A Southwest Colorado Mountain Flash Flood in an Enhanced Monsoonal Environment

National Weather Service, Grand Junction, Colorado

Abstract

Each summer the Southwest Monsoon supplies the normally arid regions of the desert southwest United States with moist air from the eastern Pacific Ocean. However, the latter part of July 1999 was unseasonably wet across southwest Colorado, with many reporting sites exceeding 200% of normal July rainfall. Saturated soil conditions coupled with heavy convective rains on 31 July 1999 produced a devastating flash flood near Ridgway, Colorado. Most of the flooding occurred in the Dallas Creek Basin, a high-altitude terrain with elevations ranging from about 2070 m MSL to the 4030 m peak of Mt. Sneffels. The convective environment was characterized by a very unstable airmass, weak wind shear, and precipitable water values approaching 170% of normal. Despite the moist and unstable pre-event environment, a synoptic trigger feature was not distinctly visible on standard height surfaces. Post-event analysis indicated that a weak shortwave existed between standard levels, but was poorly handled by all forecast models except the NGM. Consequently, atmospheric moisture fields were also better handled by the NGM, a model that seemingly tends to outforecast the ETA in monsoonal convection. The shortwave, lingering thunderstorm outflow, orographic forcing, and unique terrain factors all appear to have worked synergestically to create the unusual environment conducive for excessive rainfall in this small river basin. Rainfall on 30 and 31 July totaled nearly 96 mm, exceeding the 100-Year rainfall criterion. The resultant flash flood tripled the previous peak flow on Dallas Creek and, according to eyewitness reports, transformed the normally tranquil creek into a 100 m wide river. Although no deaths or injuries occurred, damage in this relatively unpopulated region approached $1 million. This study describes the synoptic and mesoscale features, as well as significant hydrological and geographical factors, that combined to create this record flood.