Large Scale Influences on Atmospheric River Induced Extreme Precipitation Events Impacting the Coast of the State of Washington

Along the coast of Washington State, landfalling atmospheric rivers (ARs) have a significant hydrometeorological impact by bringing precipitation to the region alleviating droughts and increasing the region’s water supply. Though sometimes beneficial, ARs can also bring extreme amounts of precipitation leading to natural disasters, such as avalanches, mudslides, and severe flooding which cause hazardous and life-threatening conditions for people living in this area. Using the Modern Era Retrospective analysis for Research and Applications, Version 2 (MERRA-2), we investigate characteristics of ARs associated with extreme precipitation events for 1980-2015 during the months of December to February, when most AR events occur. We use the TempestExtremes AR detection algorithm, which uses spatial and temporal thresholds of MERRA-2 column integrated transport of water vapor (IVT), to detect ARs. ARs associated with extreme events exceeding the 95th percentile of daily precipitation are compared to landfalling ARs that do not meet the extreme criteria to identify the features that produce extreme events. The importance of the strength and location of the Aleutian Low and North Pacific High in creating extreme events is highlighted through an investigation of synoptic variables during AR events. Important differences exist with the 500hPa geopotential height and upper level jet, creating a more favorable flow pattern allowing for higher total precipitable water vapor to reach the Washington coast. In addition, intraseasonal and interannual variability of AR induced extreme precipitation events will be assessed through a statistical evaluation of teleconnection patterns. Preliminary results indicate an enhancement of extreme precipitation events associated with certain phases of the Madden Julian Oscillation; however, El Niño–Southern Oscillation, the Pacific Decadal Oscillation, and the Pacific–North American pattern will also be explored.