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
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