Atmospheric Rivers in Southeast Alaska and British Columbia: The Bella Coola Event of 2010 and Alaska Events of 2012

Abstract:

Floods are a recurring natural hazard responsible for large socioeconomic losses globally. In mid-latitude locations, such as Western North America and Europe, heavy precipitation and floods, are connected to intense water vapor transport in extra-tropical cyclones called atmospheric rivers (ARs). This AR region is narrow (on the order of 300-500 km wide) and transports the majority of the poleward moisture flux. Given the strong link between ARs and floods on the west coast of North America, it is the aim of this research to determine if ARs are responsible for hydrohazards in British Columbia and Alaska.
Using satellite measurements, atmospheric reanalyses, and in-situ observations we undertake a hydrometeorological analysis on two major flood events, namely the Bella Coola flood in British Columbia in September 2010 in which 10 inches (250mm) of rain fell in 36 hours, and an Alaskan event that produced over 50 inches (1250 mm) of precipitation in the month of September 2012 (mostly in two landfalling ARs), and led to record river stage heights. Furthermore, the Alaskan event resulted in one fatality and $35M in damages to buildings, homes, and infrastructure.
Preliminary results suggest that AR conditions were present during these events, and are therefore likely to be important for hydrohazards more generally in British Columbia and Alaska. As the enhanced water vapor transport in the ARs encountered the steep terrain in these regions orographic enhancement of rainfall occurred resulting in record rainfall totals and floods. The occurrence of these events in September (earlier than noteworthy AR events in the U.S. West Coast farther south) may also relate to the earlier nascence of the winter circulation pattern in northern latitudes.