This information was published 12/31/2018 at 7:16 AM.
NOT THE CURRENT FORECAST
This is an archived avalanche forecast.
The Bottom Line
All forecast areas have LOW avalanche danger today. Areas of wind slab can be found, but these have proven unreactive and can be avoided in most terrain by traveling on the exposed melt-freeze crust. The potential for long, sliding falls should be as much or more of a concern today. Bear in mind that crampons and ice axes are tools for fall prevention and will be of little use after a slip due to the firm nature of the current snowpack.
Tonight’s inbound storm may produce significant snowfall on increasing wind before this forecast expires at midnight. Avalanche danger will exceed our current low rating once this takes place. Watch for the red flags of heavy snowfall and wind blown snow to guide cautious route-finding if you are out late today.
On Sunday, temperatures remained in the single digits with broken and overcast cloud cover for most of the day. Shifty wind in the morning with speeds less than 20 mph became NW around 40mph in the evening. A trace of snow fell in the morning followed by 0.4” overnight. Today, summits will be in and out of the clouds with temperatures increasing to the teens F by dusk. Wind will remain in the 30-50 mph range from the W and SW for daylight hours with up to an inch of snow falling during the day. Once nighttime arrives, the storm will begin with up to 5” falling by midnight on shifting and increasing wind. Snow continues into tomorrow morning with high wind speeds, bringing a storm total of up to 12” of snow by the end of Tuesday. Warm temperatures may mix in sleet at high elevations with rain likely down low.
Primary Avalanche Problem – Wind Slab
Wind slabs formed during a period of extreme NW wind on Saturday in the lee of terrain features. Areas in the lee of our largest fetch contain the largest and thickest wind slab, but in most of our terrain they are isolated and can be avoided. These firm wind slabs have been unreactive to human triggers and appear well bonded to the melt-freeze crust.
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.
Currently, our snowpack has very good stability. A robust melt-freeze crust that formed December 22 and 23 exists on all aspects and elevations with recently formed, firm wind slab in isolated locations. The melt-freeze crust offers excellent climbing as well as providing the opportunity for long sliding falls. This surface snow will also provide a smooth bed surface for avalanches to occur as snow arrives tonight into tomorrow.
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 12/31/2018 at 7:16 AM.
USDA Forest Service
White Mountain National Forest
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