The downpour that inundated parts of Colorado this month was a once-in-a-millennium event for those areas, according to an analysis by the National Weather Service.
Colorado residents are coping with widespread destruction from floods unleashed by torrential rains that began on September 9 and lasted for several days. The flooding killed at least eight people, forced thousands from their homes and caused nearly $2 billion in property damage.
Towns at the base of Colorado's so-called Front Range in Larimer and Boulder counties, northwest of Denver, experienced the most damage.
"As it kept raining and kept raining and kept raining, this thing kept getting more and more rare, in terms that we use for evaluating," said Geoffrey Bonnin, chief of the Hydrologic Science and Modeling Branch of the National Weather Service.
To date, monthly rainfall for September in Boulder, Colorado, totaled 17.2 inches, the most for any month since official recordkeeping began in 1893, said Byron Louis, program manager for the National Weather Service in Colorado.
Rain in that area fell nearly continuously from September 9 to September 15, Louis said.
That amount of rainfall around Boulder is likely to occur less than once every 1,000 years, according to the weather service.
Two swaths of land experienced the once-in-a-millennium rainfall. One area extended south of Boulder to more than 40 miles north in the region of Estes Park, according to a map from the National Weather Service.
The second area that was swamped extended from the Denver suburb of Aurora to lands about 40 miles north, the map showed.
"There was quite a bit of area there that really got clobbered," Bonnin said.
The extent of flooding seen in Colorado has drawn comparisons to a 1976 flood in the state that killed nearly 150 people along the Big Thompson Canyon near Loveland.
Geologists have not been able to measure how the volume of floodwater that struck parts of Colorado this month compared to the flooding in 1976 because two key gauges along the Big Thompson River were swept away, said Robert Kimbrough, hydrologist for the U.S. Geological Survey in Denver.
"It's really frustrating both gauges were taken out. That's an indicator of how large an event this was," Kimbrough said.
The U.S. Geological Survey will dispatch teams of spotters to measure the high water mark from this month's flooding, which will allow the agency to determine how the flooding compares to past disasters, he said.
(Reporting by Alex Dobuzinskis; Editing by Cynthia Johnston and Stacey Joyce)