Orographic Microbursts in a Severe Winter Windstorm

• Main Author: Tucker, Zena A.
• Other Authors: AIR FORCE INST OF TECH WRIGHT-PATTERSON AFB OH
• Format: Text
• Language: English
• Published: 1993
• Subjects: Meteorology; Geography; *WIND; *GUSTS; *STORMS; *OROGRAPHY; COASTAL REGIONS; ATMOSPHERIC TEMPERATURE; WIND VELOCITY; PACIFIC OCEAN; AIR WATER INTERACTIONS; CONVECTION(ATMOSPHERIC); WASHINGTON(STATE); DEW POINT; ISLANDS; MARINE CLIMATOLOGY; BAROMETRIC PRESSURE; MOUNTAINS; WINTER; THESES; DAMAGE; SAN JUAN ISLANDS; MICROBURSTS; GUEMES ISLAND; DOWN BURSTS; Arctic; Fraser River; Guemes; Pacific; Point Islands; San Juan
• Online Access: http://www.dtic.mil/docs/citations/ADA267651; http://oai.dtic.mil/oai/oai?&verb=getRecord&metadataPrefix=html&identifier=ADA267651

Orographic microbursts in a remarkable winter wind storm caused severe damage to the San Juan Islands and coastal regions of Washington State on December 28, 1990. The microbursts, associated with trapped lee mountain waves, produced winds in excess of 50 m/s on Guemes Island. The damage patterns of the tree fall and debris reveal small scale swaths and pockets of high winds similar to those patterns caused by convective downbursts. Analysis of large scale features reveal that an underlying cause of the strong winds was a strong pressure gradient behind an arctic front. The strong pressure gradient was due to cold air damming on the eastern slopes of the Canadian Coast and Cascade Mountains. Gap wind acceleration was noted over Puget Sound by the sharp boundary between the high speed jet of dry air emanating from the Fraser River Valley, a gap between the Coast and Cascade Mountains, and the ambient lower speed marine winds.