Avalanche Forecast for Thursday, February 13, 2020
This information was published 02/13/2020 at 7:09 AM.
The Bottom Line
Existing, large wind slabs will be further stressed by new snow and wind drifting today. Natural avalanches are possible in eastern terrain. These dry, hard slabs could run out onto low angle slopes or the floor of ravines if they occur. Large, steep slopes and gullies are of most concern and have CONSIDERABLE avalanche danger.
Route finding will be challenging today due to reduced visibility making the conservative choice of lower angle, lower consequence slopes, like ski trails and glades, a good one. Lower elevation and west facing zones also may contain small wind slabs formed by sluffing and crossloading. Evaluate these areas carefully. Our snowpack has grown quite a bit in the past week.
Yesterday, west winds continued to blow strongly enough to move snow. Cold but not frigid temperatures preserved snow on the ground. An inch of snow fell during the morning hours.
Today, the storm passing to the south and the arctic front to follow will likely produce 3” of snow, possibly a bit more if upsloping snow really gets going. Wind from the west and then northwest (30-45 mph), behind the front, will be just strong enough to move snow. Temperature will be near twenty but begin to fall later this afternoon.
Tomorrow, the next cold front settles in with clear, sunny skies and frigid temps starting out at -10F and dropping further to -15F. Northwest wind blowing 50-70mph, diminishing slightly in the afternoon, will move available snow.
Primary Avalanche Problem – Wind Slab
Wind slabs are a concern this morning even before new snow and wind loading occur today. Relatively moderate wind speeds over the past few days loaded east facing terrain with limited scouring on steep slopes. Widespread sluffing deposited snow across much of Tuckerman Ravine paths and likely others as well. These wind slabs are now fairly large, on the soft side(1F) and rest on layers of weaker (4F) snow that sits on an old ice crust. This setup has the potential to avalanche today with further loading from new snow and wind.
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.
New snow arrives this morning with relatively light wind speeds. If you are trying to take advantage of that, be sure to consider the wind slabs that are widespread in easterly terrain. The inverted structure and 18-24” size of these wind slabs led us to change plans yesterday and opt out of climbing past the mouth of Right Gully. We found shooting cracks in slabs up to 8 inches thick in the most sheltered location outside of avalanche terrain as well. The fracture potential wasn’t nearly as energetic in wind exposed locations though where the more actively loading slabs were stiffer. Unfortunately, the inverted structure did have enough energy to create the concern that led to us turning around. The floor of Tuckerman Ravine was scoured down to Feb 6/7 rain/sleet crust which serves as a good reminder that the ice crust lies beneath the large wide spread wind slabs that have grown since then.
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 02/13/2020 at 7:09 AM.
Frank Carus, Lead Snow Ranger
USDA Forest Service
White Mountain National Forest