Avalanche Forecast for Wednesday, February 20, 2019
This information was published 02/20/2019 at 7:05 AM.
The Bottom Line
Wind slabs exist in many areas of our terrain. You’ll likely find the largest of these wind slabs behind and below steep terrain features in east facing bowls and smaller slabs in sheltered areas of gullies on most aspects. A human-triggered avalanche yesterday in the Lower Snowfields (see video below or observations) is a reminder that these wind slabs are recently formed and may still be reactive. MODERATE avalanche danger exists throughout the range where wind loading has occurred due to the potential for a skier or climber to trigger a small to medium sized avalanche. There is the possibility that the weight of moving avalanche debris or a person loading just the right spot could trigger a larger avalanche in an older and more stubborn wind slab above the Feb. 8 crust. Wind scouring has removed surface snow and exposed the ice crust and old, stubborn wind slabs in many higher elevation areas and reduced avalanche hazard to LOW in places like the northern gullies in Huntington Ravine.
The 3” of light density new snow that fell Monday continued to be blown by strong WNW winds yesterday while temperatures remained in in the negative teens. Wind will diminish significantly through today to the 20-35 mph range with the temperature warming to 14F on the summit. Cloud cover will increase this afternoon ahead of another storm which should drop 3-6” of new snow by tomorrow with cold air and more upslope enhanced snow arriving afterwards.
Primary Avalanche Problem – Wind Slab
Wind slabs formed in the past 24-48 hours are the primary avalanche concern. These slabs have demonstrated a tendency to crack and fail in stability tests as well as in a recently reported human-triggered avalanche. Poor snowpack structure exists in many places with firmer wind slab over softer snow along with cold temperatures that slowed the bonding process. The reactive wind slabs that formed most recently will be difficult to distinguish from older, more stubborn wind slabs so evaluate terrain and snowpack carefully before committing to a slope.
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 useful rule of thumb for evaluating wind slab size holds that 1” of new snow can grow into a 3-5” slab. The 3” of new snow that fell Monday confirmed the accuracy of this rule of thumb. Tonight’s 3-6” snowfall and the upslope snow to follow will likely confirm its accuracy as well. The higher wind speeds and large flat expanse above our large ravines creates excellent storage for new snow and, combined with the legendary Mount Washington winds, often allows us to exceed the 1:3-5 ratio of new snow to wind slab, especially in Tucks and Gulf of Slides. Unfortunately, our best skiing opportunities at mid and higher elevations frequently exist in this wind driven snow. Copious snowfall seems to be the norm this season and our alpine areas and avalanche paths are well filled in and smoothed out. Wind slabs are likely to continue to be a problem and the only reliable solution is to give this avalanche problem type a couple of days to heal and bond to layers beneath.
The Sherburne and Gulf of Slides ski trails are snow covered to Pinkham Notch. Please avoid walking on these ski trails without snowshoes or skis. The holes punched in the snow can persist for days and create a hazard to others.
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 02/20/2019 at 7:05 AM.
USDA Forest Service
White Mountain National Forest