Avalanche Forecast for Saturday, December 01, 2018
This information was published 12/01/2018 at 7:17 AM.
The Bottom Line
Our snowpack is trending towards stability, but it is still essential to carefully evaluate snow and terrain to identify features of concern. Human triggered avalanches remain possible, and while we don’t expect them to be particularly large in size, they could still be consequential given early season hazards in our terrain. Avalanche danger remains MODERATE today for most of our terrain. We expect Low rated areas to be less sensitive to a human trigger, but it’s very much a Low does not mean No avalanche danger day.
Light wind, clear skies at higher elevations, temperatures just over 20F, and no precipitation yesterday made for a very pleasant day in the Presidential range. Minimal solar warming on southerly facing terrain was likely the only weather factor affecting our snowpack. Today is forecast to be similar though with cloudier skies this morning. Stronger wind this morning, up to 40 mph out of the northwest, should shift W and decrease through the day. We don’t expect significant wind transport of snow on the ground. Another storm arrives tomorrow, likely bringing a substantial amount of mixed wintry precipitation to higher elevations and possibly some plain rain to lower elevations.
Primary Avalanche Problem – Wind Slab
Wind of varied direction and speed late this week transported a significant amount of snow and built varied wind slabs on all aspects of our alpine terrain. Expect to find small to large wind slabs of varying hardness at the surface. This spatial variability should be on your mind when assessing stability of any slope today.
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.
A wide variety of slabs formed both during and after the storm early this week comprise our upper snowpack. These slabs lie above a crust that preceded the snow, to which we’re seeing fairly good bonding. The most consistent weak layer that continues to produce easy to moderate stability test results, with clean and planar shears, is a density change that lies a few inches above the crust and within the new snow. This layer has been gaining strength this week, but is remarkably consistent across our alpine terrain. The varied wind slabs above it are not consistent and indicate that your stability tests will have limited application to terrain other than the location of your test pit. It’s worth noting that many low elevation areas, particularly along the 302 corridor, will not have the wind slab problem present in our alpine terrain. The snowpack is healing and trending towards stability, but don’t let your guard down.
Avalanche debris out of the Lip in Tuckerman Ravine, which likely occurred late Thursday. Failed in storm snow above the crust.
The Little Headwall remains open water and is not a viable ski option from Tuckerman Ravine to Hermit Lake.
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 12/01/2018 at 7:17 AM.
Ryan Matz, Snow Ranger
USDA Forest Service
White Mountain National Forest