Avalanche Forecast for Saturday, April 25, 2020
This information was published 04/25/2020 at 7:06 AM.
The Bottom Line
Snow will warm today. As this process occurs, wind slabs that sit in avalanche terrain may lose some strength, creating the possibility of human-triggered avalanches today. This is the second day of warming for much of the snow, so we expect snow to start with a sun crust that developed yesterday that should break up quickly from ambient air temp and solar gain and allow the warmth to penetrate into the dry snow. Be wary of slopes that did not avalanche within the past several days as these are the locations that could produce a large avalanche. Today’s avalanche danger rating is MODERATE.
Yesterday consisted of scattered clouds, calm wind from the north and warm temperatures. The summit maxed at 30F and Harvard Cabin hit 43F.
Today will be a repeat of yesterday with warm temperatures likely approaching the freezing mark on the summit, light wind, and sunny skies.
Tomorrow, clouds will build in the morning in preparation for nighttime snowstorm. Wind will be shifting and increasing through the day, largely from the E. Snow may be in the air during the day, but should amount to less than an inch. Once the sun disappears, the Presidential Range will see significant accumulation, possibly over 6” by Monday morning.
Primary Avalanche Problem – Wind Slab
Wind slabs that formed mid-to-late week will see their second day of warming today. Conditions will vary depending what aspect you are on and some northerly aspects may still contain dry surface snow that didn’t warm yesterday. Wind slabs sit on a roughly 30cm thick layer of snow that has gone through multiple melt/freeze cycles. This 30cm thick layer is frozen on the top creating a fast bed surface for avalanches or sliding falls while the rest of the bottom of this layer is actually wet snow sitting at 0C. Assess the slope you plan to travel from afar to determine if wind slabs are present. If present, find a representative safe spot to conduct your snowpack assessment, determining how deep the warmth is penetrating. If snow is getting wet and heavy with possible wet loose sluff or rollerballs developing, this is a sign the snow is becoming unstable and could produce a slab avalanche.
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.
If you have not seen, the WMNF closed many trailheads yesterday. The full list can be seen here. This does not affect the trails themselves, meaning we still expect to see people recreating in the mountains. Hopefully, this means the SAR world will be quiet, but it may also present some challenges. We have found travel in the Cutler River has become difficult as normal trails and popular off trail travel routes are not seeing hiker compaction of the snow, creating unusual conditions for travel. If you head out, be prepared for different trail conditions than normal, with possible deep snow and slow going.
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 04/25/2020 at 7:06 AM.
USDA Forest Service
White Mountain National Forest