Yesterday, wind from the west started at 50mph and gradually calmed to the current 13mph. Skies had partial cloud cover all day and temperatures reached 29F at the summit, 32F at Gray Knob (4370’), 37F at Hermit Lake (3800’). High pressure over the region today will create another mild day with similar high temperatures, partly cloudy skies, and wind shifting to the south and increasing to 30mph. The next round of snowfall is forecast to begin Saturday afternoon with several inches accumulating by Sunday morning.
Primary Avalanche Problem – Wind Slab
Wind slabs can be found on many slopes and should be easily identifiable by their smooth, pillowed, appearance. These will be found on slopes in the lee of our classic NW wind, but also watch for these to have cross-loaded into gullies and behind terrain features that eddy the wind. While these wind slabs are several days old by now and have had a degree of settlement, they rest on a soft layer of snow that sits directly on top of an icy, bed surface, presenting the classic recipe for an avalanche. As today’s air temperature continues to warm, these slabs will weaken, creating the possibility of a skier or climber triggering the weak layer that exists, particularly over a thin spot. Terrain management will dictate the size of the avalanche problem you are dealing with, though with a thin snowpack, even a small slide can drag someone through exposed rocks and bushes or into a terrain trap.
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.
Warming wind slab is an avalanche problem we deal sometimes with here in this region that doesn’t fit nicely into a category. Those paying close attention will have seen our aspect rose on Wednesday look different from Thursday and again different today despite the fact we labeled the avalanche problem as a wind slab problem. With yesterday’s warming being driven more by solar gain, we identified the southern half of the compass as areas most likely to contain warming wind slab. Slightly warmer temps today with possibly more clouds following an overall warm night is leading us to believe the warming process today that will weaken our wind slab is driven more by ambient air temperature. This type of warming will affect all aspects, hence the aspect/elevation rose today identifying all wind slabs being capable of warming today and creating a warming wind slab avalanche problem. While warming encourages settlement, the snowpack must pass through a period of instability before it arrives at isothermal stability.
The Sherburne and Gulf of Slides ski trails are snow covered to Pinkham Notch and have seen heavy ski traffic.
Details on daily snowfall totals, precipitation type, total depth of snow and other information can be found on our page devoted to snow study plot data. Clickhere to check it out.
Recent snowpack and avalanche observations can be foundhere and on Instagram. Your observations help improve our forecast product. Please take a moment andsubmit a photo or two and a brief description of snow and avalanche information that you gather in the field.
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 01/24/2020 at 6:59 AM.
USDA Forest Service
White Mountain National Forest
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