This information was published 01/02/2019 at 7:08 AM.
NOT THE CURRENT FORECAST
This is an archived avalanche forecast.
The Bottom Line
Stubborn wind slabs in steep terrain should be assessed carefully if you plan to brave the cold temperatures to recreate in the mountains today. Wind slabs built by high winds yesterday may be unreactive enough to present a safer option for travel than the steep, wind-scoured icy slopes that create the potential for a long, high-speed sliding fall. Steep areas where these new wind slabs exist have MODERATE avalanche danger due to the lingering possibility of a person triggering an avalanche. Areas scoured to older snow or ice-covered snow have LOW avalanche danger. This includes the northern gullies in Huntington Ravine. Natural avalanches are unlikely in all locations today.
A low pressure system passing Monday night dropped 4” of snow capped with sleet and freezing rain. A cold front followed yesterday morning that brought 2” more snow through the day. As these snow showers fell, winds howled from the WNW and NW at over 80 mph, peaking at 123 mph. This morning, with the cold air mass in place, NW winds will diminish from around 50 mph now to a relatively light 30 mph late this afternoon. Expect a high temperature of only 6F on the summits to accompany the west wind. 2-4” of new snow with SW and W winds tonight will refresh our snowpack at higher elevations.
Primary Avalanche Problem – Wind Slab
Sustained high winds from the west in the past 36 hours created predominately firm wind slabs that are stubborn and possibly even unreactive to a human-trigger. Some softer and more reactive wind slabs may also be found but should be easy to avoid. With no new snow loading these slabs, natural avalanches are unlikely 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.
The low pressure storm system that passed directly overhead Monday night resulted in widely varying precipitation types around the range. High winds challenged accurate snowfall measurements, even at sheltered snow study plots, but it seems that around 10 cm fell at the 3800’ level. A brief period of sleet and freezing rain around dawn capped this snow, with more rain falling on the snow at lower elevations. It remains to be seen exactly how much this layer of sleet and freezing rain sheltered the snow beneath from the wind at higher elevations but drifting observed in Tuckerman Ravine indicates that at least some of the 4” of snow was accessible for wind transport. Visibility was poor with active wind loading yesterday but it appeared that the Lip avalanched sometime yesterday morning. Early morning light allowed us to observe debris in Left Gully as well.
The Gulf of Slides and Sherburne ski trails are snow covered and rideable to Pinkham Notch with variable conditions.
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 01/02/2019 at 7:08 AM.
USDA Forest Service
White Mountain National Forest
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