This information was published 01/07/2019 at 7:45 AM.
NOT THE CURRENT FORECAST
This is an archived avalanche forecast.
The Bottom Line
New snow and sustained extreme wind since yesterday afternoon have resulted in small to large wind slabs. Areas have also been scoured to older snow and melt-freeze crust. Drifted snow that is smooth and pillowy in appearance is of most concern today. Avoid travel on and below these features. All forecast areas have MODERATE avalanche danger today, with LOW avalanche danger in the Northern Gullies of Huntington Ravine and low elevation areas such as in Crawford Notch being the exceptions. Our avalanche problem varies greatly across the terrain, so be sure to make careful observations to guide your terrain decisions.
Five inches of new snow and 0.39 inches of snow water equivalent were recorded on the summit in the past 24 hours, with nearly four at Hermit Lake and three at Gray Knob. Wind has blown from the NW, in the 70-90 mph range on the summit, with four hours early this morning sustained around 90 mph and stronger gusts. Snowfall stopped last night.Wind remains NW at 70 mph and should shift N while tapering dramatically to under 20 mph by dark today. An inbound storm will bring snow starting around midnight tonight and continue through much of tomorrow, with S and SW winds to 60 mph. Total snowfall may be around 6”.
Primary Avalanche Problem – Wind Slab
Expect wind slabs formed on last night’s extreme wind speeds to vary in size, distribution, and sensitivity. You are most likely to find these slabs on the eastern half of the compass rose and while largely stubborn to a human trigger, softer pockets may be reactive. Traveling on scoured areas to avoid the avalanche problem may be an option. Though many pockets will only be capable of producing a small avalanche today, large areas of new wind slab do exist and could combine with pre-existing snow to produce a large avalanche.
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.
The past week has brought complexity back to our upper snowpack. Modest amounts of snow have been affected by multiple rounds of strong and extreme wind that has generally blown out of the west and northwest. We don’t expect an avalanche to initiate in these layers, but they could ultimately be entrained and contribute to the overall size of an avalanche in surface slabs. Surface wind slabs are the primary concern today, and determining their location following last night’s wind should be your primary field observation goal today. Wind last night has scoured a significant portion of our terrain, particularly on the west side of the range where the robust December 22 melt-freeze crust has recently been the dominant snow surface. A natural avalanche did occur sometime in the past 24 hours in Center Bowl of Tuckerman Ravine, and while we have passed peak instability, it’s certainly a day to make careful observations and terrain choices in and below avalanche terrain.
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 01/07/2019 at 7:45 AM.
Ryan Matz, Snow Ranger
USDA Forest Service
White Mountain National Forest
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