This information was published 01/21/2019 at 7:11 AM.
NOT THE CURRENT FORECAST
This is an archived avalanche forecast.
The Bottom Line
Increasing wind today will create very dangerous avalanche conditions. Wind slabs will build from the abundant recent snow accumulation that are likely to avalanche naturally. The SE half of the compass rose is likely to produce the largest avalanches today, but wind from nearly every direction in the past 24 hours means that choosing terrain to avoid wind slab avalanche danger today will be all but impossible. Avalanche danger will increase to HIGH today. Travel in avalanche terrain is not recommended. The Southern gullies of Huntington Ravine, the Left side of Tuckerman Ravine, and elevations below 3500’ are the exceptions but remain at CONSIDERABLE avalanche danger. Terrain lacking wind affected snow, like low elevations, will hold storm slabs which you are likely to trigger.
The USDA Forest Service Mount Washington Avalanche Center has issued a backcountry avalanche warning for the Presidential Range. Avalanche warning criteria may also be met in other areas outside those forecast by the avalanche center. This avalanche watch does not apply to operating ski areas.
Avalanche danger will increase through Monday, January 21, 2019, with very dangerous avalanche conditions on and below slopes steeper than 30 degrees. Natural avalanches will be likely. Travel in avalanche terrain is not recommended.
Recent heavy snowfall will combine with steadily increasing wind to create unstable conditions likely to produce large avalanches. Wind from the NNW is forecast to increase from 40 mph to 80 mph by Monday evening, with higher gusts.
The recent snow storm totaled 13” at the Hermit Lake and Harvard Cabin snow plots and 12“ at Gray Knob. After a cold start to the day on Sunday that produced lighter density snow, temperatures reached 25F on the summit yesterday afternoon, which in combination with warm air aloft allowed for a period of denser snow accumulation. Wind from the SE around 45 mph on the summit shifted W then NW overnight, bottoming out at 20 mph with a current speed of 28 mph. Today’s key weather factor will be wind which is forecast to blow from the NNW and increase slowly to 70 mph with gusts to 90 mph by dark. We may receive another trace to one inch of snow accumulation as cloudy cover persists and temperatures will only rise a few degrees above the current -14.5F on the summit. Wind will continue to increase to sustained 100 mph speeds with stronger gusts by early tomorrow before dropping off slightly through the day. Skies should become clear with temperatures rebounding to the single digits above 0F by tomorrow afternoon.
Primary Avalanche Problem – Wind Slab
Wind from the NNW forecast to increase through the day will build large unstable slabs from the recent storm snow. Expect new slabs to form on the SE half of the compass rose and be touchy to a human trigger. Additionally, reactive and small to large in size slabs formed from yesterday’s wind direction (SE) make wind slabs a concern on all aspects and elevations above 3500’ today. These windward slopes may become scoured by this evening, but hold unstable slabs this morning.
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 – Storm Slab
Storm slab from our recent heavy snowfall exists anywhere in the terrain that you don’t find wind slab. Human triggered avalanches remain likely in this reactive layer. The storm snow has an upside down character, with more dense and cohesive snow over less dense snow. This more dense snow showed an ability to propagate a crack a relatively long distance yesterday and result in avalanches entraining all storm snow.
What is a Storm Slab Avalanche?
Storm Slab avalanches are the release of a cohesive layer (a slab) of new snow that breaks within new snow or on the old snow surface. Storm-slabs typically last between a few hours and few days (following snowfall). Storm-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 snow storm of the past two days has result in widespread unstable conditions for a variety of reasons. Wind predominantly from the SE as well as a variety of other directions yesterday built slabs in lee and cross loaded areas yesterday, which we expect to remain touchy to a human trigger this morning. Other terrain holds storm slab that is similarly touchy. Northwesterly wind today will increase and transport the abundant snow available in our fetch zones to build larger slabs than those already existing in the terrain. This wind slab that builds today will likely produce large, natural avalanches. The variety of surface slabs sit on generally smooth bed surfaces, including widespread hard wind slab, some softer snow, and areas of December 22 crust. A few areas may present potential for avalanches today to step down or entrain deeper layers of snow to increase overall size, but most terrain that will be loaded by today’s wind has limited potential in this regard. That said, avalanches today could be quite large from the wind loaded storm snow alone. Today is a great day to enjoy soft snow in lower angled terrain or at a ski area.
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 01/21/2019 at 7:11 AM.
Ryan Matz, Snow Ranger
USDA Forest Service
White Mountain National Forest
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