Avalanche Forecast for Wednesday, December 19, 2018
This information was published 12/19/2018 at 7:09 AM.
The Bottom Line
Strong winds in the past 24 hours pounded the 4” of snow that fell in the past 48 hours into our steep slopes. The resulting wind slabs are likely to be very stubborn where you find them in the most sheltered terrain. Unfortunately, they may be resting on a slippery ice crust or a combination of ice crust with a weak faceted layer nearby. Steep slopes with an aspect facing away from the hurricane force northwest winds may contain larger but stubborn wind slabs. Consider these slopes to have MODERATE avalanche danger today. It may be possible for a rider or climber to trigger one of these slabs. Wind erosion has likely scoured many other slopes and gullies. These areas will have LOW avalanche danger due to smaller wind slabs.
Howling NW winds died down overnight. Yesterday’s highest gust reached 117 mph with almost 12 hours of wind in the high 80 to 100 mph range. 1.6” of new snow was recorded on the summit in the past 24 hours. Our well sheltered snow plots saw significant wind scouring and drifting in the past 24 hours. Temperature will rebound today and has already risen to 9F following a low temperature yesterday of -9F. Expect NW winds to continue to diminish to a much more hospitable 50 mph. The high temperature will reach into the low 20’sF on the summit under clearing skies.
Primary Avalanche Problem – Wind Slab
Steep leeward slopes which did not avalanche yesterday will be stubborn to the point of being unreactive today. Despite their steely and stiff nature, it’s best to continue to use normal, safe travel precautions when travelling on or below these slabs.
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.
It remains to be seen what natural avalanche activity occurred yesterday due to the lack of visibility. Four inches of snow in the past 48 hours, plus whatever snow the howling winds dug up and moved, likely generated a decent avalanche cycle on eastern aspects and may have even ripped up some of the rain crust scattered around the terrain, both in the alpine and in the ravines. If you are digging or doing stability tests, you’ll likely find fractures initiating at moderate load force but not likely to see much snow with propagation potential. The ability of slabs to propagate a crack is a better indication of slope stability in wind slabs than the tendency of the weaker interface to crack, which it almost always does.
The Sherburne and Gulf of Slides ski trails are snow covered to Pinkham Notch. Following yesterday’s wind event, you’ll want sharp edges on your skis and quick reflexes for dodging limbs on the ground.
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 12/19/2018 at 7:09 AM.
USDA Forest Service
White Mountain National Forest