Heavy rains along the Colorado Front Range over the past three days have produced flooding from Colorado Springs to Ft. Collins; the worst of the flooding has been in and near Boulder where some foothill locations have received a foot of precipitation and where several canyon streams cut across the city.
This deluge is the product of three atmospheric factors: an upper level low over the Great Basin, high pressure over the Southern Plains and a cold front that dropped south across the High Plains. Behind the latter, an easterly, upslope flow is pushing moisture toward the Front Range; in addition, monsoon moisture is flowing northward across eastern Colorado, pumped from the Gulf of Mexico and the Sea of Cortez by the combined effects of the Great Basin low and the Southern Plains high. In the Northern Hemisphere, winds move counterclockwise around low pressure zones and clockwise around high pressure domes; in this case, those wind patterns are merging to sweep abundant moisture toward the north. As that warm, moisture-laden air encounters the cold front, it rises and condenses, adding copious precipitation to the upslope flow behind the front.
Twelve days into September, Boulder has already set a precipitation record for any given month (exceeding the previous record of 9.6 inches in May, 1995). Since periods of heavy rain are expected to continue over the next few days, that previous record will be shattered and the risk of flash flooding and mudslides will persist.
