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<br /> <br />A gust front moved north and northwest from the Denver area, passing <br />Boulder between 5:30 and 5:45 p.m. with gusts of 35 to 45 mph (56 to 72 <br />km/hr) and a noticeable wall of dust. It reached Ft. Collins about <br />5:45 p.m., producing gusts to 40 mph (64 km/hr). Evidence points to <br />increased low level convergence associated with this gust front as a <br />significant factor in the explosive development of thunderstorms in <br />western Larimer County. <br /> <br />At 7:35 p.m. WSFO Denver issued a severe thunderstorm warning for eastern <br />Larimer County stating that local flooding in low areas was possible, <br />especially just to the west of Ft. Collins. The warning was based on <br />radar information and was in effect until 9:00 p.m. A special weather <br />statement issued at 9:00 p.m. mentioned a line of moderate to heavy <br />thunderstorms which could result in local flooding in the Colorado counties <br />of Larimer, Boulder, northern Clear, and extreme eastern Jackson. This <br />statement indicated that the rain could be heavy until about midnight. <br />A flash flood warning for the portion of the Big Thompson River east of <br />the canyon (i.e., from near Loveland to Greeley) was issued at 11:00 p.m. <br /> <br />FINDINGS AND RECOMMENDATIONS <br /> <br />Finding 1: Guidance forecasts of rainfall amounts and the zone forecasts <br />issued by WSFO Denver gave no indication of the abnormally heavy rainfall <br />that produced the flash flood. QPF guidance from NMC does not handle <br />convective precipitation very well. This fact is well known to NWS <br />forecasters. There is a need to provide NWS field offices with additional <br />guidance material on the heavy rainfall amounts associated with convective <br />activity as well as techniques to allow local offices to derive objective <br />estimates of rainfall rates from upper air observations. A substantial <br />research effort will be needed. <br /> <br />Recommendation: NOAA should initiate an R & D program to develop an <br />improved capability for predicting rainfall amounts associated with convec- <br />tive activity. As soon as new technology can be developed, guidance <br />for convective rainfall should be issued on an operational basis. <br /> <br />Finding 2: The warning issued by WSFO Denver at 7:35 p.m. made no mention <br />of heavy rainfall or flash flooding but did mention the possibility of <br />flooding in low areas. The 9:00 p.m. statement referred to the possibility <br />of heavy rain and local flooding. No mentiQn was made of flash flooding <br />until 11:00 p.m. Issuance of a flash flood watch is a matter of profession- <br />al judgement. The forecaster was a qualified professional and he made <br />reasonable use of the information available to him. Feedback information <br />on rainfall amounts, river stages, and conditions in the Big Thompson <br />drainage basin was not available in time to allow WSFO Denver to issue a <br />specific flash flood warning for the Big Thompson Canyon prior to onset <br />of severe flooding. <br /> <br />32 <br />