A post from Bryon Lawrence, CIRES scientist at NOAA’s Earth System Research Laboratory, bryon.lawrence@noaa.gov:

I sometimes like to draw an analogy between weather and cooking. There are recipes for significant hydrometerological events, and for the Northern Front Range and the nearby plains of northeastern Colorado, the ingredients for a major rain and flood event came together during the second week of September. A copious amount of rainfall is a key ingredient to flooding. Those who live in Colorado realize that several days of clouds and rain are unusual, especially in the Front Range in September. However, a late-summer moist flow of monsoonal moisture from the Pacific Ocean and Gulf of California; a nearly stationary mass of cooler air anchored over northeastern Colorado; and a trough of low pressure in the upper levels of the atmosphere over the Great Basin, all combined to produce several cloudy, very wet days. The focus of the heaviest precipitation was where the warmer moist air collided with the cooler air mass along and near the Front Range foothills. What made this storm truly devastating was the fact that it remained nearly stationary over Colorado for several days, blocked between high pressure to the northeast and low pressure to the southwest. Significant amounts of rain and snow are often observed when storm systems move slowly, and this storm was no exception. Soils had long become saturated by the time the heaviest rains arrived in the middle of the week. Unable to soak into ground, most of the rain that fell ran off, resulting in the dramatic floods.