05-12-2008, 01:22 PM
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#1 (permalink)
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AASI Instructor
Join Date: Aug 2007
Location: Mt. Hood Oregon
Posts: 4,085
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Spring Avalanche Danger
As we enter the springtime and our resorts close, many of us hike for our spring and summer turns. In the interest of safety, I have posted the following information from the Northwest Avalanche Center regarding general information pertaining to spring slides.
Quote:
Spring Avalanche Danger
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 snow 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, since
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.
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