Avalanche Forecast for Thursday, February 6, 2020
This information was published 02/06/2020 at 7:12 AM.
The Bottom Line
New snowfall today will become increasingly dense and possibly turn to sleet or freezing rain for a short period this afternoon. Natural avalanches capable of burying a person may occur starting this afternoon in larger avalanche paths with smaller natural avalanches likely to occur on and below the steepest slopes through the day. CONSIDERABLE avalanche danger demands cautious route-finding and a keen awareness of your position and the location of avalanche paths. The safest bet today, and tomorrow especially, will be to stay below treeline and enjoy the fresh snow on local ski trails. Sluffing on gully climbs may be continuous through the day and could lead to easy-to-trigger slabs.
An Avalanche Watch will likely be issued later today and continue through Friday at midnight. New snow and extreme wind will create very dangerous avalanche conditions tomorrow. Timing remains uncertain due to the uncertain amount of snowfall and the timing of the wind shift to the west. Avalanche watches are issued when avalanche warning criteria may be met, meaning that the danger rating is expected to reach High in many areas.
Yesterday: An inch of new snow fell yesterday morning with cold and dry conditions through the rest of the day. Two more inches (5cm) of new snow had fallen by 5:30am this morning with light snowfall reported.
Today: A period of intense snowfall this morning should taper by midday and turn into snow showers before turning to mixed precipitation for a short period. The summit temperature this morning is 11F but will warm significantly through the day, reaching the upper 20sF. Visibility will be very poor today with summit fog and snow most of the day. Wind is a relatively light 30 mph from the south on the summit this morning and should increase a bit midday.
Tomorrow: The second of two low pressure systems will impact the Presidential Range and bring heavier snowfall and much higher wind speeds than today. Wind will shift to the west and move all of the recent snowfall into east facing terrain. The timing of the wind shift, the density of the new precipitation and the degree to which an ice crust will develop are all factors in the rate and timing of our slopes being loaded with this new snow. Expect low visibility to continue.
Primary Avalanche Problem – Storm Slab
Storm slabs may be a problem today. Though total snowfall seems on track to deposit only around 6” of snow today, the increasing density of the snow combined with widely variable bed surfaces with a range of adhesion should keep the threat of natural avalanches on your radar even if wind loading does not occur. The steepest terrain will shed snow through sluffing action and builds slabs likely to be sensitive to human triggering and may release as snow loads and stresses the low density layer of snow beneath.
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.
Secondary Avalanche Problem – Wind Slab
Remember that storm and wind slabs almost always occur together during any storm. Though today will not present the usual large and hard slabs typical of our windy range, some wind effect is likely to occur even with the southerly wind direction. Consider the winds ability to build thicker slabs in upper start zones, particularly those with a more northerly aspect. Wind affected snow often becomes more cohesive, making it “slab up” and carry a crack.
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.
Forecast snowfall amount and timing of wind speed and direction shift dominated the forecast discussion this morning as we tried to pin down the size of unstable slabs and their distribution through our forecast area. At issue was the danger rating and whether widespread natural avalanches were possible and if so, when. The hard reality is that when discussion begins splitting hairs about D1.5 vs D2 and whether instabilities will be touchy or just reactive, there is a really good chance that conditions will be really heads up. The weather forecast did not scream out a clear message of large natural avalanches as it often does when wind rages so we are left with the danger rating of 3 out of 5. Considering the fact that we rarely if ever reach 5 or Extreme, you may as well think about the danger rating as reaching 3 out 4 today. Solid partners, clear and honest discussion and zero bias affecting your decision making will be more important than the avalanche rescue gear that you carry in your pack.
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 02/06/2020 at 7:12 AM.
Frank Carus, Lead Snow Ranger
USDA Forest Service
White Mountain National Forest