MOUNTAIN WEATHER FORECAST FOR THE OLYMPICS WASHINGTON CASCADES
AND MT HOOD AREA
NORTHWEST WEATHER AND AVALANCHE CENTER SEATTLE WASHINGTON
130 PM PDT FRI MAY 2 2008
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
Pacific Northwest Ski Area Association
Friends of the Avalanche Center
and other private organizations.
This forecast is prepared primarily for federal, state and
private snow safety programs in Washington and Northern
Oregon.
NOTE: The last regularly scheduled weather and avalanche
forecasts for the season were issued on Sunday, April 20,
2008. However weather and snow conditions will continue to be
monitored at the Northwest Weather and Avalanche Center with
the information that remains available. Forecasters will
issue special statements when conditions warrant.
WAZ513-518-519-019-042-501-502-ORZ011-032000-
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For your information, some information about spring avalanches
follow:
SPRING AVALANCHE STATEMENT
If you have any comments or suggestions regarding this
winter's mountain weather or backcountry avalanche forecasting
program, please direct them to:
Northwest Weather & Avalanche Center
7600 Sandpoint Way NE
Seattle, Washington 98115
You may email comments to:
nwac.sew@noaa.gov
or phone 206-526-4666 and leave a message.
Please note that in areas retaining a significant winter
snowpack, backcountry travelers face a continuing risk of
avalanches during the springtime. We strongly advise that
backcountry travelers continue to assess snowpack stability as
they travel and project the effects of anticipated future
weather on the snowpack when making route choices.
Some general notes regarding spring avalanches follow . . .
During fair spring weather the avalanche hazard is generally
lowest during the night and early morning hours when surface
snow refreezes due to heat loss to the surrounding atmosphere.
During the day, intense solar radiation and warm air
temperatures can rapidly melt and weaken surface snow layers
and produce an increasing avalanche danger during the late
morning and afternoon. Wet loose slide activity generally
starts on east and southeast facing slopes receiving morning
sunshine and progresses to west and southwest facing slopes
during the afternoon. Therefore the safest time to cross
potential avalanche terrain is during early morning hours
before the surface snow begins to warm and weaken.
This daily melt-freeze cycle is strongly affected by any cloud
cover during the night since clouds at night limit radiational
cooling and prevent freezing. This may allow melt water and
associated snowpack weakening to affect progressively deeper
layers in the snow cover. Snowpack weakening is maximized
when warm days are followed by warm overnight temperatures and
overcast skies. Backcountry travelers should exercise
particular caution under these conditions that often lead to
considerable wet loose slide activity along with possible wet
slab avalanches.
Backcountry travelers should also be aware that spring storms
might quickly produce unstable snow conditions. Although
precipitation may fall as rain at lower elevations,
substantial amounts of new snow may be deposited at higher
elevations. This new snow may form a poor bond with an old
crusted snow surface. Rapid rises in temperature following
the storm due to intense solar radiation may quickly warm and
weaken recent snow, which may need little or no disturbance to
slide. While subsequent wet loose slides may start small,
they may entrain considerable snow as they descend and may
trigger larger wet slab slides as well. Dangerous conditions
may also result from unstable cornices deposited by spring
storms, as these may be quite unstable and release during
later warm days. Also, slopes beneath glide cracks should
normally be avoided, especially during the heat of the day, as
the entire snow cover may release from melt water lubrication
and weakening.
Precipitation as rain may also create unstable snow
conditions. This is because rain falling on an already wet
snowpack causes water to quickly percolate through the
snowpack, which weakens progressively deeper snow layers. If
the water encounters a crust or an ice lens, it may flow along
this layer and lubricate it, making avalanches increasingly
likely within the snow above.
No matter what the season, backcountry travelers should avoid
slopes of questionable snowpack stability. Remember that many
areas, which undergo regular avalanche control during the
winter, may not be controlled in the spring.
Also remember that small avalanches may be dangerous, for
although wet loose snow in motion may be soft, when it stops
rapid hardening takes place. Most avalanche victims trigger
the avalanches in which they are caught, and almost half of
all avalanche deaths occur in slides traveling less than 300
feet; with some slide fatalities occurring with victims buried
only a few inches under the snow surface. During past springs
in the Northwest, several fatal accidents have occurred from
climbers or skiers releasing and being caught in relatively
small avalanches, which subsequently carried the victims into
or over a terrain trap. Hence, backcountry travelers should be
aware of both the terrain above and below intended routes.
Have a safe and enjoyable spring!
NWAC weather data and forecasts are also available by calling
206-526-6677 for Washington, 503-808-2400 for the Mt Hood
area, or by visiting our Web site at www.nwac.us.
Ferber/Northwest Weather and Avalanche Center
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