Atmospheric Rivers in Southeast Alaska: Meteorological Conditions Associated With Extreme Precipitation

Extreme precipitation events associated with atmospheric rivers (ARs) trigger floods, landslides, and avalanches that threaten lives and livelihoods in Southeast Alaska. Six rural and indigenous communities (Hoonah, Klukwan, Skagway, Yakutat, Craig, and Kasaan) identified specific needs regarding these hazards and joined the Southeast Alaska Coastlines and People (CoPe) Kuti Hub to address the shared challenge of understanding and predicting these events. This study presents a climatology (1980-2019) of synoptic, mesoscale, and local meteorological characteristics of ARs and heavy precipitation across this region. High-amplitude upper-level patterns across the northeastern Pacific Ocean favor ARs reaching Southeast Alaska, where moisture is orographically lifted, resulting in heavy precipitation. In the six communities, ARs occur 8-15 days per month, yet only 6 AR days per year account for up to 68%-91% of precipitation extremes. Furthermore, 80%-96% of days with extreme precipitation have >75th percentile integrated water vapor transport (IVT), demonstrating the strong relationship between IVT and extreme precipitation. This study also highlights the relationship between IVT direction and complex coastal topography in determining precipitation extremes. For example, in Klukwan and Skagway, 80%-90% of extreme AR days have south-southwesterly or south-southeasterly IVT. Coastal communities like Yakutat experience higher IVT and precipitation overall, and although southeasterly IVT is more common, extreme precipitation events are most common with southwesterly IVT. Collaboration with the National Weather Service in Juneau, Alaska will lead to improved situational awareness, forecasts, and Impact Decision Support Services to communities, saving lives and property in a region vulnerable to the impacts of climate change.