This information was published 01/14/2019 at 7:01 AM.
NOT THE CURRENT FORECAST
This is an archived avalanche forecast.
The Bottom Line
Human triggered avalanches are unlikely but should still be a primary hazard on your radar today. Firm, large wind slabs, like those formed late last week, are a tricky avalanche problem as they generally display minimal signs of instability under your feet. Safe avalanche terrain practices like traveling one at a time and carrying your beacon, shovel, and probe are still advised. Remember that you use these practices for the “what if” scenario of an avalanche on a slope you’ve decided is reasonably stable. All forecast areas have LOW avalanche danger today. Continuing to respect avalanche terrain and remembering that Low does not mean no avalanche danger should combine well with favorable weather for a great day in the mountains.
Summit wind under 20 mph yesterday should also be light through much of today, ultimately increasing late afternoon and tonight while blowing from the NW. Temperatures have risen a few degrees overnight, though valleys are colder than higher elevations currently, and will continue to rise to the upper teens F on the higher summits. Clear skies and no precipitation are forecast today. Tomorrow should bring similar temperatures, mostly cloudy skies, moderate wind speeds, and possibly a trace of snow by tomorrow night.
Primary Avalanche Problem – Wind Slab
Wind slabs formed from the extended period of wind following last week’s snow are generally firm in middle and upper elevations. Expect them to be unreactive to a human trigger, but look for exceptions to help guide your terrain choices. These slabs vary in size and are quite large in some terrain, particularly the Sluice and Lip of Tuckerman Ravine. This avalanche problem should be generally absent on the western half of the compass rose and at lower elevations.
What is a Windslab Avalanche?
Wind Slab avalanches are the release of a cohesive layer of snow (a slab) formed by the wind. Wind typically transports snow from the upwind sides of terrain features and deposits snow on the downwind side. Wind slabs are often smooth and rounded and sometimes sound hollow, and can range from soft to hard. Wind slabs that form over a persistent weak layer (surface hoar, depth hoar, or near-surface facets) may be termed Persistent Slabs or may develop into Persistent Slabs.
At the tail end of and following last week’s snow storm, northwesterly summit wind held in the 70-90 mph range for three days. A few hours of recorded wind were near 100 mph, and only a few dipped below 70 mph in that 72 hour period. This sustained period of extreme wind speeds heavily affected our storm snow, resulting in several natural avalanche cycles. The aftermath is areas of smooth and hard (1F) wind slab, heavily wind textured snow that is also quite hard in many places, and scouring. Similar aspects and elevations throughout the terrain vary in current conditions, largely due to upwind fetch for wind loading of the storm snow or lack thereof. While stability tests will identify weak layers in the upper snowpack, it’s unlikely that your weight will be enough to trigger an avalanche in our hard wind slab today. Remember that conditions vary greatly through the range and look for exceptions to the Low avalanche danger.
The Sherburne and Gulf of Slides ski trails are snow covered to Pinkham Notch.
Safe travel in avalanche terrain requires training and experience. This forecast is just one tool to help you make your own decisions in avalanche terrain. You control your own risk by choosing where, when, and how you travel.
Avalanche danger may change when actual weather differs from the higher summits forecast.
For more information contact the US Forest Service Snow Rangers, AMC visitor services staff at the Pinkham Notch Visitor Center, or the caretakers at Hermit Lake Shelters or seasonally at the Harvard Cabin (generally December 1 through March 31). The Mount Washington Ski Patrol is also available on spring weekends.
Posted 01/14/2019 at 7:01 AM.
Ryan Matz, Snow Ranger
USDA Forest Service
White Mountain National Forest
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