Destructive Winds Caused by an Orographically Induced Mesoscale Cyclone

Abstract

Sustained winds in excess of 70 kt and gusts of close to 100 kt were measured at the Hood Canal Bridge in western Washington on the morning of 13 February 1979 shortly before the bridge collapsed. An extraordinary blowdown of timber also occurred in the nearby area. Elsewhere in western Washington maximum one-minute wind speeds were generally 40 kt or less and peak gusts were mostly under 65 kt. Wind damage was widespread but less concentrated than in the vicinity of the bridge. Use was made of all available routine and nonroutine data sources to analyze the cause of the unusual winds. The analysis revealed that a meaoscale low pressure system or vortex formed in the lee of the Olympic Mountains as a deep Pacific storm moved ashore on central Vancouver Island. The pressure gradient on the southeast side of the mesolow exceeded 5 mb over a distance of less than 10 n. mi. The area of major destruction lay over and downwind of the region of extreme pressure gradient. At the height of the storm the winds, which blew from a south-southwesterly direction, had a fetch of 3-4 mi along the Hood Canal. The overwater fetch also appeared to contribute significantly to the extraordinary speeds observed at the bridge. Maps are presented showing both the large-scale and mesoscale features of the storm.