This information was published 01/27/2020 at 7:14 AM.
NOT THE CURRENT FORECAST
This is an archived avalanche forecast.
The Bottom Line
CONSIDERABLE danger exists for new and developing wind drifts primarily in steep east facing terrain and cross loaded gullies as moderate wind moves new snow from above the ravines, onto the steep slopes in the ravines.
Drifted snow may release naturally in the steepest terrain, and is likely to avalanche from a human trigger. Prime locations for this in Tuckerman Ravine will include the apron of Chute, to Sluice and everything in between.
Steep wind drifted slopes should be approached with caution, especially if visibility is poor reducing your ability to fully assess the terrain above or below you.
The Presidential Range received 5” of 11.6% snow Saturday on a 65 mph south east wind, and another 1” yesterday evening on a 50-60 mph west wind. A trace to 3” of new snow is possible today with wind holding west to north west at 30-45 increasing to 45 – 60 mph in the afternoon. Temperatures will be in the mid teens F. Snow showers tonight and tomorrow may bring another inch or two of snow.
Primary Avalanche Problem – Wind Slab
West-Northwest wind will transport new snow, and any available remaining snow above treeline from Saturday’s 5” into new wind slabs primarily in steep east facing terrain and cross loaded gullies. These new wind slabs will add weight and stress to already existing wind slabs formed from the Saturday snow. With current and forecast wind speeds, new wind slabs will likely be sensitive to a human trigger. Watch for shooting cracks as a warning red flag.
A lingering issue that should also steer conservative terrain choices is a weak layer of snow sitting just above the ice crust formed January 11 – 12 buried by subsequent storms. While unlikely, it’s not impossible that an avalanche today, could provide enough energy to step down to this weak layer resulting in a much larger 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.
As avalanche forecasters, we all keep a mental model of the snowpack in our heads that remains front and center from November to June. This mental model acts as an evolving image of the snow structure, which is redrawn whenever new information is applied. New information could be field time with observations and stability tests, weather data and public observations.
The current mental model of the snowpack can be described like this:
January 11-12: almost 2” of rain, 45F on the summit of Washington refroze to a melt-freeze crust on the 13th. This crust is thick enough that snow below is not likely to be a factor in current avalanche problems.
8” of snow Jan 15-16 began falling on light winds, when the wind increased it formed stiff 1F wind slabs over several inches of light 4F snow above the crust. This upside down snow structure has remained, though has proven to be unreactive.
5” of snow Jan 18-19 with NW wind built new slabs in east facing slopes. Noted that the steepest slopes in Tuckerman Ravine did not appear to avalanche naturally from either the Jan 15 or Jan 19 storm.
Jan 25: 5” of dense new snow that fell on a SE wind and was capped by a thin ice crust. Uncertainty exists on how much of this snow was transported by the wind during and after the storm, and how much remains for transport today. Uncertainty also exists about the remaining weak 4F layer at the Jan 13 ice crust. Is it possible for a human or avalanche to step down to this weak layer?
Today: New snow and west wind will certainly build new wind slabs in eastern terrain. Typically we see wind slabs as much as four times thick as the snow totals, so 3” of new snow today may build 12” thick wind slabs.
The Sherburne and Gulf of Slides ski trails are snow covered to Pinkham Notch and have been in good shape though a few of the usual wind scoured areas have rocks showing.
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 01/27/2020 at 7:14 AM.
USDA Forest Service
White Mountain National Forest
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