This information was published 01/29/2019 at 7:08 AM.
NOT THE CURRENT FORECAST
This is an archived avalanche forecast.
The Bottom Line
Wind slabs formed early yesterday have gained some stability but remain possible to human trigger. Many areas are scoured to or near to a hard melt freeze crust and lack today’s avalanche problem. This provides opportunities to avoid avalanche danger, but potential for long sliding falls on the crust mean that crampons, ice axe, and your ability to avoid such a fall are recommended for travel in steep terrain. Avalanche danger is MODERATE for most of our terrain, with the Northern Gullies of Huntington Ravine, the Right Side of Tuckerman Ravine, and elevations below 3500’ having LOWavalanche danger. The winter storm arriving this evening will significantly elevate avalanche danger overnight through Thursday. Timing of forecast snowfall currently suggests that avalanche danger won’t increase before dark today and will only increase slightly before midnight, but realize that an early arrival of the storm will increase avalanche danger sooner than expected.
Mostly clear skies yesterday came with decreasing westerly wind and summit temperatures rising from -11F to 14F. Clouds and summit fog will spread back in today ahead of a winter storm arriving this evening. Less than an inch of new snow is expected during daylight hours, with southerly summit winds in the 30-40 mph range and temperatures in the teens F. Snowfall should increase in intensity after dark and taper but continue through the day tomorrow and ultimately produce a storm total of around 12 inches. Wind through the bulk of the snowfall will be southerly in the 30-40 mph range and shift W as snow tapers off tomorrow, increasing towards 60 mph. Temperatures will steadily drop starting tomorrow morning to bottom out early Thursday at nearly -30F on the summits.
Primary Avalanche Problem – Wind Slab
Low density snow which fell late Sunday was easily transported by westerly wind, leaving significant areas of scouring and building wind slabs of varying size. The largest slabs which can be found in areas like the Headwall of Tuckerman Ravine aren’t likely to produce an avalanche larger than D2 in size, but remember that this is still large enough to bury a person. These slabs may remain reactive to a human trigger, though we expect that they’re becoming increasingly stubborn. Areas where the January 25th melt freeze crust is at or near the surface can be found on all aspects and lack today’s avalanche problem.
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.
Somewhat widespread scouring of Sunday night’s 6” of new snow occurred into yesterday morning, building some small to large sized wind slabs and leaving melt/freeze crust at or near the surface in many areas. We’re not concerned about instabilities below this crust which formed on January 25th. It’s worth noting that this crust is thinly veiled by newer snow in many areas, and from a distance may be difficult to visually identify. In areas of wind slab yesterday, the weak layer of concern seemed to be a density change above this crust, with firmer (1F-4F) slab over less cohesive snow (4F-F). A number of natural avalanches occurred either Sunday night or early Monday, including Hillman’s Highway and Dodge’s Drop in Tuckerman Ravine and Odell Gully in Huntington Ravine, none of which exceeded D2 in size. Stability of these recent wind slabs has been increasing since this natural avalanche cycle over 24 hours ago. Forecast snow and wind late today will contribute to existing slabs and is unlikely to create an avalanche problem in terrain currently lacking wind slab, possibly elevating avalanche danger slightly but keeping us within the range of our forecast danger ratings. If this storm arrives sooner than forecasts currently suggest, new storm and wind slabs could become a significant travel concern if you’re out late today.
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 01/29/2019 at 7:08 AM.
Ryan Matz, Snow Ranger
USDA Forest Service
White Mountain National Forest
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