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--.- CHAPTER 3 FLOOD OF APRIL 29, 1999 INTRODUCTION The flooding that occurred from April 29, 1999 through May 5,1999 was the result of an unusually long period of moderate to heavy rainfall over a broad area of southern Colorado. Up to 13 inches of rain was reported west of Interstate 25 and 5 inches to 9 inches of rain in the eastern plains. A large cutoff low-pressure area that stalled over the Great Basin caused the extended period of heavy rain. The large upper low stalkd over the Grl~at Basin for three days, from April 29, 1999 through May 1, 1999. The low-pressure area pulled in southeast winds up against the eastern face of the Rocky Mountain5, transporting large amounts of moisture up from the Gulf of Mexico at tremendous speeds. Wind profilers indicated sustained southeast winds at 40 to 50 knots just off the surface for much of the event. Since the terrain of southeast Colorado generally rises to the northwest, a strong southeast wind is a strong upslope wind. The strong orographic lifting of this strong southeast wind was the primary cause of the intense rainfall that occurred during this event. In fact, the areas th,lt received the heaviest rainfall coincided with areas that create the greatest orographic lifting during southeast wind events. This area runs along, and adjacent to, the Rocky Mountains from the Pikes Peak region south to the Wet Mountains. It includes Manitou Springs and Colorado Springs in western EI Paso County and many small mountain towns in western Pueblo County. These are the areas that received the highest rainfall that led to the extreme flooding downstream. Although local precipitation totals, on the order of 12 to 13 inches, are not uncommon within 24 hour periods, they generally occur over smaller geographic areas. The 1999 event extended over a much larger area and was not a localized flash flooding event like the one that occurred in 1997. Analyses of data provided by the network of cooperators (COOP), the Automated Local Evaluation in Real Time (ALERT), and spotter data after the event indicated that total rainfall from the storm around Manitou Springs and Colorado Springs, in westem EI Paso County, ranged from 10 to 14 inches over an elliptical area parallel to the Rocky Mountains. A center of 6 to 10 inch rainfall extended southward from tbis area, along the Wet Mountains in western Pueblo County. Elsewhere, total rainfall for thi5 event was generally in the 3 to 6 inch range across southeast Colorado. These rainfall figures reflect an extreme event for southeast Colorado. Many of the rainfall reports received were indicated to be daily or storm total records. What were extremely unusual about this event were the duration of the heavy rainfall and the time of year that it occurred. Rains of this magnitude occurring for over a day or so are very rare. In addition, rainfall of this magnitude nonnally occurs later in the season; usually in the late May to late August time frame. This storm was a rarity in both duration and time of occurrence. Post Flood Assessment Report 13 Chapter 3 - Flood of April 29, 1999 Draft Revised 09/09/99