This information was published 02/02/2019 at 7:14 AM.
NOT THE CURRENT FORECAST
This is an archived avalanche forecast.
The Bottom Line
Westerly wind since Wednesday’s snowstorm has created wind slabs that remain possible to human trigger. Today’s avalanche problem of stubborn wind slabs is low probability but high consequence, meaning that complacency due to the lower likelihood of avalanches may lead you to travel in terrain which still could produce a large avalanche. Some scouring to a crust may allow options to avoid this avalanche problem, but it may be difficult to visually identify this crust today. Reduce your risk by reducing your time spent under or on steep areas of larger slab, especially where snow has accumulated beneath steeper terrain features. MODERATE avalanche danger exists for most of our forecast areas, with the Northern Gullies of Huntington Ravine the one exception with LOWavalanche danger.
West wind slackened from 80 mph to the current 60 mph by yesterday afternoon while shifting WNW. Temperatures remained quite cold, with a high of -13F on the summit and highs below 0F at our snow plots which are just below 4000’ in elevation. Today will be warmer, with a forecast summit high of 2F. Summit wind should remain WNW and increase to 70 mph by this afternoon. Snow showers are likely to produce an inch or so of accumulation, predominantly falling this afternoon, but the notoriously tricky to predict upsloping affect may produce more snow by this afternoon or evening. No precipitation is forecast for tomorrow as temperatures rise to 20F on the summit and wind tapers to under 40 mph.
Primary Avalanche Problem – Wind Slab
Wind slabs formed through yesterday morning on consistently W wind following Wednesday’s storm. These slabs can be found on the eastern half of the compass rose, with the largest slabs in directly east facing terrain. Expect them to be stubborn to a human trigger and generally firm, with a few exceptions being softer in sheltered terrain. Additional snow today, which will be affected by WNW wind, could add volume to these slabs or develop smaller and more reactive slabs on the surface. Potentially small snow totals may minimize these new slabs. Terrain most exposed to wind will exhibit scouring to or near to the January 25th rain crust. These typical Presidential Range stubborn wind slabs aren’t likely to avalanche from a human trigger, but the possibility does remain and should certainly keep you from letting your guard down.
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.
Field observations yesterday combined with the history of sustained 70-80 mph wind events lead us to believe that the relatively large wind slabs formed since Wednesday are generally hard and stubborn. The supportive nature of these hard slabs can offer substantial bridging strength, reducing the likelihood of a human triggered avalanche. That said, the slow bonding inherent to our current cold temperatures,the presence of an ice crust in the upper snowpack, and generally well developed slide paths with few anchors remaining are factors that help keep a relatively large avalanche a possibility today. Remember that these wind speeds also produce significant spatial variability, making it difficult to apply stability test results across terrain. Scouring has occurred in places and travelling on exposed crust may be an option to avoid the avalanche problem. Scouring in some upper avalanche start zones, like the Headwall of Tuckerman Ravine, has combined with sluffing to create the largest wind slabs relatively lower down in our avalanche paths. These combined sluff and wind slabs produced a number of natural avalanches since Wednesday, check out our observations page for more details.
The Sherburne and Gulf of Slides ski trails are snow covered to Pinkham Notch. The annual MWV IceFest this weekend draws climbers from all over New England to the mountains this weekend. The White Mountain Avalanche Education Foundation will be at the event tonight and Frank will give a brief presentation about terrain management and avalanches in Huntington Ravine.
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 02/02/2019 at 7:14 AM.
Ryan Matz, Snow Ranger
USDA Forest Service
White Mountain National Forest
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