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    Why Was the September 2013 Colorado Flooding So Bad?

    By By Alex Sosnowski, Expert Senior Meteorologist.
    September 22, 2013, 12:56:30 AM EDT

    While other floods over the years in Colorado may have been more intense, the Flood of 2013 occurred over a much larger area and was significantly longer lasting than most.

    Flash floods in Colorado are uncommon, but not rare. The rugged terrain combined with access to moisture from the Gulf of Mexico and the tropical Pacific tends to stack the deck.

    According to the National Center for Atmospheric Research (NCAR), since 1864, there have been at least 22 floods with a 1999 dollar value of $2 million or greater.

    Thunderstorms along the Rockies and foothills are quite common during the summer months.


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    Moisture is drawn in from the Gulf, Pacific or both. The sun heats the ground uniformly, but the higher elevations give rising thermals an extra boost, compared to lower elevations over the Plains to the east and Great Basin to the west. As that air rises, it cools and forms towering clouds. These clouds then release the moisture in the form of heavy rainfall.

    Typically, these storms are short-lived and are pushed along by strong winds high in the atmosphere.

    During the Flood of July 1976, a small cluster of thunderstorms erupted. However, rather than move along, the storms remained nearly stationary for several hours and centered over the Big Thompson River Canyon.

    According to the National Weather Service in Boulder, Colo., the 1976 storms dropped 12 inches rain in three hours and 8 inches of rain in 1 hour.

    The single wall of water rushing downstream along the Big Thompson River caught hundreds of people off guard and led to 145 fatalities and $40 million in damage in 1976.

    The flood during 1965 was intense, very damaging and encompassed multiple days (June 14-20) but was not as extensive as 2013. According to NCAR, 21 people lost their lives and damage topped $500 million. Rainfall up to 12 inches was reported during the overnight hours of June 14-15 near Fort Collins.

    A little over 48 years later, a similar, but much longer-lasting and more widespread flooding event set up over Colorado, New Mexico, western Kansas and southern Wyoming. In Colorado, the event spanned Sept. 8-15, 2013. In parts of New Mexico, the flooding continued into Sept. 16.

    The key weather players during the September 2013 flooding event were a large swath of tropical moisture over the Rockies (referred to as the Monsoon by locals), a large area of high pressure over the Midwest and a storm in the upper atmosphere over the Great Basin.


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    The moisture over the Rockies was literally being squeezed from both sides by the high to the east and the dry air rotating in from the Great Basin around the upper-level storm. This squeezing resulted in a much more vertical profile of moisture than would have occurred without either system present. The high over the Midwest also drove additional air thousands of feet uphill from the Plains to the foothills and Rockies. This action released extra moisture and further enhanced the rainfall.

    The high over the Midwest acted like a giant roadblock and turned what would have been a several-hour event into a week-long ordeal.

    The result was a plume of heavy rain that re-fired on an almost daily basis from New Mexico to Colorado and southern Wyoming.

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    While the Flood of 1976 was more intense over a small area and the Flood of 1965 was intense and lasted for days, the Flood of 2013 lasted nearly a week and covered hundreds of square miles in multiple states. Rainfall exceeded 12 inches at a number of locations.


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    Officials are hopeful that the number of fatalities will remain much lower during the 2013 event, but there are many still unaccounted for.

    According to the Larimer County, Colo., Sheriffs Office, as of Tuesday midday, the latest numbers for unaccounted individuals was down to 197 people from nearly 400 early Monday.

    Statewide, however, more than 500 people remain unaccounted for as of Tuesday morning, the Colorado Department of Emergency Management told AccuWeather.com.

    Despite the enormity of the damage and the dangers to residents, the excessive rainfall has provided relief for the drought-stricken state and will alleviate the wildfire threat moving forward.

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