This information was published 01/27/2019 at 6:58 AM.
NOT THE CURRENT FORECAST
This is an archived avalanche forecast.
The Bottom Line
Wind drifted snow exists on slopes in the lee of a west wind as well as beneath terrain features and sits on top of a robust crust. Incoming snow today will increase both the size and distribution of these wind slabs. A bit of uncertainty exists in the forecast snow totals, but if we receive the upper end of the forecast 2-4”, these areas of wind slab will become possible to trigger by a skier or climber in steep terrain, earning a MODERATE rating. The exception to this is the Northern Gullies in Huntington which have a LOW rating due to their aspect. Until the snow starts to accumulate, the current avalanche hazard may be overshadowed by the potential of long, sliding falls on the icy bed surface. If traveling in avalanche terrain today, a kit complete with climbing equipment for the morning and avalanche rescue gear for the afternoon would not go amiss.
Thursday’s rain transitioned immediately to snowfall, with around 5” of snow falling on the summit and 1.5” at Hermit Lake (3800’) between then and now. Prevailing westerly wind over this same time period has trended downward in speed. Low pressure racing across the region today will drag a warm front this morning followed by a cold front later this afternoon. Both frontal passages are bringing moisture, but the bulk of the forecast 2-4” of snow today will arrive with the cold front this afternoon into evening. Current wind from the south at 35 mph will increase through the morning to around 50 mph, possibly moving toward SW at times, before becoming west as the cold front passes. Another 2” of snow may fall tonight before skies clear tomorrow and temperatures fall below 0F.
Primary Avalanche Problem – Wind Slab
Wind slabs have formed from snow showers over the past few days on terrain in the lee of a west wind. These wind slabs are small in size and interspersed with areas of exposed melt-freeze crust. Snow that is likely still available for transport above ravine elevations, combined with the forecast 2-4” today, will continue to build these wind slabs. Today’s shift in wind and eventual increase in speed will push the likelihood of triggering one of these wind slabs toward possible, particularly if we receive the upper end of forecast snow totals.
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.
Thursday’s rain event has eliminated concerns of weak layers deeper in our snowpack for now. The melt-freeze crust was supportable above treeline yesterday while places in the trees still offered opportunities to post hole. Snow that arrived late on Thursday fell on a wet snow surface, allowing it to bond well to the robust crust that now exists. The likely weak layer today will not be the interface between the crust and wind slab but a density change between layers above the crust. That being said, any fall today, whether caused by a slip and fall or triggering the wind slab, will have dire consequences due to the fast nature of the hard crust. Self-arresting on the bed surface that exists is unlikely; fall prevention is a much better bet for today, performed by equipment and terrain selection.
The Sherburne and Gulf of Slides ski trails are snow covered to Pinkham Notch.
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 01/27/2019 at 6:58 AM.
USDA Forest Service
White Mountain National Forest
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