Avalanche Forecast for Monday, February 25, 2019
This information was published 02/25/2019 at 7:05 AM.
The Bottom Line
Snowfall and sustained extreme wind speeds today will continue to build slabs in terrain on the eastern half of the compass rose. If you are able to find the ability and motivation to battle the wind and cold into the alpine today, be sure to respect the possibility of a large natural avalanche from east facing terrain like the Headwall of Tuckerman Ravine and the Gulf of Slides. Remember that avalanche terrain includes runout zones like the floor of Tuckerman Ravine which may be threatened by natural avalanches from the steeper terrain above you. Today’s wind speeds will also scour significant wind exposed areas which will lack today’s avalanche problem as a result. Avalanche danger is CONSIDERABLE today, with the Northern Gullies of Huntington Ravine having MODERATE avalanche danger due to likely scouring. Human triggered avalanches will be possible where wind drifted snow accumulates on steep open terrain.
Snow accumulation yesterday totaled nearly 8” at our snow plots and 5” at the summit. Snowfall continues currently and another 4-8” may fall today and tonight at higher elevations. Wind, the key factor driving today’s avalanche problem, has been from the west at speeds near to or over 100 mph since before midnight last night. These wind speeds will continue and even increase on the higher summits while shifting NW today, with a slight decrease forecast for tomorrow. Temperatures should steadily drop from the current single digits above 0F to a summit low around -15F tomorrow morning.
Primary Avalanche Problem – Wind Slab
West and Northwest wind around 100 mph today couples with the snowfall of yesterday and today to build wind slabs in easterly terrain. We expect these wind speeds to build hard and stubborn wind slabs that could be quite large, with continued loading through the day making natural avalanches possible. This avalanche problem will be absent in windward terrain which will be scoured today. Elevations below 3500’ may also hold wind slab today, which you can expect to be somewhat softer and more reactive to a human trigger.
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.
The past week has brought a number of storms with moderate snowfall totals and varied wind directions, ultimately creating layers of stubborn wind slab over the February 8th melt/freeze crust. The current period of westerly wind around 100 mph, forecast to continue into tomorrow morning, should affect all snow that fell yesterday in our middle and upper elevations. These wind speeds tend to eventually scour a majority of our alpine terrain. Today is unique for a period of extreme wind, with continued loading from today’s snowfall keeping the potential for natural avalanches and our danger ratings elevated. Today’s new snow forecast is a combination of the usual upslope enhanced snow that we frequently get when a cold air mass moves in after a storm and streamers of moisture laden clouds from the Great Lakes area. This “lake effect” snow forecast comes with a fair degree of uncertainty and wide range of total amount forecast. If the new snow fails to materialize, our peak avalanche activity and hazard rating will pass by early on, leaving scoured areas and unreactive to stubborn wind slabs as the primary concern.
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 02/25/2019 at 7:05 AM.
Ryan Matz, Snow Ranger
USDA Forest Service
White Mountain National Forest