Avalanche Forecast for Tuesday, March 12, 2019
This information was published 03/12/2019 at 7:08 AM.
The Bottom Line
Wind in the 70-90 mph range from the west continues to affect our new snow and load terrain on the eastern half of the compass rose. The resulting firm wind slabs are possible to trigger in most of our terrain. East and Southeast facing terrain, like the Headwall of Tuckerman Ravine and to a lesser extent the Gulf of Slides, should have the largest new wind slabs and will receive the greatest wind loading today. Due to this continued loading, human triggered avalanches will be likely and natural avalanches will be possible, giving the Headwall of Tuckerman Ravine a CONSIDERABLE avalanche danger rating today. Other forecast areas have MODERATE avalanche danger, with LOW avalanche danger for the Northern Gullies in Huntington Ravine due to scouring of new snow.
Snowfall in the past 24 hours has totaled 4.4” on the summit, bringing the 48 hour total to just over 8”. Our snow study plots recorded nearly 4” of new snow yesterday as well. Sustained W wind of 70-90 mph yesterday has begun to shift NW, where it should remain through the day with little change in wind speed. Another trace to 2” of snow will fall today, predominantly this morning, as cloud cover decreases through the day. Summit high temperatures should be in the single digits F above zero, rising only slightly from the current -1F. Tomorrow brings decreasing wind speeds, temperatures rising to around 20F on the summit, and a slight chance of mixed precipitation in the evening or after dark.
Primary Avalanche Problem – Wind Slab
Wind slabs built on westerly wind since late Sunday are our primary avalanche problem. Given the extreme wind speeds, expect these slabs to generally be stubborn to a human trigger but deserving of respect due to being recently formed. East facing terrain in the lee of recent and current wind will hold the largest new slabs, with cross loading and terrain-driven variation in wind direction likely loading all aspects on the eastern half of the compass rose. Our recent wind speeds tend to scour upper start zones and build slabs slightly lower in our avalanche paths. Significant graupel has been noted at snow plots, which tends to pool relatively low in avalanche paths where it may act as a weak layer.
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.
New snow and wind in the past 48 hours are the primary factors driving instability in our snowpack at this time. The snow initially fell on southerly wind, loading northerly terrain, before shifting W and increasing since late Sunday. Our recent and forecast wind speeds tend to build firm and stubborn wind slabs and also scour the most wind exposed lee terrain which would be loaded on lower wind speeds. The recency of formation and continued loading of these hard slabs means that though stubborn, they’re likely not well bonded to older layers. Our windward west facing terrain should be quite scoured at middle and upper elevations and generally lack this avalanche problem. Lower elevations have received less new snow, and while pockets of wind thicker wind slab may exist, this terrain also tends to lack today’s primary avalanche problem. The new slabs lie on a number of layers of older wind slab, formed predominantly last week, that seem to have become unreactive. We don’t expect an avalanche in the new wind slab to step down to these older layers, but it’s not impossible.
The Sherburne and Gulf of Slides ski trails are snow covered to Pinkham Notch. Don’t miss our final Avalanche Awareness presentation of the season, one week from today at Plymouth State University, details are on our events page.
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 03/12/2019 at 7:08 AM.
Ryan Matz, Snow Ranger
USDA Forest Service
White Mountain National Forest