|Widespread flooding in Boulder. Courtesy Ed Szoke.|
According to the Western Region Climate Center, Boulder has an average annual precipitation of 18.72 inches and an average monthly precipitation in September of 1.53 inches. Until this event, records for the wettest month and day were 9.59 inches in May 1995 and 4.80 inches on July 31, 1919. Both of these records have been obliterated.
Rainfall estimates from the Advanced Hydrologic Prediction Service show rainfall maxima in or near Boulder of more than 8 inches for the 24-hour period ending 1200 UTC (0600 MDT) 12 September and more than 6 inches for the 24-hour period ending 1200 UTC (0600 MDT) 13 September.
Reports to the Community Cooperative Rain, Hail, & Snow Network (CoCoRaHS) show accumulations of more than 8 inches at several locations in the Boulder area over the 24-hour period ending 7 AM 12 September and approaching 6 inches over the 24-hour period ending 7 AM 13 September (reports are still trickling in).
My colleagues in the Boulder area have reported as much as 14 inches over the 2-day period which jives with these reports.
To the northwest of Boulder, the gauge along the North Fork of the Big Thompson River at Drake reached what appears to be a record crest of 10.55 feet.
Lying in the climatological lee of the Rockies, the Boulder area is relatively dry, but experiences episodes of heavy precipitation (often in the form of snow) during periods when the flow develops an easterly component and can tap into moisture from the Gulf of Mexico, as was the case, for instance, at 1200 UTC (0600 MDT) 12 September and for much of the period.
In addition, precipitable water (a.k.a. integrated water vapor) values in the sounding taken from Denver International Airport, were likely the highest ever observed in September (records go back to 1948). Prior to this event, the highest value observed was just under 1.25 inches, whereas they reached as high as 1.33 inches at 1200 UTC (0600 MDT) 12 September.
Those are easily identified factors, but more work is needed to understand this event and why it was so productive and prolonged. With over 1000 meteorologists in the Boulder area, the causes of such an extreme event will certainly be investigated in depth.