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 Sep. 12 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.
How Unusual?
Peak Rain Totals
From the afternoon of September 9 through midday on September 13, 14.62 inches of rain had fallen in Boulder, Colo. This included an incredible 9.08 inches of rain on September 12, alone!
According to the Western Regional Climate Center, the previous all-time record rainfall for any calendar day in Boulder was a 4.80 inch deluge on July 31, 1919., in records dating to 1893. September 12th's total almost doubled the previous record wet day.
Furthermore, the previous wettest September day was only 3.05 inches on Sep. 4, 1909. In fact, only two other September days featured over two inches of rain, none previously since 1938.
The ten previous wettest calendar days in Boulder since 1893 all occurred between early April and early August, according to National Weather Service.
Average September rainfall in Boulder is only 1.63 inches. So, Boulder picked up almost nine times their average September monthly rainfall...in almost four days! The previous wettest single month in Boulder was 9.59 inches in May 1995.
On September 12, a storm total of 11.5 inches came in from Aurora, Colo., an eastern Denver suburb.
The average annual precipitation (rain + liquid from melted snow) in Denver is 14.92 inches. That's over three-quarters of the yearly average precipitation occurring in the span of a day or so!
This brings to mind two major flash flood events in northern Colorado in recent times:
- Ft. Collins Flood (July 28, 1997): 6-10 inches of rain the day after another heavy rain event; $200 million damage; five killed
- Big Thompson Canyon Flood (July 31, 1976): A lone thunderstorm dumped 12 inches of rain in 4.5 hours above the Big Thompson Canyon; $35.5 million damage; 139 killed
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