This information was published 12/14/2018 at 7:06 AM.
NOT THE CURRENT FORECAST
This is an archived avalanche forecast.
The Bottom Line
Areas of multi-layered wind slab can be found sitting on top of a melt-freeze crust. Safe travel in steep terrain will require evaluating snow and terrain carefully to identify features of concern. East-facing aspects keep a MODERATE rating today as they received more snow from direct loading and have seen less settlement. Areas with a LOW rating today are a mix of crusts, hard wind slab, and sun-affected snow that may harbor isolated pockets of unstable snow.
Snowfall was last recorded on the Presidential Range Wednesday morning. This snow that fell was relatively light density (6%) and arrived on an increasing, strong, NW wind. Thursday’s high pressure brought light wind from the south and increasing temperature along with clear skies. As the high departs today, clouds will develop along with increasing wind from the west and increasing temperatures. It looks as if the high summits will be cold enough to receive up to 2” of snow tonight with lower elevations seeing mixed precipitation or rain.
Primary Avalanche Problem – Wind Slab
Wind slab is the avalanche problem today, though it is not contained to snow that arrived on Tuesday night. Strong wind appears to have transported much of this snow into the woods, leaving older wind slabs on the radar today. While some of these formed more than a week ago, largely cold temperatures have slowed the bonding process. These firm wind slabs are stubborn to a trigger, though could break large when they do. Areas with a Moderate rating saw less scouring and have more of the soft wind slab that may be reactive to a skier or climber. Keep in mind that it may not be the first set of tracks on a slope that finds the trigger point.
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.
Much of the melt-freeze crust from December 2 and 3 is visible in avalanche terrain, particularly on the western half of the compass. Wind slab is prevalent on eastern aspects, such as the Headwall of Tuckerman and the main gullies in Gulf of Slides, with a mix of texture and hardness. Areas of softer wind slab have a much smoother appearance and present the bigger challenge to safe travel. These are present in the lee of terrain features as well as places that are down-wind of our fetch. Large areas of hard (Pencil hard) wind slab that offer minimal boot penetration can be found in shady aspects and have interfaces that fracture in a non-planar, irregular character. South-facing aspects received solar gain on Thursday and prior which has promoted settling in the weak layers.
Ski trails like the Gulf of Slides and Sherburne are skiable all the way to the Pinkham parking lot. Few rocks exist, though the occasional open waterbar is good reason to check your speed. Developing facets under the melt-freeze crust has created tough travel in places with deep post-holing. Travel speeds can get significantly slowed by this.
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 12/14/2018 at 7:06 AM.
USDA Forest Service
White Mountain National Forest
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