This information was published 03/02/2019 at 7:06 AM.
NOT THE CURRENT FORECAST
This is an archived avalanche forecast.
The Bottom Line
Wind slabs formed in the past week have become unlikely but not impossible to human trigger. Time has helped strengthen bonds between layers in our upper snowpack, resulting in LOWavalanche danger for all forecast areas. Low does not mean no avalanche danger, and practicing good habits today will help protect against the unlikely but potentially high consequence possibility of an avalanche. These good habits should include assessing consequences of an avalanche wherever you travel, traveling one at a time from safe zone to safe zone, and bringing your beacon, probe, shovel, and ability to use them. This will also help habits become more ingrained for days of elevated avalanche danger. It’s also a good time to respect the potential for a long sliding fall on the hard snow surface, making crampons, ice axe, and an ability to not fall requisite for steep terrain. Those who come into the mountains with good preparation should enjoy some fun snow conditions and mild weather today.
Moderate wind speeds, mostly clear skies, and temperatures approaching 10F on the summit made for a pleasant day yesterday. Today is a little warmer, with the summits and our terrain currently in the lower to mid teens F and which is forecast to rise a few degrees. Clouds will move in some time this morning as skies become mostly cloudy. Wind should be light on the summits. A chance at new snow tonight, totaling a trace to 2”, will combine with a slight increase in NW wind. Partial clouds, westerly wind around 30 mph, and a lull in precipitation tomorrow should give way to significant snowfall late Sunday night into Monday.
Primary Avalanche Problem – Wind Slab
Snow early last week fell on sustained westerly wind over 100 mph, creating hard wind slabs that are large and specifically located on the eastern half of the compass rose. These wind speeds always create weird wind patterns, and you’ll find smaller pockets of wind slab in other areas as well. Both large and small areas of wind slab have become unlikely to human trigger. Remember that unlikely does not mean impossible and use normal caution if you travel in avalanche terrain 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.
In the past week the Presidential Range has received approximately a foot of snow. Most of this snow fell on Monday with sustained westerly wind well over 100 mph. These wind speeds typically result in widespread scouring and isolated pockets of hard wind slab, but they are typically not accompanied by the simultaneous snowfall which occurred on Monday. This combination of extreme wind and significant new snow loaded easterly terrain with mostly hard wind slabs that are now stubborn to unreactive but large. Greater scouring occurred on the west side where a far less developed snowpack still exists. Beneath the snow of the past week, older wind slabs seem generally well bonded to the February 8th melt/freeze crust. In some easterly terrain, wind loading of the almost 1” of snow that fell early Thursday has produced a softer but shallow surface snow that is generally not an avalanche problem but which makes for good turns.
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 03/02/2019 at 7:06 AM.
Ryan Matz, Snow Ranger
USDA Forest Service
White Mountain National Forest
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