Avalanche Forecast for Sunday, December 23, 2018
This information was published 12/23/2018 at 7:08 AM.
The Bottom Line
New snow that fell at the tail end of the rain event, combined with extreme winds, has formed isolated areas of wind slab that should be small and easy to avoid by staying on wind scoured slopes. All elevations and aspects have LOW avalanche danger today. In all locations, long, sliding falls will create equal, if not greater, hazard for the day. Ice dams and undermined snow will add to the list of objective hazards that are present in the range today.
The rain event over the past few days introduced close to 3” of rain to our snowpack. Temperatures fell below freezing late Saturday morning, continuing to drop to a current 1F on the summit. Before precipitation shut off, 2.1” of snow fell on the summit, about ¾” at Grey Knob, and a trace at Hermit Lake. This snow arrived on 40 mph west winds that shifted WNW and increased to 80+ mph for the remainder of the day. High pressure across the region today will create clear skies, decreasing wind speeds from NW 74 mph at 6am to around 30 mph by dark, and temperatures in the teens F. Clouds will develop late in the day with a chance of snow, though accumulations should be minimal. The next round of high pressure will move in Monday afternoon, setting up a dry pattern for the holiday week.
Primary Avalanche Problem – Wind Slab
Wind slab that formed yesterday afternoon is likely small, isolated, and stubborn to a trigger by skiers and climbers due to high wind speeds. The largest and thickest areas of wind slab can be found in the direct lee of WNW wind with the largest fetch, such as low in the Headwall area of Tuckerman. These wind slabs should be easily discernible from other refrozen, scoured surfaces.
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.
Following a snowy November and cold start to December, the inevitable reset button was hit for our snowpack this past week. Three inches of rain with warm temperatures saturated the upper part of our snowpack and was followed by plummeting temperatures, creating a hard and fast sliding surface. While the avalanche problem exists today, it is isolated in nature and other objective hazards deserve as much respect. Long, sliding falls will be possible on any slope but flat surfaces: use crampons and an ice axe properly. Ice dams will form when a flash freeze follows rain events. Climbers should be particularly aware of this on all aspects and elevations. Holes in the snow due to undermining were visible yesterday in many gullies and are a good indication of thin spots in the snowpack.
The Sherburne and Gulf of Slides ski trails are snow covered to Pinkham Notch, though expect ice, the occasional rock, and open waterbars to add to the challenge.
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 12/23/2018 at 7:08 AM.
USDA Forest Service
White Mountain National Forest