This information was published 01/09/2019 at 7:06 AM.
NOT THE CURRENT FORECAST
This is an archived avalanche forecast.
The Bottom Line
Heavy snow with growing wind speeds are the clear indicators of today’s avalanche problem. Unfortunately, low visibility will make direct observation of this growing avalanche problem difficult. Lower angled terrain may provide excellent riding conditions but avoiding the runouts of avalanche paths will be key for safe travel today, especially as wind picks up this afternoon. Periods of heavy snow today, combined with snow on the ground from yesterday, will provide plenty of snow for avalanches that will be large enough to easily bury a person. The newly formed slabs will likely be touchy as well. Avalanche danger will rise to CONSIDERABLE today, especially where wind slabs build. Lower elevation, wind sheltered terrain will have MODERATE avalanche danger with smaller human-triggered avalanches still possible in steep terrain.
A few inches (7cm) of snow fell on Mount Washington yesterday with a couple inches more in the mountains south of the notches. Precipitation type remained snow at higher elevations with mixed precip and rain in the valleys and was accompanied by light wind from the south through west. The second low pressure system passing today will deepen offshore and create bands of snow which will bring up to 8” of snow to our forecast area by late afternoon with more falling into the evening. Temperatures will fall through the day as wind slowly ramps up and shifts to the northwest and reaches strong loading speeds in the afternoon. All the warm moist air in the region will keep conditions ripe for thick fog as well as continued upslope snow showers through the night and into tomorrow.
Primary Avalanche Problem – Wind Slab
Wind slab avalanches will grow in size as well as in likelihood of triggering today as new snow is loaded into slopes and gullies, generally on the east side of the range. Crossloading of other aspects will also be likely. Natural avalanches will become increasingly likely later in the day.
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 – Dry Loose
Dry Loose avalanches, a.k.a. sluffs, could be a problem in steep terrain and may help trigger a larger wind slab avalanche where these exist from previous wind loading and sluffing. Examples of this would be in the middle of Sluice and below Headwall ice in Tuckerman Ravine.
What is a Dry Loose Snow Avalanche?
Dry Loose avalanches are the release of dry unconsolidated snow and typically occur within layers of soft snow near the surface of the snowpack. These avalanches start at a point and entrain snow as they move downhill, forming a fan-shaped avalanche. Other names for loose-dry avalanches include point-release avalanches or sluffs.
The 3” of new snow which fell in the past 24 hours was barely affected by wind yesterday. That new snow fell on mostly stubborn wind slabs and some exposed ice crust in more wind hammered areas. The more dynamic winter weather at higher elevations over the past week has made for a more varied and dynamic upper snowpack. Most of the instabilities prior to today are stubborn wind slabs built over the weekend and while stubborn and primarily glued to underlying crusts, a heavy load today could break a larger slab loose.
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 01/09/2019 at 7:06 AM.
USDA Forest Service
White Mountain National Forest
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