The 2013 Colorado floods was a natural disaster occurring in the U.S. state of Colorado. Starting on September 9, 2013, a slow-moving cold front stalled over Colorado, clashing with warm humid monsoonal air from the south. This resulted in heavy rain and catastrophic flooding along Colorado's Front Range from Colorado Springs north to Fort Collins. The situation intensified on September 11 and 12. Boulder County was worst hit, with 9.08 inches (231 mm) recorded September 12 and up to 17 inches (430 mm) of rain recorded by September 15, which is comparable to Boulder County's average annual precipitation (20.7 inches, 525 mm). This event has also been referred to as the 2013 Colorado Front Range Flood, reflecting a more precise geographic extent in and along the Colorado Front Range mountains.
The National Weather Service's Hydrometeorological Design Studies Center stated in a document that the annual exceedance probability (AEP) for the entire rainfall event was as low as 1/1000 (0.1%) in places.
The flood waters spread across a range of almost 200 miles (320 km) from north to south, affecting 17 counties. Governor John Hickenlooper declared a disaster emergency on September 12, 2013, in 14 counties: Adams, Arapahoe, Broomfield, Boulder, Denver, El Paso, Fremont, Jefferson, Larimer, Logan, Morgan, Pueblo, Washington and Weld. By September 15, federal emergency declarations covered those 14 counties as well as Clear Creek County.
On October 15, 2013, the State of Colorado launched a website to assist flood survivors and victims with resources and information at coloradounited.com.