Avalanche Forecast for Saturday, December 15, 2018
This information was published 12/15/2018 at 7:07 AM.
The Bottom Line
Our avalanche terrain, which harbors existing wind slab, will go through a period of warming today. For this reason and the period of instability it will bring to our snowpack, avalanche danger will be MODERATE for our forecast areas. This scenario of warming wind slabs has many nuances that can be hard to predict. While wind slabs are larger in size and distribution on east facing aspects, the greater amount of solar gain may have more of an impact on shallower wind slabs that reside on south aspects. Heightened avalanche conditions exist on specific terrain features today that can be avoided with good route planning and careful navigation.
Following a mostly clear Thursday, winter weather brought increasing wind speeds and snowfall to the summits on Friday. The summit received a total of 1.3” of snow with no new snow at Hermit Lake or Grey Knob. During hours of recorded snowfall, wind started at WSW just under 50 mph and increased to 60 mph shifting due west. Today, upslope snow showers may bring up to another inch of snow to higher elevations with skies clearing by afternoon. Temperatures increased from the high-teens F to 23F on the summit where they currently reside with elevations below 3300’ above freezing at 6am. Temperatures will increase today with a forecast high on Mount Washington of 29F. Wind speed will decrease gradually through the day.
Primary Avalanche Problem – Wind Slab
Warming wind slab is the avalanche problem today. This particular scenario has a great amount of uncertainty due to temperatures being close to the freezing mark, clearing that will allow solar gain but a short window of actual daylight due to being close to the Winter Solstice, and varying thickness of existing wind slab. While today’s warming will eventually lend stability to the overall snowpack, the first time wind slabs see warmth does decrease stability of the slab for a period of time. South and east facing aspects will be of greater concerns, but for differing reasons. South aspects will see the greatest amount of warming due to length of time in the sun while east facing slopes have the largest size and distribution of wind slab that can be affected.
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 surface of our snowpack is a mixture of melt-freeze crust and wind slab. Wind slabs range from recently formed to those formed over a week ago. Our snowpack has been very dynamic this season and today will add another change in conditions that will keep avalanche danger elevated. Keeping an eye on the weather today will be key to safely navigating avalanche terrain. Wind slabs that warm are notoriously hard to predict the behavior of, particularly with the various thickness and distribution we have currently in our terrain.
Ski trails in the Pinkham Notch area are snow covered with the exception of the rare waterbar that is only half frozen.
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 12/15/2018 at 7:07 AM.
USDA Forest Service
White Mountain National Forest