Avalanche Forecast for Saturday, December 29, 2018
This information was published 12/29/2018 at 7:11 AM.
The Bottom Line
Loose wet avalanches that you could trigger should be your primary concern on steep snow slopes this morning. These are most likely in softer areas of our currently wet snow. Realize that even these relatively small avalanches can be a big deal in high consequence terrain and choose your route accordingly. Dropping temperatures and a refreezing snowpack mean that wet avalanches will diminish as a problem through the day, but a trace to 2” of new snow forecast means that quickly developing wind slabs should be on your radar for later in the day. All forecast areas have MODERATE avalanche danger today for this variety of avalanche problems. Also, remember that once refrozen, our current snow surface will become quite hard, making effective use of crampons and ice axes necessary to prevent a long sliding fall in steep terrain.
Snowfall yesterday morning totaled just over 2” on the summit. Precipitation shifted to freezing rain and plain rain as temperatures pushed above freezing in the area. Approximately 0.5” of rain fell, and precipitation is now shifting back to snow as we return to below freezing conditions. Expect additional snow accumulations in the trace-2” range, with W wind becoming NW and remaining in the 60-80 mph range with stronger gusts. Temperatures will fall steadily towards 0F through tomorrow morning, wind will decrease significantly overnight, and we should ultimately see at least partial clearing tomorrow with no precipitation.
Primary Avalanche Problem – Wet Loose
Wet loose sluffs should be on your radar this morning for steep mid and upper elevation terrain. Potentially reactive to a human trigger, these relatively small avalanches are most likely in softer wet snow like the pockets of wind deposited snow from the past week. It’s worth keeping larger wet slab avalanches on your radar as well, which your weight and/or a wet sluff could trigger. These wet avalanche concerns will diminish through the day as our upper snowpack refreezes.
What is a Wet Loose Avalanche?
Wet Loose avalanches are the release of wet unconsolidated snow or slush. These avalanches typically occur within layers of wet snow near the surface of the snowpack, but they may quickly gouge into lower snowpack layers. Like Loose Dry Avalanches, they start at a point and entrain snow as they move downhill, forming a fan-shaped avalanche. Other names for loose-wet avalanches include point-release avalanches or sluffs. Loose Wet avalanches can trigger slab avalanches that break into deeper snow layers.
Secondary Avalanche Problem – Wind Slab
New snow and wind today may build pockets of new and reactive wind slab in alpine terrain on the eastern half of the compass rose. The size of these wind slabs will depend largely on snowfall totals, so be on the lookout for potentially dangerous wind slabs if we see the upper end of forecast snow accumulation.
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.
The current warm-up has been far less drastic than the significant rain storm of a week ago. While not significantly affecting snow coverage, the new liquid moisture in our upper snowpack will refreeze today and increase stability. Minimal new snow falling as this refreeze occurs may become small wind slabs, but also don’t be surprised if the extreme wind today ultimately does a disappearing act with this new snow in all but the most sheltered terrain. As of yesterday morning, our upper snowpack was a mixed bag of areas scoured to the hard refrozen surface and varying wind slabs which formed in the past week. The upper snowpack will become a mix of supportable and breakable crusts with the recent wetting and current refreeze.
The Sherburne and Gulf of Slides ski trails are snow covered to Pinkham Notch though recent melting has resulted in sections of marginal coverage.
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 12/29/2018 at 7:11 AM.
Ryan Matz, Snow Ranger
USDA Forest Service
White Mountain National Forest