Avalanche Forecast for Friday, December 21, 2018
This information was published 12/21/2018 at 7:24 AM.
The Bottom Line
Rain falling on snow can result in wet avalanches though these avalanche types are notoriously hard to predict. This type of avalanche can be quite destructive, though slower moving than dry slabs, due to their greater density and pushing power. Rain on snow also weakens the snowpack for foot travel and makes for miserable post-holing conditions. And then there is hypothermia. If those reasons aren’t enough to choose something other than going into the Presidential Range today, then consider the possibility of a large natural avalanche caused by an erupting waterfall blowing out the snowpack in the Lip and generating a wall of slush, water and boulders. Photos of a similar historical event can be seen here with a view of the debris track here and the crown here taken last year. Though there is the possibility of natural avalanches in other isolated areas of steep terrain, it will be unlikely in most places. The Headwall area of Tuckerman Ravine has CONSIDERABLE avalanche danger due to a confluence of terrain factors, a thicker snowpack, and a stream channel under the snow. The floor of Tuckerman Ravine is threatened by a natural avalanche from this area. Human triggered avalanches are possible elsewhere in avalanche terrain throughout the range which has MODERATE avalanche danger today.
Skiers and climbers enjoyed the gift of light wind, warm temperatures and sunshine yesterday before today’s lump of coal. Rain began overnight and will continue through the day with rain falling heavily at times. The most recent forecast totals indicate around an inch and half will fall, but we could get more due to banding and orographic lift. Temperature on the summit will slowly fall from a high in the lower 40’s starting around midnight tonight and reach freezing levels by tomorrow morning when snow showers begin. The potential for long sliding falls on a hard icy snowpack will become a problem tomorrow.
Primary Avalanche Problem – Wet Slab
Wet Slab avalanches will become a concern as recent wind deposited snow becomes wet and loses strength. Softer snow will be more likely to fail though larger areas of harder old wind slab may be more consequential. This problem will increase through the day as water and heat penetrate the snowpack.
What is a Wet Slab Avalanche?
Wet Slab avalanches are the release of a cohesive layer of snow (a slab) that is generally moist or wet when the flow of liquid water weakens the bond between the slab and the surface below (snow or ground). They often occur during prolonged warming events and/or rain-on-snow events. Wet Slabs can be very unpredictable and destructive.
Secondary Avalanche Problem – Wet Loose
Wet loose avalanches could be triggered in steep terrain today. These sluffs are most likely to occur in areas of softer snow. They could entrain enough snow to cause problems in bigger terrain or on slabby ice climbs.
What is a Wet Loose Avalanche?
Wet Loose avalanches are the release of wet unconsolidated snow or slush. These avalanches typically occur within layers of wet snow near the surface of the snowpack, but they may quickly gouge into lower snowpack layers. Like Loose Dry Avalanches, they start at a point and entrain snow as they move downhill, forming a fan-shaped avalanche. Other names for loose-wet avalanches include point-release avalanches or sluffs. Loose Wet avalanches can trigger slab avalanches that break into deeper snow layers.
Warming yesterday allowed recently built, firm wind slabs to further settle. The majority of the thicker wind slabs were firm and stubborn while thinner, softer wind slabs were more scattered. No avalanche activity has been observed or reported in the past two-three days. In steep terrain, you can find buried crusts with small facets nearby. Stability tests and a fair amount of traffic supported the theory that a crack would not propagate in these layers.
The Sherburne and Gulf of Slides ski trails are snow covered to Pinkham Notch. The Little Headwall streambed has remained open in many places so far this season and will become a full on waterfall today. Don’t plan on skiing out from the Bowl anytime soon.
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 12/21/2018 at 7:24 AM.
USDA Forest Service
White Mountain National Forest