A period of excessive rainfall between September 10 through 15 has triggered excessive flooding in Northeastern Colorado. Rainfall has exceeded five inches over a very large area, and some local rainfall amounts have exceeded 15 inches. Damage has occurred as a result of rushing water and inundation, affecting homes, businesses and infrastructure over a large area. At least seven fatalities have been reported, with many more unaccounted for. At least 17,500 homes have been damaged by flash-flooding, with some 1,500 completely destroyed. At least 11,700 have left their homes. Response efforts are ongoing with both state and federal support. It will take some time to assess the precise impacts of this event, and as always our first thoughts are with those lost and directly affected.
Excessive rainfall occurred in Northeastern Colorado between September 10 through 15. Upper-level flows enabled an unusual moisture feed, together with aggressive ascent in the atmosphere (forced in part by the Rocky Mountains). These enabling conditions, combined with a series of surface disturbances and frontal boundary, triggered a series of heavy rainfall events in the area. Rainfall rates were reported by the National Weather Service (NWS) to exceed two inches per hour. Total rainfall amounts were reported to exceed 12 inches in Boulder over a three day period, with reports of up to 15 inches in Boulder County. Radar estimates of total rainfall show five inches over a very large area north of I-70, with local amounts exceeding 15 inches in some areas of the foothills. The excessive rainfall occurred over already saturated soils. Local watersheds were overwhelmed, and severe flooding occurred as a result. Such a scenario has been described as rare by the state climatologist.
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The map above depicts radar-estimated precipitation in inches. SOURCE: NOAA
At least seven fatalities have been reported so far, and two more are presumed dead after homes were taken by floodwaters. State emergency officials report many unaccounted for (although communications disruptions may be partly to blame). More than 2,100 people have been evacuated, and at least 11,700 have left their homes.
While damage assessment reports are preliminary, officials expect severe damage to infrastructure and communities. Damage due to rushing waters together with inundation over a wide area can be expected. According to the Colorado Office of Emergency Management, around 1,500 homes have been destroyed, and at least 17,500 have been damaged by flash-flooding.
Major roads have been washed out, especially those adjacent to flooded creeks and rivers. Communications and power systems have been disrupted over a large area, together with gas and sanitary systems. Officials indicate that the most severely affected areas could be unreachable in the near-term, and uninhabitable for up to a year.
Governor Hickenlooper has declared a disaster emergency in the state, and President Obama has declared a major disaster in Colorado, enabling federal aid to support local and state response efforts. The National Guard have been activated from both Colorado and Wyoming to support. FEMA is sending two teams of 80 for the search and rescue effort, including ongoing efforts in Larimer County. Rescue efforts were impeded for a period of time as a result of grounded helicopters under restrictive weather, although weather conditions are expected to clear today.
According to the National Weather Service Precipitation Frequency Data Server, this event exceeds a 1-in-1000 year rainfall event. Local officials have compared this event to the Big Thompson Canyon Flood of 1976 and the Lawn Lake Flood of 1982.
Sources: Agence France Presse, Reuters, Associated Press, U.S. National Weather Service, U.S. Storm Prediction Center
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