This information was published 02/18/2019 at 6:58 AM.
NOT THE CURRENT FORECAST
This is an archived avalanche forecast.
The Bottom Line
Slabs that formed late last week into Saturday remain possible to human trigger today with the greatest likelihood being for small avalanches in softer wind drifted snow. Large avalanches in firmer wind slab are not yet ruled out. A small skier triggered avalanche on a low consequence terrain feature yesterday is a pertinent example what you might find today. Check this out on our observations page and thanks for this and similar observations. Consider the consequences of either a small or large avalanche in terrain you choose today, and realize that low visibility might affect your ability to assess terrain. All forecast areas have MODERATE avalanche danger.
Clear skies and light to moderate wind yesterday gave way to clouds overnight. Snow began falling early this morning, with 1-2” forecast for today. Wind should remain light through the day, shifting NW and increasing this evening as snowfall tapers off. The current summit temperature of 7F should hold through the day. Overnight, temperatures will drop towards -10F by tomorrow morning as cloud cover decreases and NW wind approaches 70 mph with stronger gusts. Tomorrow should bring mostly clear skies with summit temperatures in the single digits F below 0 and continued NW wind around 70 mph.
Primary Avalanche Problem – Wind Slab
Wind slabs on many aspects, formed late last week on varied wind speed and direction, have generally become stubborn to a human trigger. You’re more likely to trigger an avalanche in softer pockets where your skis or boots sink into the slab. The possibility of a large avalanche does remain, particularly where the snow has a hollow sound. New snow on increasing NW wind may build new reactive but small slabs by late today or tonight.
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 robust crust that was widespread in our terrain has mostly been covered by wind slabs formed since Wednesday at middle and upper elevations. The layered slabs above the crust formed on multiple periods of westerly and southerly wind and as recently as Saturday, with small new slabs likely to develop late today. As of yesterday, surface slabs were generally firm (1F +/-) and supportable under skis, with softer pockets of greater ski penetration and certainly boot penetration existing also. We suspect these softer pockets could behave similarly to the small skier triggered avalanche in the Ammonoosuc Ravine yesterday. A firm over soft structure has been identified in the slabs formed above the crust, keeping large avalanches a relevant though less likely concern. Keep this possibility as well as the spatial variability inherent to wind slabs in mind as you make snowpack observations today.
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 02/18/2019 at 6:58 AM.
Ryan Matz, Snow Ranger
USDA Forest Service
White Mountain National Forest
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