Avalanche Forecast for Thursday, November 29, 2018
This information was published 11/29/2018 at 7:09 AM.
The Bottom Line
Wind speeds are increasing and continuing to transport snow at ravine elevations. East facing terrain with available snow upwind, such as Gulf of Slides, Tuckerman Ravine and to a lesser extent, Huntington Ravine holds the greatest potential for enough loading today to raise concerns for avalanches to occur without a human trigger. The Central area of Huntington and the Headwall area of Tuckerman Ravine retain a CONSIDERABLE rating due to the potential for larger avalanches failing naturally as a result of continued wind loading. Wind slab avalanches pose a threat in other areas as well, though the MODERATE rating reflects the smaller size of potential avalanches on less developed slide paths. Evaluate snow and terrain carefully if venturing into these areas and consider the consequences of any avalanche in our steep and obstacle-filled, early season terrain.
The current temperature on the summit is 12F with a northwest wind blowing at 52mph. An inch or two of new snow fell in the past 24 hours with rime growing on surfaces above treeline. A north-northwest wind slowly ramped up in the past 24 hours from the 20-30mph up into the 40 and 50 mph range early this morning. The northwest wind will continue to ramp up some more, reaching the 50-70 mph range with higher gusts possible. Limited snow shower activity may contribute another trace to an inch of new snow. Temperatures will remain roughly where they are in the mid-teens now through the day.
Primary Avalanche Problem – Wind Slab
The most recent snowfall will be transported by northwest winds and deposited into wind slabs and further strain existing weak layers beneath. Plumes of snow at ridges or snow pouring down steep terrain are indicators that loading is continuing and should serve as a warning sign that the potential for natural avalanches is still present. You’ll find softer wind slabs in the terrain as well. These were built by lower wind speeds yesterday and the day prior by an east wind and are likely not thoroughly healed.
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 wind shift and increase is the big concern today. Another snowpack issue to keep on your radar while navigating the minefield of various wind slabs is the presence of a crust of refrozen snow beneath it all that is the result of the warmup on the 24th. Though field stability tests yielded easy failures in the new snow interfaces above this layer, the early facets beneath this crust are worth considering as a contributor in the volume of snow available to a running avalanche. Snow appeared to be bonding fairly well to the ice crust. Strange weather creates a strange snowpack and the snow and wind pattern of the past few days fits that description. Stay heads up out there and assume the worst in the snow pack until you find lots of evidence to the contrary.
Your observations are helpful to our goal of providing as much information as possible to our readers. They don’t have to be professional level, but if you have photos of snowpits, avalanche activity or even obscure slide paths, submit your observation here.
If you are a regular consumer of our avalanche forecast, you may have noticed the change of title from Advisory to Forecast. The change is mostly semantic and is meant to keep a consistent product throughout the country’s avalanche centers and the avalanche education world’s efforts.
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 11/29/2018 at 7:09 AM.
USDA Forest Service
White Mountain National Forest