Avalanche Forecast for Saturday, February 16, 2019
This information was published 02/16/2019 at 7:03 AM.
The Bottom Line
New wind slabs that you could trigger on skis or on foot can be found on many aspects today. Shifting winds have created these widespread wind slabs in our middle and upper elevation terrain.The new wind slabs vary in size and sensitivity to triggering due to these changes in wind speed and direction, with large and reactive slabs likely in specific areas. If you travel in avalanche terrain today, be ready to make continual assessments of the upper snowpack to guide your travel decisions. All forecast areas have CONSIDERABLE avalanche danger. Natural avalanches remain possible in areas that receive further wind loading today. Low elevations which received rain and a refreeze last night are the exception and lack today’s avalanche problem.
Snow yesterday totaled 5” at our snow plots. An afternoon changeover to rain occurred below about 3500’ in elevation where temperatures were above freezing. Southerly wind held between 40 and 50 mph with stronger gusts until a shift to W wind yesterday evening. Several hours of W wind near 60 mph ultimately diminished to the current 45 mph, which is forecast to shift NW and increase this afternoon. Temperatures are currently below freezing throughout our terrain and should remain so through the day, with a trace to 1” of snow forecast. Mostly clear skies, no precipitation, and decreasing wind are forecast for tomorrow.
Primary Avalanche Problem – Wind Slab
South wind yesterday shifted west overnight, transporting the 5” of new snow and building wind slabs on a wide range of aspects. The wind will shift again to the NW and move more snow on the ground into avalanche starting zones. These wind shifts make wind slab the primary avalanche problem on all but SW and W aspects. Be on the lookout for soft and reactive wind slabs on most middle and upper elevation aspects with a fair amount of spatial variability due to these shifting winds. Layers formed earlier this week beneath the new wind slab could be pulled into moving avalanche debris and contribute to the size of a slide today.
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.
Five inches of new snow yesterday brings our total since Wednesday to nearly 20” for middle and upper elevations. This snow was affected by alternating periods of strong westerly and southerly wind, both in the past 24 hours and in the days prior. The resulting layers of wind slabs forming from this varied wind direction lie on a hard and slick rain crust, with some areas scoured to the crust. We expect slabs formed prior to yesterday to be stubborn though potentially able to contribute to size of an avalanche today, with slabs formed since yesterday being reactive to a human trigger. Low elevations are a different story, with rain and a subsequent refreeze last night.
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 02/16/2019 at 7:03 AM.
Ryan Matz, Snow Ranger
USDA Forest Service
White Mountain National Forest