Avalanche Forecast for Tuesday, March 5, 2019
This information was published 03/05/2019 at 7:01 AM.
The Bottom Line
New wind slabs are likely to avalanche from a human trigger today. Recent storms have brought modest snowfall totals, but the sustained wind since yesterday afternoon can easily build slabs several feet thick from just several inches of snow. Continued wind loading this morning means that natural avalanches are also possible. All forecast areas have CONSIDERABLE avalanche danger. You may not find an avalanche problem on lower elevations which received less recent snow. Look for relatively soft snow on the surface as today’s avalanche problem wherever you travel today.
Over 4” of snow fell at higher elevations yesterday and last night, as summit wind became W and WNW while increasing to the current 50-70 mph range. Snowfall ended overnight. Summit wind should decrease through today to under 30 mph. Partial clearing this morning will trend towards cloudy skies. Upslope snow showers tonight and tomorrow may produce a total of 1-3” of snow on W and NW wind under 45 mph. It’s a cold one, with a current summit temperature of -13F expected to rise but remain below 0F today.
Primary Avalanche Problem – Wind Slab
Wind slabs formed since yesterday afternoon will be reactive to a human trigger today. Shifting wind from W to WNW last night combined with cross loading likely distributed these wind slabs of varying size for all middle and upper elevation terrain on the eastern half of the compass rose. These new slabs will be truly large in east facing terrain with a significant upwind fetch zone like the Headwall of Tuckerman Ravine and the Gulf of Slides.
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.
A series of modest snowfall events since last Thursday, with the heaviest snow falling yesterday on ideal loading wind speeds, have created new wind slabs with reactive layers that you’re most likely to find on the eastern half of the compass rose. These layers sit on the mostly firm and unreactive wind slab formed early last week. On some southerly aspects, this old wind slab was capped with a sun crust late last week. These areas were producing small natural avalanches yesterday morning, with easterly and northerly aspects also displaying small avalanche activity and generally poor bonding between layers of soft new snow. Wind has since increased and built much larger wind slabs that are likely above this weak structure. Keep this fresh wind slab over less cohesive snow setup in mind today but also remember that the distribution of wind slab avalanche problems is always variable. You may find scoured areas in westerly terrain without an avalanche problem and even have some icy February 8th crust exposed. Low elevations may also lack an avalanche problem due to lesser new snow, but be on the lookout for reactive new slabs anywhere you travel 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 03/05/2019 at 7:01 AM.
Ryan Matz, Snow Ranger
USDA Forest Service
White Mountain National Forest