This information was published 01/06/2019 at 7:28 AM.
NOT THE CURRENT FORECAST
This is an archived avalanche forecast.
The Bottom Line
Avalanche danger will increase today as snowfall and wind create new wind slabs in our terrain. As snow is transported, the likelihood of human-triggered avalanches will rise along with the potential for natural avalanches driving our CONSIDERABLE rating for today’s avalanche problem. Exposed melt-freeze crusts and older unreactive wind slabs do exist, providing opportunities to avoid traveling through the avalanche problem, though bear in mind these are isolated and may be directly under significant loading taking place above.
Temperatures on the summit reached 31F yesterday with all elevations below going above freezing under cloudy skies. Westerly winds averaged 30 mph, though daylight hours saw speeds drop below 20 mph at times. Today will feel much more like typical early January as a cold front passes, with NW wind increasing from a current 50 mph to sustained 75 mph by dark. Temperatures will drop through the day to around 0F. Snow showers today may bring up to a total of 4” by midnight, with up to 2” by midday. Wind will diminish tomorrow with no snow expected until the early hours of Tuesday.
Primary Avalanche Problem – Wind Slab
Prime wind loading speeds from the NW plus potentially more snow (2-4”) may combine to create touchy new wind slab in leeward locations as well as cross-loading on many slopes. Snowfall totals come with a degree of uncertainty today, though some snow leftover from Thursday’s storm remains available for transport above many start zones. Today’s new wind slab will form on soft older wind slab in many locations, increasing the amount of snow the could be entrained in an avalanche. Today’s ratings are based both on the potential to trigger an avalanche in snow that arrives today as well as the potential size that could be produced in our terrain.
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.
Our terrain has a drastically different snowpack depending on aspect. Wind slab that formed late this past week can be found on the eastern half of the compass. This cohesive slab can mostly be found above a less cohesive layer, though has proven unwilling to propagate a crack. That being said, skiers in steep terrain found significant sluffing yesterday, leading us to believe an avalanche that starts small has the potential to entrain a large amount of snow. This soft snow will also be moved further downslope into lower start zones today. Slopes with westerly aspects have been scoured to rain crusts and hold very little, if any, avalanche danger. These rain crusts do provide the potential for long sliding falls with rock and tree-filled runouts below.
Ski tracks disappear through the choke of the Chute from skier-induced sluffing from above the top of the hourglass.
Variability in the fetch above Tuckerman Ravine. Wind slab exists on E and SE aspects of the summit cone, much of the Bigelow Lawn is scoured to December rain crust, and soft snow available for transport today lies on and just below the rollover of the Headwall.
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 01/06/2019 at 7:28 AM.
USDA Forest Service
White Mountain National Forest
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