Northwest Avalanche Center

2017 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: forecasters@nwac.us or phone 206-526-6165 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 the 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. Loose wet 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 as it may increase the wet snow avalanche potential. 

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 loose wet slides may start small, they may entrain more snow as they descend and may trigger larger wet slab slides as well.

Be aware of the hazards overhead regarding cornices that were formed in late winter or spring. Cornices become more unstable and likely to fail during warm weather. Also, cornice fall may trigger loose wet or wet slab avalanches on the slopes below. 

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 areas which undergo regular avalanche control during the winter are most likely not controlled in the spring. 

Also remember that small avalanches may be dangerous. Several fatal accidents have occurred during past springs from climbers or skiers being caught in relatively small avalanches which subsequently carried the victims into or over a terrain trap. Hence backcountry travelers need to be aware of both the terrain above and below intended routes.

Have a safe and enjoyable spring!

NWAC