Avalanche Forecast for Wednesday, December 5, 2018
This information was published 12/05/2018 at 7:02 AM.
The Bottom Line
Winds from the northwest blew yesterday at an ideal speed to transport 1.5” of very low density snow that fell at upper elevations. Low visibility did not allow field observations but anticipate new wind slabs to have formed on east-facing aspects, beneath buttresses and steep slopes. These small wind slabs are sitting on a glazed, refrozen crust and could be reactive. The predominant surface in steep terrain, out of the trees, will be a supportive icy surface that’s well suited for setting sliding speed records. The guiding principle for safe travel has little to do with self-arrest and everything to do with not falling down. The avalanche danger is Low in all forecast areas today. The steepest and most sheltered areas may hold thicker wind slabs that push the upper end of this rating.
The past three days brought classic New England weather that ran the gamut from snow, sleet and freezing rain with summit temperatures ranging from 36F to -5F. After the warm up turned the upper snowpack to slush, 4” of snow fell, which was soon soaked again with rain then capped with sleet and further glazed with freezing rain. Yep, a pretty classic New England tour of precipitation types. Yesterday, 1.5” of 5% density snow on the higher summits blew in on top of this surface as temperatures dropped to a low of -5F. A trace fell at Hermit Lake. Today, high pressure centered over the area will allow wind to diminish with a light northwest and west wind. Mostly clear skies, good visibility and light wind should make for a fine but cold day. Clouds may begin to move in during daylight hours this afternoon. Temps will reach 10F on the summits, high 20’s in the valleys.
Primary Avalanche Problem – Wind Slab
Be wary of unstable wind slabs in steep terrain. You could trigger a small avalanche from areas where snow has accumulated and formed firm slabs of snow over softer snow. You may be lured onto this softer snow in order to avoid traveling on the bulletproof, icy surface. If so, consider the potential for one of these wind slabs to pop off and send you sliding.
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.
There is currently 110cm of snow on the ground at Hermit Lake and 93cm at Gray Knob. At least two crust layers are present in the snow pack which has actually seen remarkably few warming events over the past 6 weeks. Look for facets to develop near these crusts as temperatures drop again this week. A cold front may bring some upslope snow to the higher summits Thursday night. The icy surface will not encourage bonding of new snow, especially considering the cold temperatures that seem likely on Thursday.
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 12/05/2018 at 7:02 AM.
USDA Forest Service
White Mountain National Forest