Avalanche Forecast for Friday, December 28, 2018
This information was published 12/28/2018 at 7:18 AM.
The Bottom Line
Expect freshly forming wind slabs to be touchy to human triggers today. As new snow and wind drifts accumulate through the morning, natural avalanches will become likely, and may become more likely as snow turns to sleet and rain this afternoon. Avalanche danger may rise to CONSIDERABLE today in all mid and upper elevation forecast areas not facing directly southwest.
You can avoid these avalanches by staying on low angle or lower elevation terrain that does not have deep new snow or wind drifted snow. Travelling in longer avalanche paths will expose you to the risk of natural avalanches from above. Gusty and erratic wind that loads slopes with more snow could trigger these avalanches unexpectedly.
Light snow began falling around 2am this morning and has picked up in intensity with sleet mixing in already, despite the ground level air temperature of 7F. Snow will continue through the morning on westerly then southwesterly wind around 60 mph. So far this morning, wind has been gusty and a bit erratic. Sleet will likely continue to mix in this morning, adding weight to the forming wind slabs, before a change over to freezing rain and rain later in the day. Precipitation will switch back to snow tonight with upslope snow showers delivering another couple of inches tomorrow.
Primary Avalanche Problem – Wind Slab
Small to medium sized wind slab avalanches will be able gather enough snow to be dangerous, particularly considering the icy bed surface beneath. These wind slabs will grow increasingly unstable through the day as new mixed precipitation types falling on the snow adds stress to the weak snow near the icy bed surface.
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.
Air and upper snowpack temperature recordings this morning are not favorable for new snow to bond to the existing, icy snow surface. This hard rain crust is widespread through our terrain and was the result of the 3” rain event on December 21-22. Despite losing almost 25cm of snow during that event, our avalanche paths are still well developed but with rocks, boulders, and ice cliffs now showing again. Given the glazed over snowpack and cold temperatures associated with this morning’s new snow, it would be wise to view the exposed boulders, bushes and cliffs in our avalanche paths as potential obstacles and not as anchors. Today’s avalanche problem is more of an angry badger backed into a corner than a burly grizzly bear.
The Sherburne and Gulf of Slides ski trails have patches or water ice and some recently exposed rocks mixed with the refrozen rain crust. New snow will obscure these obstacles.
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 12/28/2018 at 7:18 AM.
Frank Carus, Lead Snow Ranger
USDA Forest Service
White Mountain National Forest