This information was published 01/01/2019 at 7:14 AM.
NOT THE CURRENT FORECAST
This is an archived avalanche forecast.
The Bottom Line
Wind from the west and northwest will ramp up this morning and build new unstable slabs through the day, resulting in increasing avalanche danger. The Headwall of Tuckerman Ravine will have HIGH avalanche danger, with natural avalanches that threaten the floor of the ravine likely. All other areas will have CONSIDERABLE avalanche danger with natural avalanches being possible. You’re likely to trigger an avalanche on steep, wind drifted slopes today. Avoid travel on or below this wind loaded terrain.
Snowfall began after dark yesterday on the heels of a fairly unremarkable weather day. The storm has produced nearly 5” of mixed snow and sleet at Hermit Lake. Overnight winds were 30-60 mph and varied in direction between S and SW. Several hours of mixed snow, sleet, and freezing rain should occur this morning before switching back to all snow. Upslope snow showers are forecast to produce another 2-4 inches of snow today before precipitation ends this evening. Current summit temperature of 22F should drop by 10 degrees through the day. Wind will shift through W to NW and increase rapidly this morning to around 70 mph.
Primary Avalanche Problem – Wind Slab
Wind will increase to create new and touchy slabs in terrain on the eastern half of the compass rose through the day. We expect wind slabs to vary in size and be large enough to easily bury a person in specific areas like the Headwall of Tuckerman Ravine or the Main Gully in Gulf of Slides. Mixed precipitation types, including significant sleet, add an element of uncertainty to the size of new slabs, but a slick bed surface makes natural avalanches likely today.
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.
Secondary Avalanche Problem – Dry Loose
Sluffing may be a concern in areas less affected by wind or where new mixed precipitation lacks cohesion to form a slab. Though small in size, sluffs could be easily initiated by a person travelling in steep terrain. Rain at lower elevations on the west side this morning indicates that sluffs could be of the wet variety in steep lower elevation terrain.
What is a Dry Loose Snow Avalanche?
Dry Loose avalanches are the release of dry unconsolidated snow and typically occur within layers of soft snow near the surface of the snowpack. These avalanches start at a point and entrain snow as they move downhill, forming a fan-shaped avalanche. Other names for loose-dry avalanches include point-release avalanches or sluffs.
New snow and mixed precipitation will be transported by W and NW wind today to build our primary avalanche problem. Prior to this storm, our upper snowpack was stable and provided a surface of both smooth, hard wind slab and icy refrozen snow. This means that avalanche concerns are limited to the new snow but that a slick bed surface elevates the likelihood of natural and human triggered avalanches. Varied storm snow measurements and precipitation types lend a significant amount of uncertainty to today’s avalanche problem, particularly in relation to size. We do expect today’s sustained wind to effectively transport the mixed frozen precipitation along with the 2-4” of new snow and for the resulting new slabs to be touchy.
The Sherburne and Gulf of Slides ski trails are mostly 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 01/01/2019 at 7:14 AM.
Ryan Matz, Snow Ranger
USDA Forest Service
White Mountain National Forest
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