Colorado Flash Flooding: How It Happened

By: Jon Erdman

Published: September 12,2013

 

 

 

 

 

 

 

A collection of ingredients came together to produce a destructive, deadly flash flood event in northern Colorado, including Boulder and parts of the Denver metro areas, late Wednesday and Thursday.
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The atmospheric setup, features leading to the destructive and deadly flash flooding in northern Colorado on Sep. 11-12, 2013.

Two-day northern Colorado radar rainfall estimates through the morning of Sep. 12, 2013.

First, the ground was saturated with heavy rainfall both on Sept. 9 and 10. Radar estimates suggest parts of the Front Range, including the Denver metro, picked up well over 1 inch of rain each afternoon and evening.  Street flooding was particularly severe Tuesday in the east Denver metro, including Aurora.
The atmospheric setup at right consisted of an upper-level low pressure center over the Great Basin, blocked from moving east or north by a large dome of high pressure aloft over the Pacific Northwest and southwest Canada. Winds aloft were spreading apart, enhancing lift in the atmosphere for rain and thunderstorms.
This low pressure circulation tapped a plume of moisture from the Pacific Ocean off western Mexico into northern Colorado.
According to Dr. Jeff Masters from Weather Underground (blog), the amount of moisture in the atmosphere, or "precipitable water", over Denver as of 6 a.m. MDT Thursday reached record values for the month of September (1.33 inches). Records date to 1948.
Finally, winds 15,000 feet above the ground were generally blowing from southeast to northwest and were light. This means the relatively low-topped rain and thunderstorms could not move off the Foothills and Front Range from west to east, but instead "trained" or lingered along or near the Foothills and Front Range Urban Corridor. This is a classic scenario for major flooding in northern Colorado.
Parts of the Front Range I-25 corridor were still officially in drought as of this week, according to the Drought Monitor analysis.
(MORE:  The Underrated Power of Flood Water)
   MORE: Flooding Photos

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Local residents view dangerously high Boulder Creek following overnight flash flooding in downtown Boulder, Colo., Sept 12, 2013. (AP Photo/Brennan Linsley)