This information was published 04/22/2020 at 7:14 AM.
NOT THE CURRENT FORECAST
This is an archived avalanche forecast.
The Bottom Line
Wind transport of new snow will continue into today, creating wind slabs in east facing terrain at ravine elevations and above. Wind slab formed quickly overnight and will continue to form today as NW wind transports snow onto steep slopes and near terrain features. Natural avalanches are also possible, especially in areas downwind of a large fetch. Careful snowpack evaluation is essential today as human triggered avalanches are likely. Route finding will be key to avoid the patchwork of wind slab hazards. The avalanche danger level is CONSIDERABLE. Long sliding falls are a possibility in wind scoured terrain, crampons and an ice axe should be on your list of gear when entering the mountains today.
Yesterday, clear skies in the morning eventually darkened and became overcast as precipitation began on the summit at 1:00 PM. Snow accumulated in the afternoon for a total of 3.8” at the summit and 5.5”at Harvard cabin. Wind shifted from the S to the W in the evening and gradually increased from the teens into the 50-70 mph range.
Today, snow has continued into this morning and is forecast to continue throughout the day, 1-3” is expected to accumulate. W wind will shift to the NW and increase from 55-75 to 70-90 mph, and gusts may reach the triple digits. Temperatures have fallen into the single digits F at the summit and will remain there.
Tomorrow, clouds will recede and make way for clear skies. Wind will remain from the NW and gradually decrease from 60-80 mph to 35-50 mph. Temperatures are forecast to be in the upper teens F.
Primary Avalanche Problem – Wind Slab
Yesterday’s snowfall combined with today’s early morning snow showers has accumulated 3.9 inches of new snow at the summit and 5.5” at Harvard cabin. From the beginning of the storm, wind speeds have ramped up from 35 mph to current speeds near 70 mph. As snow is transported from the windward side into the ravines, locations that will most likely form wind slabs will be below the steepest portions of slopes and in gullies that face south due to cross loading.
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.
Field time has been limited recently resulting in a gap in knowledge of how recent weather conditions have affected the avalanche terrain. With this degree of uncertainty, it is best to be on the cautious side while traveling in the Presidential Range. Visibility may be limited for the majority of the day and route finding above tree line could be difficult due to blowing snow. If traveling through avalanche terrain today remember to practice good travel techniques in order to mitigate risk, such as only exposing one person at a time to avalanche hazard, crossing avalanche paths higher up, and pausing at critical decision points.
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 04/22/2020 at 7:14 AM.
USDA Forest Service
White Mountain National Forest
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