A Climatology of Windstorms in the Western Pacific Northwest,

Transcription

1 A Climatology of Windstorms in the Western Pacific Northwest, Wolf Read Oregon Climate Service College of Oceanic and Atmospheric Sciences Oregon State University

2 Research Goals Compile data for high wind events for the time period , essentially the era of modern surface airways data. Comparative analysis of similar scenarios. Determine patterns, if any. High Wind, NWS Definition: Average Velocity of 35 kt or greater And/or gusts of 50 kt or greater

3 Research Methods Reviewed thousands of original unedited surface observation forms. Reviewed published and unpublished surface maps. Reviewed satellite photos. Reviewed literature that pertained to significant storms (Coparanis, Cramer, Lynot, Mass, Reed). Salem, McNary Field, October 12, 1962: 1-Min Wind 1-Sec Gust

4 Storm Classification by Track Nearly all high wind events in the PNW are associated with the close passage of an intense extratropical cyclone. Six main track types defined. Class boundaries based roughly on mean separation between official stations.

5 Why is Storm Track Important? Track determines where high winds will strike. Real-World Example February 7, 2002:

6 Minimum Central Pressure The centreal pressures of high-wind-generating cyclones varied greatly, ranging from 954 to above 1000 mb.* Most high-wind scenarios occur with central pressures below 995 mb. Sub-970 mb events are rare. Storm of the decade. Sub-960 mb once every two to three decades. Sub-955 mb once a century? Known events: December 12, 1995 and perhaps January 9, * Depending on the criteria, about 40 to 50 significant high-wind-generating cyclone events have been identified for the time period (about one a year).

7 Central Pressure Vs. Max Wind Velocity Maximum wind speed appears to be weakly related (at best!) to a windstorm s minimum central pressure, for at least two related reasons: 1) Peak gradients during these storms rarely exceed 20 millibars over 100 statute miles* regardless of central pressure. 2) Ageostrophic flow through terrain features appears strongly dependent on maximum pressure gradients. * Corresponds to a geostrophic potential of about 180 kt, which, under ideal conditions, translates to about a 90 kt 1-min wind over terrain with its attendant high-friction.

8 Typical Maximum Winds Most high-wind-generating storms appear to reach a wind velocity threshold that s difficult to exceed. Peak gradients again the explanation, with the typical high-wind-generating cyclone having maximum values in the range of 12 to 16 millibars per 100 statute miles. Only the Columbus Day storm of 1962, and perhaps the February 13, 1979 Kitsap windstorm appear to have exceeded the extreme wind threshold. Table 1: Max wind velocity thresholds (kt). Strong 10-yr event, extreme 20-yr event. Extreme rarely exceeded, even in 50-yr. Coast Interior Strong Extreme Strong Extreme 50 G G G G 70

9 Central Pressure and High-Wind Reach The area affected by a storm s high winds appears to be reasonably related to minimum central pressure. On the right, the red fields cover region of probable high winds during four windstorms with similar tracks. The deeper the storm, the further its reach!

10 Unusual Storm 1: The Columbus Day Storm (CDS) Relative to other storms, max pressure gradients do not explain the CDS s extreme winds. Stronger gradients have occurred, but without the hurricane-force winds of the CDS, such as on November 10, Interior Gradient: 12 mb / 100 mi Max Gust: 110 kt CVO, 92 kt TTD Interior Gradient: 24 mb / 100 mi Max Gust: 65 kt RBG Upper-Air Flow 12 OCT NOV 1975

11 Unusual Storm 2: February 13, 1979 Mesoscale Event Lee troughing east of the Olympic Mountains induced a mesoscale low over the north Kitsap Peninsula. Average wind reportedly reached 80 kt at the Hood Canal Bridge! 6 mb Gradient / 8 Miles

12 Top 5 Seattle Windstorms R a n k Date 1 12 Oct Jan Jan Nov Jan 1953 Peak gusts in mph. Everett Seattle City Boeing Field Sea- Tac Renton Tacoma Avg MM MM 52 58