Avalanche Forecast for Sunday, March 10, 2019
This information was published 03/10/2019 at 7:20 AM.
The Bottom Line
Snow falling today will combine with wind to create new unstable slabs and increasing avalanche danger through today and tonight. Wind from the south today will load terrain on the northern half of the compass rose. After dark, wind shifting to the westwill transport snow from our largest fetch zones into easterly terrain and continue to enlarge new slabs. Watch for quickly developing wind slabs today and realize that the size of new wind slabs depends on the amount of snow we actually receive.. Avalanche danger will rise to MODERATE today as human triggered avalanches in these growing slabs become possible. If you’re in the mountains late today, be aware that a wind shift to W and increasing speeds may result in avalanche danger reaching CONSIDERABLE due to the increased chance of natural avalanches in wind loaded terrain.
Temperatures slowly climbed to the upper teens F on the summit yesterday under clear skies and decreasing NW wind. New snow today should total 2-4 inches and fall on S wind that will increase this morning from under 30 mph to the 50-60 mph range. Snowfall may taper slightly after dark as wind ultimately shifts W and increases. A period of mixed precipitation, particularly sleet, may occur tonight though forecasts are trending towards all snow and totaling at least another inch. Consistently W wind around 70 mph is forecast tomorrow and we may receive an additional trace to 2” of snow.
Primary Avalanche Problem – Wind Slab
New snow on increasing southerly wind will build new wind slabs though the day that will become possible to human trigger as they grow in size. Expect the largest wind slabs to develop on northerly aspects but watch for cross loadinging to also build slabs on easterly and westerly aspects. The wind shift to W is forecast to occur around midnight tonight, but if you’re out late today, watch for an earlier shift in wind and corresponding wind loading of easterly terrain from our large fetch zones. Additionally, a remote possibility does exist for an avalanche to entrain existing snow in isolated areas and contribute to overall size.
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.
Secondary Avalanche Problem – Dry Loose
Steep, wind sheltered terrain will be prone to dry loose sluffing as new snow accumulates today. While small in size, always remember that a small loose snow avalanche in the wrong place can knock you off your feet and be a big deal. Additionally, sluff tends to pile relatively low in terrain, especially below ice climbs, and ultimately helps to build cohesive slabs in these locations.
What is a Dry Loose Snow Avalanche?
Dry Loose avalanches are the release of dry unconsolidated snow and typically occur within layers of soft snow near the surface of the snowpack. These avalanches start at a point and entrain snow as they move downhill, forming a fan-shaped avalanche. Other names for loose-dry avalanches include point-release avalanches or sluffs.
The wind slabs that formed over the past week have finally had time to gain stability; today’s avalanche problem is primarily based on inbound snow and wind. In the past two days with many skiers and climbers in the terrain, just one human triggered avalanche has been reported. These older wind slabs do seem to have a firm over soft structure but have become largely unreactive to a human trigger. Today starts with low avalanche danger for most terrain. As new slabs build and avalanche danger increases today, it’s important to remember that today’s moderate avalanche danger rating is based fully on the forecast of incoming weather. Watch for snowfall totals and wind direction that differ from the forecast if you’re playing in or around avalanche terrain today.
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 03/10/2019 at 7:20 AM.
Ryan Matz, Snow Ranger
USDA Forest Service
White Mountain National Forest