Avalanche Forecast for Saturday, December 8, 2018
This information was published 12/08/2018 at 7:03 AM.
The Bottom Line
Recently formed wind slabs will vary in size and character across our terrain but likely be touchy to a human trigger today. Periods of sustained wind yesterday and earlier in the week has transported our relatively meager recent snowfall into large wind slabs. Today is a great day to realize that wind can easily build wind slabs quadruple the thickness of recent snowfall totals, an estimate that our terrain often exceeds. The Headwall area of Tuckerman Ravine and the Central area of Huntington Ravine have Considerable avalanche danger today, with human triggered avalanches likely. All other forecast areas have Moderate avalanche danger. Evaluate snow and terrain carefully to identify features of concern. Realize that our upper snowpack is quite variable and that a few feet may be the difference between relatively stable snow and triggering an avalanche.
Yesterday’s W wind, blowing in the 50-60 mph range for most of the day on the summit, has shifted NW and decreased to under 40 mph overnight. Wind should remain NW and between 30-40 mph before increasing again this evening. This wind transported the 10+ inches of snow which has fallen in the past week, half of which of which we received since Thursday. We may see a trace of snow late today as cloud cover increases slightly. Temperatures are forecast to remain just below 0F on the summit today. Tomorrow is forecast to be 10 degrees warmer and bring no additional precipitation.
Primary Avalanche Problem – Wind Slab
Wind slab built yesterday on W wind is our primary avalanche problem today. Existing across the eastern half of the compass rose in our terrain, it will vary in size due to aspect and upwind “fetch” or area of snow that was available for transport. East facing terrain in the ravines likely holds the thickest and largest wind slabs due to direct loading, with cross loading affecting other terrain not directly scoured by W or NW wind. An avalanche in the new wind slab could ultimately entrain snow deposited earlier this week in some areas and produce a large avalanche, while smaller avalanches in terrain receiving less wind deposition are a concern as well.
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 approximately 10” of snow which has fallen since a melt/freeze crust formed last Sunday and Monday has been transported by our usual W and NW wind, including yesterday’s significant wind event. Size of slabs formed over the past week vary greatly, with the crust likely exposed at the surface where scoured by wind. The newest wind slab was particularly reactive yesterday at lower ravine elevations, with cracks shooting ahead of ski tips and hand shear tests failing repeatedly on isolation with Q1 (clean) shear quality. Wind transport has continued to build this layer, and while we suspect it has become slightly less reactive it should remain touchy to a human trigger.
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 12/08/2018 at 7:03 AM.
Ryan Matz, Snow Ranger
USDA Forest Service
White Mountain National Forest