atmosphere, efficient <br />updraft structure, inverse wind shear and terrain-locking sub-cloud layer forcing resulted <br />in an average storm rainfall of 11.58 inches, <br /> <br />These terrain-locked storms that have ready access to copious amounts of plains moisture <br />created by the strong upslope component sub-cloud layer winds have produced the deadliest <br />Front Range flash flooding events. The continued forcing of the moist sub-cloud layer air into the <br />storm updraft appears to have lead to 3-6 hour storm duration rainfall. The Big Thompson, <br />Cheyenne, Plum Creek and Frijoles Creek storms appear to have been steady state super-cell <br />thunderstorms. The recent Fort Collins event appears to have been caused by a "train-echo" <br />effect of successive storms moving repeatedly off the terrain across the same basin or area. <br />Both steady state super-cells in inverse wind shear and "training storms" produce highly <br />localized extreme precipitation events when the storms are locked into a terrain feature. <br /> <br />A third form of foothills flash flooding storm is the severely shearedlhigh elevation storms <br />exemplified by the Virginia Creek storm of August 1981 and the recent Buffalo Creek storm of July <br />1996. Both of these storms formed at elevations above 10,500 feet and produced flash floods in <br />the higher foothills above 7,500 feet. The primary differences in these storms from the <br />foothills/plains storms was the absence of inverse wind shear, much stronger cloud-layer winds <br />and strong dynamic cloud-layer forcing that produced severe weather events on the adjacent <br />plains. It is possible that the production of hail by these two storms in the foothills limited the <br />amount of rain that was produced. Note that these storms produced an average of only 4.19 <br />inches of rain or about a third of the storms connected to a plains low-level jet. <br /> <br />Finally, the Cheyenne storm of August 1, 1985 appears to be an aberration of the super-cell <br />storms described storms. It shares the severely sheared environment of the Virginia Creek and <br />Buffalo Creek storms but was "locked over a basin" by a low level circulation generated by the <br />interaction of the storm updraft with a low level jet and upper level jet like the Big Thompson, <br />Frijoles and Plum Creek storms, In addition this storm was connected to a strong, moist low-level <br />jet. Large hail and two tomadoes were additional severe events produced by the Cheyenne super- <br />cell storm, It is likely that the severely sheared vertical wind structure of this storm limited the <br />amount of rainfall produced. Clearly, the interaction of the storm atmosphere's vertical wind <br />profile and the strength of the low level winds play critical roles in determining the <br />strength and duration of the extreme precipitation event Neither of these factors is <br />considered quantitatively in the existing site-specific PMP methodology. <br /> <br />3. Flash flooding events west of the Continental Divide (CD-West storms) <br /> <br />The eight storms that produced an extreme precipitation event west of the Continental Divide and <br />their attendant atmospheric structure are summarized in Table 2. The best examples of the west <br />CD storms are the Dallas Creek and Saguache Creeks storms of July 1999 and the Opal, <br />Wyoming storm of August 1990. Good radar data was available for the Saguache and Dallas <br />Creek storms while cloud-to-ground lightning data was used to assist in the re-construction of the <br />remaining storms except for the Muddy Creek storm of June 1994. <br /> <br />3 <br />