Major Extratropical Cyclones of the Northwest United States, Part I:Historical Review, Climatology, and Synoptic EnvironmentClifford Mass and Bridget Dotson 1Department of Atmospheric SciencesUniversity of WashingtonSeattle, Washington 98115Submitted to Monthly Weather ReviewNovember 20081 Corresponding author:Professor Clifford F. MassDepartment of Atmospheric Sciences, Box 351640University of WashingtonSeattle, WA [email protected](206)685-09101Abstract2IntroductionAlthough the cool waters of the eastern Pacific prevent tropical cyclones from reaching the Northwest U.S, this region often experiences powerful midlatitude cyclones capable of producing hurricane-force winds. In fact, some Northwest cyclones have winds comparable to category two or three hurricanes, are generally larger than tropical storms, and have effects amplified by tall trees, thus making such storms a major threat to life and property. Even though Northwest extratropical cyclones have frequently resulted in widespread damage and injury, national media attention has been far less than for their tropical cousins. Only a handful have been described in the literature (Lynot and Cramer 1966, Reed 1980, Reed and Albright 1986, Kuo and Reed 1988, Steenburgh and Mass 1996), and there are many questions regarding their mesoscale and dynamic evolutions, including interactions with terrain. Reviewing the NOAA publication Storm Data and newspaper accounts, suggests a conservative estimate of damage andloss due to cyclone-based windstorms over Oregon and Washington since 1950 of 10 to 20 billion (2008) dollars. Perhaps the richest resource describing the large cyclones that strike the region is the extensive series of web pages produced by Wolf Read1. Over fifty storms are described in great depth in that work, as well as articles reviewing the basic characteristics of the intense low-pressure systems that bring great damage to the region.The Pacific Northwest is particularly vulnerable to strong cyclone-based windstorms due to its unique vegetation, climate, and terrain. The region’s tall trees, many reaching 30 to 60 m in height, act as force multipliers, with much of the damage to buildings and power lines not 1 http://www.climate.washington.edu/stormking/3associated with direct wind damage, but with the impact of falling trees. Strong winds, predominantly during major cyclone windstorms, account for 80% of regional tree mortality, rather than old age or disease (Kirk and Franklin 1992). Heavy precipitation in the autumn, which saturates Northwest soils by mid-November, enhances the damage potential, since saturated soils lose adhesion and the ability to hold tree roots. The substantial terrain of the Northwest produces large spatial gradients in wind speed, with enhanced ageostrophic flow near major barriers that produce localized areas of increased or more sustained wind and damage. The most damaging winds from major Northwest storms are overwhelmingly from the south and generally occur when a low center passes to the northwest or north of the location in question. The closest analogs to major Northwest cyclones are probably the explosively developing extratropical cyclones of the north Atlantic that move northeastward across the U.K. and northernEurope. Cyclones striking both regions develop over the eastern portion of a major ocean and thus exhibit the structural characteristics of oceanic cyclones, as documented by Shapiro and Keyser (1990). Several of these events have been described in the literature, including the 15-16 October 1987 storm (Lorenc et al. 1988, Burt and Mansfield 1988), the Burns' Day Storm of 25 January 1990 (McCallum 1990), the Christmas Eve Storm of 24 December 1997 (Young and Grahame 1999), and the series of three storms that struck northern Europe in December 1999 (Ulbrick et al 2001). Browning 2004, Browning and Field 2004, and Clark, Browning and Wang 2005 present evidence that a limited area of strong winds associated with evaporative cooling anddescent (termed a sting jet) contributed the strongest surface winds during the October 1987 storm. In the discussion section below, the characteristics of Northwest windstorms and the great4extratropical cyclones of northern Europe are compared. A major difference between the landfalling major cyclones of these two regions is the substantial terrain of the Northwest, which is generally absent over western European shores. Several studies have examined the interactions between cyclones or other synoptic features and the terrain of the West Coast. Ferber and Mass (1990) described the acceleration that occurs southwest of the Olympic Mountains as strong southerly flow produces a windward ridge on its southwest flanks and a lee trough to its north, creating a hyper-pressure gradient over the coastal zone and near-shore waters. Steenburgh and Mass (1996) examined the interaction of the 1993 Inauguration Day Storm with Northwest terrain, finding little evidence of terrain-induced coastal acceleration but noting that troughing in the lee of the Olympics resulted in a several-hour extension of strong winds over Puget Sound. Bond et al (1998) using flight level data from the NOAA P3 during the December 12, 1995 windstorm, found little evidence of coastal wind enhancement along the Oregon coast. Several papers (Loesher et al 2006, Olson et al 2007, Colle et al 2006, Overland et al. 1993, 1995) examined the barrier jets that develop seaward of the high coastal terrain of southern Alaska as low-pressure systems approach and cross the coast. Major questions remain regarding storm-related coastal wind enhancement seaward of lower coastal terrain and for varied stability profiles. Another issue is the relative importance of geostrophic, antitriptic, and isallobaric dynamical balances in explaining the strong winds over and near orographic coastal zones.This paper documents the climatology of strong Pacific Northwest cyclones, examines thesynoptic environments in which they develop, describes some intense events with large societal 5impacts, considers a well-simulated recent event (the 2006 Chanukah Eve storm), and identifies some outstanding scientific questions about their development and dynamics.Historical ReviewThis section reviews a selection of strong midlatitude cyclones that have produced substantial damage and economic loss over the northwest U.S. The goal is to provide insights into the general characteristics