OR-AMS March 22, 2007 Meeting Summary

 

“Major Windstorms” by Tyree Wilde, NOAA-National Weather Service

 

  We were treated to two presentations, both related to extreme weather events.  So, 22 guests showed up at the KPTV/FOX-12 studio in Beaverton.

 

  Tyree explained the components of a windstorm: offshore development and the presence of an upper level low.  Most PNW windstorms occur October through March.  The jet stream and forecast of the storm track is critical to determining the impacted areas.  The strongest winds are ~300 miles wide.  The lifecycle is ~48 hours.  Rapid storm development and complexity are factors that are hard to predict.  A major windstorm is a once in ten-year event.  Common damage includes fallen trees, down power lines, closed roads, and damaged houses.

 

  It is worth noting the major windstorms of the last 50 years.  The Columbus Day Storm, the Granddaddy of all Oregon windstorms, on October 12, 1962, was the strongest, non-tropical windstorm to ever hit the continental United States.  The CD Storm formed from the remnant of Typhoon Frieda in the western Pacific Ocean.  The pressure center was 960 mb.  The wind speeds were amazing: 150 mph at Cape Blanco (with 179 mph gusts), 138 mph at Newport, and 116 mph at Portland.  The damage cost was $235 million (or $1.4 billion in today’s dollars) and 46 were killed. 

 

  The November 13-14, 1981 windstorm packed a “1-2 punch.”  The pressure center was 2 mb lower than the CD Storm.  The wind speeds were 98 mph at Tillamook, 75 mph at Brookings, and 71 mph at Portland.  The damage cost was $50 million and 11 died.

 

  The December 12, 1995 windstorm had a pressure center of 958 mb.  The wind speeds were 119 mph at Sea Lion Caves, 107 mph at Newport, and 74 mph at Portland.  The damage cost was $8.2 million and two deaths were reported.

 

  Finally, the December 14, 2006 windstorm didn’t display the “classic storm track” (it was more southwest).  The pressure center was 970 mb.  The wind speeds were 114 mph at Mt. Hebo (coast), 106 mph at Newport, 91 mph at Lincoln City, and 70 mph at Portland.  Over 1 million people were without power in Oregon and Washington for days.

 

 

“Rain Events of November 2006” by Drew Jackson, KPTV/FOX-12

 

  The second presentation documented the multiple impacts of the intense rain events of early November 2006.  The “Pineapple Express” pattern (i.e., jet stream pushing much moisture from the tropics) dominated our weather in early November.  The rainfall totals during the peak of one storm, November 6th and 7th, were record-breaking at many sites: 14.3 inches at Lee’s Camp (Oregon Coast Range), 4.3 inches at Tillamook, 15.5 inches at June Lake, 9.9 inches at Saddle Mountain (Oregon Coast Range), and 7.57 inches at Portland.  During November 2-8, 2006, the north Oregon Cascades saw 25 to 32 inches. 

 

  The rain totals are comparable to the February 1996 flood event (but without the low-elevation snow).  The Wilson River saw new record flows, 38500 cfs, on November 6th.

 

  The big news story was Mt. Hood and the glacial outburst floods.  Warm intense rain during the autumn season (when glaciers are at their minimum size with minimal snow cover) is the main cause in glacial dam breaks.  These outburst floods pick up loose debris (think Mt. St. Helens in 1980) as they rapidly flow downhill and little water is left by the time the flow slows to a halt. 

 

  State Highway 35, just east of Mt. Hood, was severely impacted by outburst floods.  Local river channels were aggrading (i.e., filling with sediment).  Highway 35 has seen big washouts in 1968, 1981, 1998, and 2002. 

 

  The Sandy River saw many washouts, as did Eliot Creek and Polallie Creek.  The White River saw 1000 km3 of debris moved, compared with 400 km3 in past events. 

 

  There are three theories as to the cause: (1) Glacial retreat (due to climate change) which exposes more sediment that can be flushed out in an intense rain.  (2) Return to a “wet cycle” (long-term trend) where more rain is occurring.  (3) Coincidence with a series of several hot dry summers (enhancing glacial retreat) and a “Pineapple Express” weather pattern, which happened a lot during the 1990s to the present.  Drew is leaning toward cause #3.  Finally, Drew left us with food for thought: some areas experienced flooding worse than the 1996 major flood event.

 

  After the talk concluded, and several questions were answered, Drew and Mark Nelsen took the guests on a tour of the studio—a first time for some folks.  New state-of-the art computers and graphics were demonstrated.  We appreciated the hospitality of our hosts.

 

 

Note-taker: Kyle Dittmer, OR-AMS President