Avalanche Forecast for Thursday, January 3, 2019
This information was published 01/03/2019 at 7:11 AM.
The Bottom Line
New snow and shifting wind will create touchy and widespread wind slab avalanche conditions today. As winds shift, very light density snow on the ground will continue to be picked up and loaded onto the weak layer of new snow that’s been falling since midnight. These avalanches are likely to be big enough to bury you in many locations. Approaching steep slopes or gullies from below would be a bad idea due to the touchy nature of the new slabs and the potential for an avalanche to release naturally. Avalanche danger will rise to CONSIDERABLE in all higher elevation forecast areas. Cautious route-finding is essential today. Lower elevation areas such as Webster slides and Mt. Willard may rise to MODERATE avalanche danger due to less new snow and wind loading.
Snow began falling at a moderate intensity around midnight last night and will continue steadily through the morning. As of 6am, 3.5” (9cm) of 6% density snow had fallen at Hermit Lake and Harvard Cabin. It seems that we are on track to meet or exceed the forecasted total snowfall figures which have ranged from two to six inches over the past 36 hours. The low pressure system responsible will be fast moving but the snow associated with it is very light density and will be easily blown into wind slabs by the 40-55 mph winds. The new snow came in on wind from the SSW but will be shifting to the NW through the day. While these wind speeds are fairly low velocity by Mount Washington standards, it should be noted that snow this light in density is already drifting in protected locations around Hermit Lake. Anticipate challenging visibility through the afternoon due to thick summit fog.
Primary Avalanche Problem – Wind Slab
Don’t be fooled by the relatively small amount of snow expected to fall today. Wind slabs will grow in size through the morning and into the afternoon and will be touchy. The light density snow will be easily loaded and cross-loaded into drifts and pillows but will also create a sensitive failure layer near the old firm wind slabs and icy old surface beneath. Anticipate the potential for a crack to propagate widely or wall to wall across a slope or gully.
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 are possible if you can find a wind sheltered spot, such as those at lower elevations, where a wind slab hasn’t already formed. Debris from a dry loose avalanche could serve as a trigger to a larger wind slab avalanche.
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.
Much of our avalanche terrain consists of firm wind slab sitting on older rain crust. New snow is falling on 4-5” of snow which was capped by sleet and freezing rain on Monday night. The snow beneath the sleet and freezing rain crust is dry and loose and could contribute to the total volume of snow in areas where an avalanche today breaks into that crust. Any step down avalanches will just be adding insult to injury since today’s snow and wind slabs are likely to be more than capable of carrying, burying or otherwise taking you somewhere you don’t want to go. Safe travel in avalanche terrain today will be challenging due to the active wind loading, low visibility and touchy nature of new slabs.
The Sherburne and Gulf of Slides ski trails are snow covered to Pinkham Notch. Ice patches, water bars and rocks are lurking, especially lower on the trails.
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 01/03/2019 at 7:11 AM.
USDA Forest Service
White Mountain National Forest