This information was published 12/16/2018 at 7:05 AM.
NOT THE CURRENT FORECAST
This is an archived avalanche forecast.
The Bottom Line
Areas of somewhat soft snow in the alpine, that skiers and riders will naturally be drawn to, are generally soft wind slab and are the same locations where it will be possible to trigger an avalanche today. There are a number of snow surfaces and layers to keep an eye on, and it’s a time to continue responsible travel in avalanche terrain. Bring beacon, shovel, and probe along with your brain and a good partner; travel one at a time, and evaluate snow and terrain carefully. Careful terrain selection today can be used to manage the primary avalanche problem today. All forecast areas have MODERATE avalanche danger today with exceptions being the Right side of Tuckerman Ravine and the Northern gullies in Huntington Ravine which are rated LOW.
It’s currently a few degrees below freezing on the summit of Mt. Washington, with a temperature inversion keeping lower elevations colder this morning. It looks as though the inversion will lift and allow temperatures to rise to around the freezing mark in much of our terrain today, with cloud cover forecast to slowly increase through the day and into the night. The current N wind, under 20 mph on the summit, may increase slightly while shifting through E to SE as a weather system approaches. We should receive some new snow, though measurable accumulation is not expected before this avalanche forecast expires at midnight tonight. Snow totals by tomorrow night could be several inches or more.
Primary Avalanche Problem – Wind Slab
Relatively soft areas of wind slab which formed late last week remain possible to human trigger and should be your primary avalanche concern today. Firmer, generally larger, and more stubborn wind slab also exists in our terrain and should be respected though are less likely to be human triggered. Both of these types of wind slabs can be found on the eastern half of the compass rose in alpine terrain. We’re on the cusp of temperatures and sun combining to warm these existing slabs and decreasing stability, so be suspect if you encounter snow that has been affected by warming.
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.
The Presidential range currently holds a variable and somewhat complex snowpack. Areas of recently formed and relatively soft wind slab alternate with firmer wind slab that is generally older.The December 3rd melt freeze crust is also present at the surface along with a few areas of more recent sun crust. The softer wind slabs tend to be both more reactive and smaller, with some avalanche activity since late Friday, and the firmer wind slabs which are mostly older are more stubborn and generally larger. It’s worth noting that facet development around the December 3rd crust seems to be occurring. More developed and widespread facets have been observed at low elevation gullies in the Crawford Notch area, which may create a greater stability concern if they persist until a significant storm loads our low elevation terrain.
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 12/16/2018 at 7:05 AM.
Ryan Matz, Snow Ranger
USDA Forest Service
White Mountain National Forest
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