Synoptic Climatology of Extreme Snow and Avalanche Events in Southern Alaska

Previous research has analyzed the snow avalanche climate and synoptic patterns for the mountain ranges of the Western U.S.; however, at present there is a dearth of detailed studies for southern Alaska. This study assesses the snow avalanche climatology of southern Alaska, using all available long-term snow data extending from present back into the early twentieth century for snowfall data, and back to the 1970s concerning avalanche data from two ski resorts. Snowfall distribution in southern Alaska during large snowfall events were defined as greater than 12 inches, which are commonly associated with big avalanche cycles. Output from high-resolution WRF simulations were also used to provide further insight into factors that contribute to differences in precipitation patterns over the rugged terrain around Prince William Sound, Alaska. Synoptic data were taken from the 20th Century Reanalysis Project, encompassing a grid over the Northern Pacific Ocean and adjacent East Asia and Northwestern North America. Results indicate orography, low-level wind direction, moisture content, and temperature fluctuations all dictate the winter precipitation patterns in Southern Alaska. Years with fewer large snowfall events also coincide with warmer sea-surface temperatures in the Pacific Ocean associated with the Pacific Decadal Oscillation. Large snowfall events in Prince William Sound can be classified into three distinct categories based upon the location of the surface low and upper level pattern. The three categories are separated as 1) Surface low in the Gulf of Alaska with a 500mb trough in western Alaska, 2) 500mb low in Northern Alaska, and 3) Surface low in the eastern Aleutian Islands with a 500mb ridge in the Gulf of Alaska. Composite anomalies of upper-level flow reveal that major snow avalanche events are associated with shifts and the intensity of the Aleutian low, but other synoptic features such as adjacent ridges associated with cold-core highs at the surface can promote episodes of faceted-crystals in the snowpack at daily to weekly timeframes prior to avalanching. Increased knowledge in understanding the multivariate nature of both snow and avalanches can be used to improve forecasting skill and calculate probabilities and recurrence intervals of snowstorms or avalanches on a seasonal timescale.