Avalanche Forecast for Wednesday, April 17, 2019
This information was published 04/17/2019 at 7:18 AM.
The Bottom Line
Small wind slabs formed Monday night could produce a human triggered avalanche and large areas of hard refrozen snow make long sliding falls a real concern today. The recently formed wind slabs should be avoidable in most of our terrain. Look for smooth, white pockets of snow as the avalanche problem today. Human triggered avalanches are unlikely due to the isolated and avoidable nature of the drifted snow, however not impossible and if recreating today you may be drawn to travel on these areas of new snow. A small avalanche could result in a long sliding fall. As temperatures rise this afternoon, be thinking about sluff management as the snow surface softens. All forecast areas have LOW avalanche danger.
Yesterday the summit recorded more than 12 hours of WNW wind over 90 mph which has drifted and heavily scoured the 3’ of snow that fell Monday night into Tuesday morning. Summit temperature reached only 14F, firmly freezing the past weekend’s wet snowpack. NW wind relaxed overnight to 50-60 mph and is expected to remain NW through the day today decreasing to 10-25 mph. Summit temperatures today are expected to reach the mid 20sF, and at 3000’ mid 40sF under clear skies. Clear skies today turn cloudy overnight with a chance for mixed precipitation on Thursday.
Primary Avalanche Problem – Wind Slab
Wind slabs from the 3” of snow Monday night and Tuesday morning can be found in sheltered locations, especially on the eastern half of the compass rose at elevations above 3500’. These small pockets of wind slab are likely stubborn to a human trigger and can largely be avoided by traveling on the refrozen, grey old snow. A human triggered avalanche is not impossible today. Triggering even a small wind slab in steep icy terrain could result in a high consequence sliding fall.
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.
Secondary Avalanche Problem – Wet Loose
The likelihood of wet loose avalanches will increase as the day progresses particularly on steep slopes that receive the most direct sunlight. As the snow surface warms, the softening surface snow will become less stable and may sluff under a ski or boot. These wet loose sluffs should remain small in size, however moving sluffs can entrain skis or snowboard and carry a person downhill if not managed carefully.
What is a Wet Loose Avalanche?
Wet Loose avalanches are the release of wet unconsolidated snow or slush. These avalanches typically occur within layers of wet snow near the surface of the snowpack, but they may quickly gouge into lower snowpack layers. Like Loose Dry Avalanches, they start at a point and entrain snow as they move downhill, forming a fan-shaped avalanche. Other names for loose-wet avalanches include point-release avalanches or sluffs. Loose Wet avalanches can trigger slab avalanches that break into deeper snow layers.
The rain wetted snowpack of Monday has been solidly refrozen above 3800’ by yesterday’s unseasonably cold weather. At the same time, continuous strong WNW wind scoured much of the recent 3” of new snow leaving a patchwork of very textured frozen surface with pockets of new wind slab. In Tuckerman and Huntington ravines, areas of drifted new snow appear to be well bonded to the frozen bed surface. The exposed melt freeze crust may soften at middle elevations today given the warming temperatures, sunshine and low winds. Any softening that occurs will be more aspect driven than elevation driven, leading to wet-loose sluffing becoming more of a problem on slopes receiving direct sunlight.
It’s worth noting that while the summits received the low end of forecasted possible new snow accumulation yesterday, direct observations into the higher, steeper terrain were hampered by fog and blowing snow. Larger, thicker wind slabs may exist higher in the ravines in locations with significant upwind fetch like the Headwall of Tuckerman Ravine. Such wind slabs should be carefully evaluated or avoided altogether in high consequence terrain.
The Sherburne and Gulf of Slides ski trails are snow covered to Pinkham Notch, though this is changing by the day. Expect ice patches, opening stream crossings and the occasional bare patch.
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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. Click here to check it out.
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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 04/17/2019 at 7:18 AM.
Jeff Fongemie, Snow Ranger
USDA Forest Service
White Mountain National Forest