The pre-cold-frontal low-level jet within oceanic extratropical cyclones represents the lower-tropospheric component of a deeper corridor of concentrated water vapor transport in the cyclone warm sector. These corridors are referred to as atmospheric rivers (ARs) because they are narrow relative to their length scale and are responsible for most of the poleward water vapor transport at midlatitudes. This presentation will discuss landfalling ARs along adjacent north- and south-coast regions of western North America. SSM/I satellite observations of long, narrow plumes of integrated water vapor (IWV) are used to detect ARs over the eastern Pacific from 1997-2005. The north- and south-coast regions experienced 301 and 115 ARs, respectively. Most ARs in the south (north) occurred during the cool (warm) season, despite the fact that the cool season is climatologically wettest for both regions. Composite SSM/I IWV analyses show landfalling wintertime ARs extending northeastward from the tropical eastern Pacific, whereas the summertime composites were zonally oriented and, thus, did not originate from the tropics. Companion SSM/I composites of daily rainfall show significant orographic enhancement during the landfall of winter (but not summer) ARs.

The NCEP-NCAR reanalysis dataset and regional precipitation networks are used to assess composite synoptic characteristics and over-land impacts of landfalling ARs during winter and summer. The ARs (as defined by the SSM/I IWV plumes) are accompanied by strong vertically-integrated horizontal water vapor flux impinging on the West Coast in the pre- (post-) cold-frontal environment in winter (summer). Even though the IWV in the ARs is greater in summer, the vapor flux is stronger in winter due to much stronger flow associated with more intense storms. The landfall of ARs in winter and north-coast summer coincide with anomalous warmth, a trough offshore, and ridging over the Intermountain West, whereas the south-coast summer ARs coincide with relatively cold conditions and a trough near the coast. ARs have a more profound impact on near-coast precipitation in winter than summer, because the terrain-normal vapor flux is stronger and the air more nearly saturated in winter. During winter, ARs with the largest SSM/I IWV are tied to more intense storms with stronger flow and vapor flux, and more precipitation. ARs generally increase snow-water equivalent (SWE) in autumn/winter and decrease SWE in spring. On average, wintertime SWE exhibits greater than normal gains during AR storms, except for lower elevations where warmer-than-normal conditions yield rain.