Avalanche Forecast for Wednesday, December 12, 2018
This information was published 12/12/2018 at 7:05 AM.
NOT THE CURRENT FORECAST
This is an archived avalanche forecast.
The Bottom Line
New wind slabs will add complexity and danger to existing wind slabs in the steep, sheltered and primarily east facing aspects of Ravine elevations. Due to current wind speeds and low density snow, anticipate these new wind slabs to be reactive to human-triggering. Natural avalanches seem unlikely due to the small amount of new snow and subsequent smaller sized new wind slabs. Any avalanche activity today in the new snow could potentially step down into the other wind slab and create an avalanche that is larger than expected. For these reasons, avalanche danger today is MODERATE, primarily on the steep, easterly aspects at Ravine elevations or the highest peaks. Evaluate snow and terrain carefully to identify this problem before committing to a slope. Avalanche danger below 3,500’ is LOW with natural or human-triggered avalanches unlikely.
It is currently 3F on the summit of Mount Washington with 2 cm of very light snow reported so far at Hermit Lake and 1cm at Gray Knob. Cold, dry conditions since Sunday night have prevailed in the high country until moderate snow showers began this morning. It remains to be seen how much of the forecasted trace to two inches of snow we receive until weather clears by noon. Winds have blown from the northwest in the mid-30 mph range since snowfall began around midnight. Northwest and north winds in the 30 to 45 mph range, with gusts up to 65 mph, will continue to move snow through the day.
Primary Avalanche Problem – Wind Slab
New wind slabs, forming this morning, are the primary avalanche problem. It is possible for person to trigger these slabs today which will likely be sensitive due to the low density snow just beneath. Wind slabs which formed earlier this week are also a factor and should be carefully assessed as well. Triggering one or both of these layers could generate a larger than expected avalanche in isolated areas.
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.
No unintentional human-triggered avalanches have been reported in the past several days but a ski cut on a low consequence steep area in Tuckerman Ravine caused a 20-30 cm by 6-7 m slab to propagate and fail. No deeper instabilities or larger avalanches have been reported recently in the more widespread and much thicker, firm wind slabs that sit above the December 2nd melt freeze crust. The current problem are the wind slabs built December 9-10 as well as those developing this morning. Cold temperatures have slowed stabilization but no faceting has been reported or observed in the snowpack.
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 12/12/2018 at 7:05 AM.
USDA Forest Service
White Mountain National Forest
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