This information was published 04/24/2020 at 7:03 AM.
NOT THE CURRENT FORECAST
This is an archived avalanche forecast.
The Bottom Line
Most of east-facing avalanche terrain holds wind slabs formed mid week. These sit on a firm surface that acted as a bed surface for a natural avalanche cycle within the past few days. Some paths have reloaded after the cycle, some did not avalanche, and some are now wind-scoured, providing an icy surface for sliding fall potential; a mix of surfaces exist that are discernible from a distance. Avalanches today have the potential to be large in size, easily capable of burying a person. The possible size of avalanches combined with the likelihood due to structure and possible warming drives today’s MODERATE avalanche danger. Identify features of concern and be sure to assess the stability of the slope you plan to use.
Yesterday, skies cleared, temperatures warmed into the teens F, and wind from the W decreased from 70mph to a current 20mph. Today, no snow is forecast. Wind from the W and NW will blow 10-25mph. Temperatures will increase into the 20sF on the summit, flirting with above freezing in many locations. Skies will be clearing. Tomorrow will be a repeat of today with warm temps, low wind, and clear skies.
Primary Avalanche Problem – Wind Slab
Wind slabs that formed over the past few days will warm today due to ambient temperature and also solar gain. This warming process will initially increase instability of the wind slab, perhaps putting the sensitivity of the wind slab on the reactive side of stubborn. Field time yesterday showed the wind slab has a definite upside-down structure, has weak strength and could propagate. Some slopes had avalanched sometime over the past 2 days with some slopes reloading and some remaining scoured with old surface visible. When conducting slope assessments, be wary of paths that have not avalanched as these are the places that could produce a very large avalanche. Exposing one person to investigate whether or not there is recent avalanche debris that is covered by blown-in snow will help answer this question.
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.
Despite the calendar saying it’s almost May, we are still in a winter mindset in the mountains. Check out this video for a visual of Huntington Ravine yesterday and what we found in the snowpack. Under today’s wind slab problem, there is about a 30cm layer of snow that has been warmed. In places where this 30cm layer is insulated by snow above, it is not completely refrozen and contains wet snow below a thin melt/freeze crust. Below this layer is dry winter snow. We are not at our isothermal snowpack yet, but it would take an unusual event to affect any layer below the 30cm thick wet layer. In places where scouring has occurred, exposing this wet layer to the elements, it has refrozen and this is the surface that would offer the potential of a long sliding fall.
The Sherburne and Gulf of Slides ski trails are snow covered to Pinkham Notch.
The Lion Head Winter Route remains the easiest route to the summit from Pinkham Notch but requires an ice axe, crampons (not just micro-spikes) and possibly a rope. This is a mountaineering route and requires solid skills for a safe, timely ascent.
Details on daily snowfall totals, precipitation type, total depth of snow and other information can be found on our page devoted to snow study plot data. Clickhere to check it out.
Recent snowpack and avalanche observations can be foundhere and on Instagram. Your observations help improve our forecast product. Please take a moment andsubmit a photo or two and a brief description of snow and avalanche information that you gather in the field.
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 04/24/2020 at 7:03 AM.
USDA Forest Service
White Mountain National Forest
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