Avalanche Forecast for Tuesday, November 27, 2018
This information was published 11/27/2018 at 6:50 AM.
The Bottom Line
New snow and wind will continue today, making natural avalanches possible. Dangerous avalanche conditions exist on and below steep slopes. Avalanche danger due to these wind slabs will remain elevated today and tomorrow as snow continues to fall and is redistributed by the wind. The new snow fell on a slick bed surface of refrozen snow created by recent warm temperatures and rain. This surface is likely to make wind and storm slabs even more sensitive to human triggering. All forecast areas have CONSIDERABLE avalanche danger. Today seems like a good day to visit your local ski area, glade or ski trail and give the storm a chance to clear out and the new snow to settle and bond.
The Presidential Range is receiving yet another early winter storm. Overnight temperatures in the teens plus orographic effect at higher elevations has made more lower density snow on the summits than in the adjacent valleys. As of midnight, the summit of Mount Washington has recorded .65″ of water for somewhere around 8″ of snow. Hermit Lake reported 31cm of 10% snow at 6:30am. 8-14″ more snow is in the forecast for today on the summit. Lower elevation locations also received a sizable amount of snow. In addition to significant amounts of new snow, wind in the 50-60 mph range blew from the east and east-southeast through the night but will diminish slowly through the day. Lower elevations may see some rain on the new snow which could make wind and storm slabs weaker and more prone to failure though areas above 1500′ should see all snow.
Primary Avalanche Problem – Wind Slab
Large wind slabs developed on west aspects and behind terrain features as strong easterly winds blew last night. Be on the lookout for cross-loaded slopes as well as softer and more reactive wind slabs as winds diminish today.
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
Given the strong winds recorded last night, it seems unlikely that you’ll find much snow that wasn’t affected by wind. If you do, remember the refrozen crust that you may find below will make for an effective sliding surface for an avalanche. Even a short ride could carry you into rocks or trees or into a terrain trap.
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.
Low visibility and continued snowfall will hamper safe observations today but it is highly likely that west facing avalanche paths grew in size and length overnight. While we still have an early season snowpack, there has certainly been enough snow and avalanche activity to create bed surfaces that are almost full length. That said, trees and bushes remaining in paths will make any ride more dangerous. High east facing terrain likely received a good hammering by the wind and may even have been scoured down to old surface in a few places. Look for smaller wind slabs scattered around the usual slopes and gullies rather than the thick wind slabs that are normally built by our more typical west wind. Continued snowfall and wind will combine with an unusual wind pattern will create a tricky avalanche problem today. Keeping it low angle until visibility improves and wind slabs heal is probably a good idea.
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 11/27/2018 at 6:50 AM.
Frank Carus, Lead Snow Ranger
USDA Forest Service
White Mountain National Forest