Avalanche Warning in the Washington Cascades

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AVALANCHE WARNING
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NORTHWEST WEATHER AND AVALANCHE CENTER

NWAC Program administered by
USDA-Forest Service
with cooperative funding and support from
Washington State Department of Transportation
National Weather Service
National Park Service
Washington State Parks and Recreation Commission
(including Snowmobile and Snowpark Programs)
Pacific Northwest Ski Area Association
Friends of the Avalanche Center
and other private organizations
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BACKCOUNTRY AVALANCHE FORECAST FOR THE OLYMPICS, WASHINGTON
CASCADES AND MT HOOD AREA
These forecasts apply to back country avalanche terrain below
7000 feet. They do not apply to highways or operating ski areas.
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0830 AM PST Friday, November 28, 2003
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ZONE AVALANCHE FORECASTS.....

WASHINGTON CASCADES AND OLYMPICS......
...............AVALANCHE WARNING...................
Considerable avalanche danger above 4 to 5000 feet and moderate
below early Friday morning, rapidly and significantly increasing
later Friday morning and early afternoon, becoming high above 3
to 4000 feet and considerable below with natural avalanches
likely. Avalanche danger decreasing later Friday afternoon north
and Friday night in the south becoming considerable above 5000
feet and moderate below early Saturday. Further decreasing danger
expected on Saturday, becoming moderate above 4000 feet and low
below except moderate below 7000 feet in the extreme southern
Cascades.

MT HOOD AREA....
...............AVALANCHE WARNING...................
Considerable avalanche danger above 5000 feet and moderate below
Friday morning, rapidly and significantly increasing mid-late
Friday afternoon and night, becoming high above 4000 feet and
considerable below with natural avalanches likely. Avalanche
danger decreasing Saturday, becoming moderate below 7000 feet.

SUNDAY OUTLOOK...
Slightly increasing danger expected Saturday night and Sunday,
mainly in the southern Cascades or in areas receiving heavier
showers in the extreme north Cascades.

SNOWPACK ANALYSIS.....
During the past week, substantial amounts of cool and relatively
low density unconsolidated snow have been deposited over an old
rain crust formed by heavy rain reaching high elevations in early
November. Some faceting near the old crust has weakened the bond
of this recent snow to the crust, and some new weak layers of
either low density low wind deposited snow or surface hoar exist
within the new snow as well. Along with some relatively large
cornices and increasing snow or winds overnight, this snow pack
structure is producing an increasing considerable danger above 4
to 5000 feet early Friday, with human triggered slabs probable on
lee slopes. Despite some terrain or vegetative anchoring still
prevalent with a still generally shallow snow cover at lower
elevations and along the Cascade east slopes, this snow pack
structure remains very susceptible to the destabilizing effects
of warming, high winds and heavy loading-all of which appear
likely to develop and shift southward Friday and Friday night.

FRIDAY...FRIDAY NIGHT.....
Substantial warming, strengthening winds and increasingly heavy
rain or snow should spread into most areas of north-central
Washington Friday morning before gradually decreasing Friday
afternoon, with heaviest precipitation and strongest winds
shifting slowly southward into southern Washington and northern
Oregon later Friday afternoon and night. This weather should load
and weaken the existing snowpack and associated weak layers, with
natural or human triggered avalanches increasingly likely by
later Friday morning in the north and Friday afternoon and night
in the south. Isolated larger slides are also possible, with some
slides involving all of the new snow received above the crust
formed almost three weeks ago. In some favored lee deposition
areas above 4 to 5000 feet, potential fracture lines of 3 to 5
feet or more are possible-mainly in terrain of the Olympics and
Cascades near and west of the Cascade crest where heaviest recent
snowfall has accumulated.

SATURDAY....
Decreasing showers and significant cooling likely in the
Washington Cascades and Olympics Saturday should help to refreeze
and strengthen upper wet snow layers and shift the danger from
the old snow pack to some generally small new wind slabs over the
refreezing crust. However, much less cooling combined with a
slower end to the precipitation in the Mt Hood area should
produce a slower and less significant stabilization of the recent
heavy snowfall, with a moderate danger and some wet slabs within
the old snow pack remaining possible over the weekend.

SATURDAY NIGHT...SUNDAY...
Generally light showers are likely in the northern Washington
Cascades Saturday night and Sunday morning, while some light rain
or snow spreads northward into the Mt Hood area and southern
Washington Cascades. Although most precipitation amounts are
expected to be relatively light, this should produce a slight
increase in the danger-especially in areas receiving heavier rain
in the south or more significant shower activity in the north.

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Backcountry travelers should be aware that elevation and
geographic distinctions are approximate and a transition zone
between dangers exists. Remember there are avalanche safe areas
in the mountains during all levels of avalanche danger. Contact
local authorities in your area of interest for further
information.

NWAC Mountain Weather Forecasts and mountain weather data are
also available by visiting our Web site at http://www.nwac.noaa.gov.

Moore/Forest Service Northwest Weather and Avalanche Center