Oregon-AMS Meeting Summary

 

“Extreme Weather: December 2007 High Impact Event” by Tyree Wilde & Andy Bryant, NWS

 

  We had 28 attend this technical meeting.  FOX-12/KPTV hosted the meeting.  Our guest speakers were Tyree Wilde, Warning Coordination Meteorologist, and Andy Bryant, Service Hydrologist, from the Portland NWS Office. 

 

   This event was unusual in that three storms combined to make for a high-impact event. We saw multiple hazards: snow, wind, high seas and surf, major coastal and river flooding, and major debris flows.  This event was exceptional in that it was of long duration – 36 hours.

 

  A Federal Disaster Declaration was made for nine counties in Oregon, 12 in Washington, and two tribes.  Damage was estimated to be $205 million.

 

  The storm was initially a snow-producer, then high winds, then the high impact wind-and-rain—a one-two punch.  Weather charts showed 2-inch precipitable water in the west Pacific.  A 985 mb low was noted on Sunday, December 2nd.  The peak of the storm (Sunday night) had an offshore low of 955 mb and a high of 1055 mb (Rocky Mountains).  The east-west pressure gradient was enormous.  A low level, 110 knot jet was observed over the immediate coast which significantly aided the strong winds.

 

  Initially, the NWP models did a good job on the wind and sea-waves but was a little light on the precipitation amounts.  The models initially had the axis of heaviest precipitation over western Washington.  Subsequent model runs did move the axis of heavy precipitation southward over northwest Oregon and southwest Washington as the event unfolded.   The observations were incredible: 15-18 inch snow in the Cascade Mountains, up to six inches of rain in the Willamette Valley of Oregon, and marine winds of 60-70 knots, a five-foot storm surge, and 44-48 foot sea-swells.  The big story was the wind: 102-129 mph in coastal communities.  The highest report was 1290 mph at Bay City in Tillamook County.  Other high wind reports were received from elevated sites (146 mph at Radar Ridge (~1500 ft, Pacific County) and 114 mph Cape Meares (145 feet near Tillamook)).  Associated damage included downed trees (~20 million board feet), closed roads near the coast, and extensive power outages.  Major debris flows (~40,000 cubic yards) on December 11th near Clatskanie, OR blocked Highway-30 for days. 

 

  The other major news was the extended major floods that occurred.  About 12-20 inches fell in the Oregon Coast Range.  Major floods occurred on the Willapa River (Pacific County, Washington), Grays River (Wahkiakum County, Washington), Chehalis River (Lewis County, Washington), Nehalem and Wilson Rivers (Tillamook County, Oregon).  There were also several rivers farther north in western Washington that had major flooding.  The major flood of the Chehalis made national news headlines, as the river engulfed Interstate-5 for many days, given their 20-inch storm total and seven-foot rise in three hours.  The big flood story in Oregon was the Vernonia major flood on December 2nd – 4th.  The Nehalem River near Vernonia had a peak flow of 18,500 cfs (major flood is 9000 cfs), which compares to the peak flood of 13,500 cfs from the February 1996 major flood event.  Preliminary analysis suggests that the Vernonia precipitation was a 250-year event and the flood a 300-year event.

 

  After the talk concluded, Tyree and Andy answered many questions.  We appreciate Tyree and Andy sharing with us the details of a major storm event!  Afterwards, Chief Meteorologist Mark Nelsen (FOX-12) gave a personal tour of their weather center.

 

Note-taker: Kyle Dittmer, OR-AMS President