SPRING AVALANCHE STATEMENT IN EFFECT
If you have any comments or suggestions regarding this winter's mountain weather or backcountry avalanche forecasting program, please direct them to:
Northwest Avalanche Center
7600 Sandpoint Way NE
Seattle, Washington 98115
You may also email comments to: firstname.lastname@example.org or phone 206-526-4666 and leave a message.
General notes regarding spring avalanches follow . . .
During fair spring weather the avalanche danger 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, sun effects 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 avalanche activity generally starts on east and southeast facing slopes receiving morning sunshine and progresses to the 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 cooling and may prevent re-freezing. Lack of a thorough surface re-freeze may allow melt water to affect and weaken 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 avalanche conditions. Although precipitation may fall as rain at lower elevations, substantial 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 effects may quickly warm and weaken any recent snow, which may need little or no disturbance to slide. While subsequent wet loose slides may start small, they may entrain more snow as they descend and may trigger larger wet slab slides as well. Dangerous conditions may also result from cornices formed during spring storms; the cornices may become unstable and fail following warming. Slopes beneath glide cracks should normally be avoided as the entire snow cover may release from melt water lubrication and weakening. Glide avalanches are difficult to predict as they are not necessarily tied to the warmest part of the day or following the heaviest rain.
Rain may also increase the likelihood of avalanches. Rain falling on an already wet snowpack causes water to quickly percolate through the snowpack and weaken 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, are likely not controlled in the spring.
Also remember that small avalanches may be dangerous. Although the 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. Several fatal accidents have occurred during past springs 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!