![]() ![]() Lackmann and Gyakum (1999) examined the composite synoptic evolution associated with 46 heavy precipitation events over Washington State in which each of four observing sites received at least 12.5 mm (~0.5 in.) of daily precipitation. A thorough understanding of the synoptic conditions conducive to extreme precipitation events along the west coast of North America is essential for making future projections.Ī series of major floods over the western United States during the 1980s and 1990s stimulated research on the nature of heavy precipitation over the region. 2011) since ARs are generally on the southern flank of the jet stream. Such changes could potentially have a large impact on precipitation in the Pacific Northwest and California ( Salathé 2006 Dettinger 2011 Mass et al. It has been theorized that changes in equator-to-pole temperature gradients ( Yin 2005) and Hadley cell expansion could push the storm track poleward in the Northern Hemisphere ( McCabe et al. 2005 Held and Soden 2006 Trenberth et al. Some studies have suggested that anthropogenic global warming could impact the severity and frequency of extreme precipitation events in the future ( Groisman et al. 2008 Viale and Nuñez 2011 Dettinger et al. AR water vapor is often quickly converted to heavy precipitation upon landfall on coastal and inland mountain ranges, inundating local watersheds and causing severe flooding in low-lying areas ( Ralph et al. In a seminal paper on the topic, Zhu and Newell (1998) found that greater than 90% of the hemispheric meridional water vapor flux is transported by such features, with four to five evident at any given time. Most extreme precipitation events along the North American west coast are associated with narrow plumes of above-normal water vapor that stretch from the subtropics or tropics to the coast and are often referred to as atmospheric rivers (ARs). More recently, heavy rainfall and resulting flooding during January 2009 closed Interstate 5 and other major routes in Washington State, flooded major river drainages throughout the Northwest, and heavily damaged the Howard Hanson Dam in the Washington Cascades, putting 10–20 billion dollars of assets and infrastructure, as well as tens of thousands of people, at risk ( Mastin et al. For example, during December 1996 and January 1997, heavy rain (25–100 cm in two weeks) produced severe flooding over portions of California (CA), Washington (WA), and Oregon (OR), causing 3.9 billion dollars (all amounts are in U.S. Since 1955, these storms were responsible for roughly two-thirds of the presidential disaster declarations in Washington State and Oregon, and nearly one-quarter of the declarations in California (see ). ![]() The northwest United States frequently experiences extreme precipitation events during the winter months, resulting in billions of dollars of damage as well as loss of life. High precipitable water values are more frequent during the summer, but are not associated with heavy precipitation due to upper-level ridging over the eastern Pacific and weak onshore flow that limit upward vertical velocities. ![]() ![]() A small subset of extreme precipitation events over the southern portion of the domain is associated with a very different synoptic evolution: a sharp trough in northwesterly flow and post-cold-frontal convection. Composite evolutions indicate negative anomalies in sea level pressure and upper-level height in the central Pacific, high pressure anomalies over the southwest United States, large positive 850-hPa temperature anomalies along the coast and offshore, and enhanced precipitable water and integrated water vapor fluxes over southwest to northeast swaths. Precipitation areas of major events are generally narrow, roughly 200 km in width, and most are associated with atmospheric rivers. Most regional flooding events are associated with precipitation periods of 24 h or less, and two-day precipitation totals identify nearly all major events. NCEP–NCAR reanalysis data were used to construct synoptic composite evolutions of these events for each coastal location. To answer these questions, this study uses 60 years of National Climatic Data Center (NCDC) daily precipitation observations to identify the top 50 events in two-day precipitation at six coastal stations from northern California to northwest Washington. Outstanding questions about such events include whether there are a range of associated synoptic evolutions, whether such evolutions vary along the coast, and the associated rainfall duration and variability. Extreme precipitation events impact the Pacific Northwest during winter months, causing flooding, landslides, extensive property damage, and loss of life. ![]()
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