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). 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. 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 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. 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. Strong winds, predominantly during major cyclone windstorms, account for 80% of regional tree mortality, rather than old age or disease (Kirk and Franklin 1992). 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 associated with direct wind damage, but with the impact of falling trees. The Pacific Northwest is particularly vulnerable to strong cyclone-based windstorms due to its unique vegetation, climate, and terrain. 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.
#November 11 2015 storms in central united states series
Perhaps the richest resource describing the large cyclones that strike the region is the extensive series of web pages produced by Wolf Read 1. Reviewing the NOAA publication Storm Data and newspaper accounts, suggests a conservative estimate of damage and loss due to cyclone-based windstorms over Oregon and Washington since 1950 of 10 to 20 billion (2008) dollars. 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. 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. 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. Major Extratropical Cyclones of the Northwest United States, Part I: Historical Review, Climatology, and Synoptic EnvironmentĪlthough 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.
Table 2: Major cyclones that crossed between the Olympic Mountains and central Vancouver Island.A Recent Example: The Chanukah Eve Storm of December 14-15, 2006.
Synoptic Composites of Northwest Windstorms.
0 kommentar(er)
