Avalanche Forecast for Sunday, December 9, 2018
This information was published 12/09/2018 at 7:04 AM.
The Bottom Line
Recent wind-loading and avalanche activity are keeping our danger rating at MODERATE today. While the majority of the existing wind slab avalanche problem is firm and stubborn, the potential size of some of these slabs makes the consequences of triggering something severe. The potential for a person to trigger one of these large, firm wind slabs remains possible, but just barely. Areas of softer wind slab in more sheltered spots may also present a problem. It’s important to remember that you only need to be wrong once in your assessment of a slope. Even stubborn, firm wind slabs can fail if the right trigger point is found. Do the right thing and have everyone in your party bring beacons, probes and shovels and travel one at a time through high consequence terrain.
Yesterday brought cold temperatures and strong WNW and NW winds. Temperatures bottomed out at -12F with wind speeds in the 60 mph range. Most of the snow on the ground had already been deposited but the wind gusts were still adding a bit of load to the slopes. Today temperatures will moderate with with high temperatures around 10F though west winds will continue to keep things chilly. Summits will be in and out of the clouds and fog until a weak cold front brings upslope snow showers tonight.
Primary Avalanche Problem – Wind Slab
Triggering a firm wind slab may be possible today on steep, generally east facing aspects and in gullies where snow has accumulated into slabs. These wind slabs will have a variety of thickness from 6” to several feet thick. Triggering seems most likely where the weight of a person can reach down to the layer of weak, soft snow beneath. Cracks in these hard slabs can propagate from thin areas to thicker parts of the slab and bring down a large volume of snow. Softer slabs may also exist in the terrain in especially well sheltered areas and would be more sensitive to triggering.
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 usual smorgasbord of surface conditions exist in the terrain. The melt freeze crust is not as exposed as you might think, though small areas exist in the terrain and are quite slippery and fast. The 20cm of snow that has fallen at higher elevations this past week covered most of that icy surface and now exists as firm, wind packed snow. This surface isn’t always supportable unless you are wearing skis or snowshoes due to the soft snow that was deposited by light winds on Thursday. See our observations page for recent and more detailed information about what’s going on in the snowpack.
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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 12/09/2018 at 7:04 AM.
USDA Forest Service
White Mountain National Forest