“Severe flooding began during the evening of September 11, when several areas received up to 2 inches of rainfall per hour,” says Dr. Yucheng Song, senior scientist, AIR Worldwide. “In Erie, which is located in Boulder County, the Erie Parkway was submerged under a foot of water. By the morning of September 12, a flash flood warning had been issued for the areas affected by the Waldo Canyon wildfire last year, where the lack of foliage within the burn scar has increased the risk of excess runoff.”

All government buildings in Boulder, including the evacuated University of Colorado and Naropa University, are closed.

Also closed are portions of Boulder Canyon, Big Thompson Canyon, Lefthand Canyon, Poudre Canyon and many highways including Highway 36, factors that are hampering rescue efforts. Smaller towns such as Longmont and Lyons have been completely choked off from surrounding areas by unnavigable roads.

Water had peaked overnight at Boulder Creek at almost nine feet by September 12, with an average flow of 380 cubic feet/second (the average is 64 cubic feet/second), setting a record downpour experienced in September. The Big Thompson River at Loveland, which is at 7.5 feet, is experiencing its highest flow in 14 years at 2,300 feet/second.

As of Sunday, there were five recorded fatalities—four in Boulder County and one in El Paso County—and 1,254 people unaccounted for in Colorado, although that number is expected to shrink significantly in coming days.

According to AIR, this region often experiences heavy precipitation this time of year, but it has also been an unusually wet summer, with over 10 inches of rain inundating parts of Colorado during July and August.

Dr. Song says, “Currently, a surge of moist air that originated in the Gulf of Mexico has combined with a low-pressure system over Utah and a cold front. The area's mountainous topography causes the air to cool as it moves upslope, producing more precipitation. This upslope flow will continue over the next few days due to a high pressure system to the east that will keep the system virtually stationary over the next few days.”

Water continues to rise today as the rain is expected to continue through at least Tuesday, with strong warnings along the Interstate 25 corridor.

Colorado has a history of flood events, says AIR, and thus flood mitigation plans: for example, Boulder County has codes and ordinances that limit or prohibit building in floodplain areas, and channelization and detention ponds have been built in the county. Still, about 70 percent of residential construction in Colorado is wooden, with about 40 percent having basements, which increases the risk for contents and building damage.

Most commercial buildings in the flooded areas, however, are made of steel and concrete, and are built to stricter standards than homes.

Most flood policies in Colorado are obtained through the National Flood Insurance Program (NFIP). A FEMA spokeswoman says that the NFIP insures 15,700 policies in 11 counties affected by the storm, and more than 22,000 statewide.

Swiss Re has said that 2013 is already the second costliest year for insured flood losses on sigma records. In August, rainfall in the Plains and Tennessee Valley that led to flash floods and inundated basements caused $25 million in economic losses.

Early July flooding in Ontario caused more than $850 million in initial insured property damage, and flooding in Europe, Canada and Australia caused nearly half the total insured natural catastrophe losses in the first half of the year.