Avalanche Forecast for Sunday, February 17, 2019
This information was published 02/17/2019 at 7:07 AM.
The Bottom Line
It is possible for skiers and climbers to trigger a large avalanche today. Wind slabs that exist on the eastern half of the compass have a good deal of variability in their hardness. A double-edged sword exists in avalanche terrain with avalanche paths offering the less likely chance of producing a large avalanche while sheltered areas or treed slopes with softer snow give an increased likelihood of producing smaller avalanches. All areas have MODERATE avalanche danger due to the the potential of initiating a slide today as well as the possibility of producing a destructive avalanche. Terrain with a southern aspect, such as the Northern Gullies in Huntington, likely contains less developed wind slab and may offer more opportunities for safe travel. Good terrain management will provide ample opportunities for quality skiing and climbing.
Wind speed increased through the day yesterday with a shift to the NW. Steady speeds close to 70 mph were recorded late in the evening after starting the day in the 40 mph range. About 1” of snow fell at upper elevations. Today will be a pleasant mid-winter day with summit temperatures rising to 10F with wind from the NW becoming W and dropping from the current 60 mph to the 10-20 mph range by mid-afternoon. No snow is forecast today, though developing clouds this afternoon mark the incoming low pressure system that will bring steady but light snowfall tomorrow.
Primary Avalanche Problem – Wind Slab
Wind slabs that vary in hardness, from soft through hard, can now be found on a variety of aspects on the eastern half of the compass at middle and upper elevations. While overnight wind from the NW transported the inch that fell Saturday, it has also increased bridging strength, likely creating stubborn wind slab on exposed slopes like those in typical avalanche paths. Sheltered areas that are protected from high wind speeds contain softer wind slab that provide a more likely place to initiate a crack. While treed slopes may provide anchoring early in the season, today they may lure skiers onto the more reactive areas of today’s avalanche 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.
Our upper snowpack at mid and high elevations consists of layered wind slab resting on top of a crust that developed over a week ago. Field observations yesterday in Huntington and Tuckerman found a reactive surface wind slab in wind sheltered areas on top of a more stubborn layer of wind slab below. This all sits on top of a hard bed surface. Natural avalanche activity on NE aspects, that occurred this past Wednesday, indicates the possibility of a slide entraining all snow above the crust and producing a large avalanche. Lower elevations saw a brief period of rain on Friday, creating a breakable crust on the surface. While temperatures and cloud cover will negate warming above 3500’, the effect of warming down low may weaken the crust and weaken any slabs that may exist in steep terrain. Bright and direct sun on steep slopes and in wind protected areas would be the red flag to watch for.
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 02/17/2019 at 7:07 AM.
USDA Forest Service
White Mountain National Forest