Avalanche Forecast for Wednesday, February 13, 2019
This information was published 02/13/2019 at 7:02 AM.
The Bottom Line
Dense new snow and growing wind slabs will make natural avalanches possible on steep slopes throughout the Presidential Range today. Human-triggered avalanches are likely and will be large enough to bury and kill a person on open slopes and gullies. More snow will fall through the day as the wind shifts west and increases in speed. Existing wind slabs will increase in size as this occurs and make natural avalanches likely in larger east facing terrain such as the Headwall area of Tuckerman Ravine and possibly the Gulf of Slides. Those areas, which have more snow available upwind, will have HIGH avalanche danger. Today is a good day to avoid recreating on or beneath steep slopes over 30 degrees. Areas beneath larger east facing slopes like the Bowl in Tuckerman Ravine or the terrain traps below slopes in the Gulf of Slides are best avoided today.
The summit Observatory weather station is reporting .63” snow water equivalent so far with a SE wind overnight blowing 50-60 mph with gusts to 82 mph. Nine inches of snow has fallen at Hermit Lake with 8” reported at Gray Knob. Temperatures have slowly warmed on the summit from 5F when the snow started late yesterday afternoon to the current 12F. Temperatures aloft have warmed as well causing precipitation types to change over to sleet this morning. More snow is forecast today with SE winds continuing at 35-50 mph gusting to 60. Wind will shift to the west mid-day and continue to blow at speeds more than capable of blowing snow into east facing avalanche terrain. This west wind will ramp up through the evening and overnight ultimately shifting NW as it reaches the 55-75 mph range.
Primary Avalanche Problem – Wind Slab
Wind slabs will be a factor across many aspects today due to the shift in wind direction. If you are thinking of tangling with steep terrain today, don’t be fooled by what appear to be low wind speeds. Wind slabs developed on higher wind speeds overnight and the brief lull in wind speeds this morning won’t change the likelihood of a natural avalanche very much.
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
Dense snow over an icy bed surface make widespread storm slabs dangerous in most all avalanche terrain today. Due to the density of the snow, lower wind speeds in sheltered terrain may not be able to easily move the new snow. Anywhere that this new snow exists on a slope over 30 degrees will likely be easy to trigger. Consider the size of slopes above and the terrain traps and cliffs that may be below you.
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.
The icy bed surface beneath the new snow and sleet that are creating today’s avalanche problem is a strong factor to include in your risk assessment today. This icy bed surface adds complexity to today’s travel plan and would make any avalanche easy to trigger, potentially wider or wall to wall in a gully, and really hard to escape. The steepest terrain will likely sluff and avalanche on the icy bed surface with some regularity today before slabs begin to adhere, bridge across terrain, and then grow larger in the afternoon. My money is on a large natural avalanche across the Bowl of Tuckerman Ravine tonight or early tomorrow. Anticipate a similar danger rating in mid-elevations tomorrow as wind ramps up and upslope snow continues to fall through today.
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 02/13/2019 at 7:02 AM.
USDA Forest Service
White Mountain National Forest