Avalanche danger will increase further today due to more new snow and wind loading. East facing terrain will develop wind slab avalanches which could occur without a human trigger. Don’t be fooled by the small amount of snow in the forecast. The shift in wind direction and speed will access soft drifts tucked away in the alpine and contribute more snow to wind slabs that will create avalanches easily large enough to bury a person in the Gulf of Slides and Tuckerman Ravine. Other areas with less available snow could still produce dangerous avalanches.
Low pressure systems passing near the area followed by upslope showers will produce 3-5” of new snow. Wind speeds will be relatively light this morning but will increase steadily through the day with effective loading speeds in the 50-60 mph range beginning in the early afternoon. Temperatures which have become seasonably cold in the past couple of days will grow even colder with mid-single digits during daylight hours dropping to -8F tonight.
Primary Avalanche Problem – Wind Slab
New snow and light to moderate wind this morning will create sensitive wind slabs on top of our lingering wind slab problem. Remember that small amounts of new snow over the past several days have continued to stress existing wind slabs and today’s new snow will only create more stress and strain on already weak bonds. The base of steep areas and lee terrain are both habitats for today’s wind slab problem.
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.
The disparity that exists in the QPF between NWS and the MWObs is largely irrelevant to the rating today. Wind loading which occurred yesterday built wind slabs which have not yet stabilized. Temperatures and time are not on our side, even this morning with human triggered avalanches certainly possible due to the recency of these slabs development. The icy layer deep in the snowpack has resisted bonding and may even be developing some early facets, though overcast conditions overnight have not made for ideal facet farming. More snow tonight, cold temperatures and further high winds will keep our stability concerns alive tomorrow and while the threat of natural avalanches may pass tonight, the fresh and thick wind slabs will keep the threat real tomorrow. Expect a solid Moderate rating and a continued need to be really wary of what appear to be stubborn hard slabs. We have limited observations from the west side but sheltered locations appear to hold enough snow to recreate on. Scouring has likely taken it’s usual toll in more exposed location and it is doubtful that typical streambeds are entirely closed in, though they may be close. Gulf of Slides has certainly filled in with the problematic ice bulge trigger points close to but not yet entirely covered.
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 01/08/2020 at 7:15 AM.
Frank Carus, Lead Snow Ranger
USDA Forest Service
White Mountain National Forest
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