Avalanche Forecast for Saturday, January 25, 2020
This information was published 01/25/2020 at 6:59 AM.
The Bottom Line
Avalanche concerns today lie within the wind slabs that formed between January 16 and 19. It is possible that a human could trigger an avalanche today as warm air continues to weaken the slab that has so far shown good stability despite a poor structure. Identifying features of concern (rollovers, unsupported wind slabs, edges and thin spots of the wind slab) and avoiding these and traveling one-at-a-time should provide safer travel. As the snow moistens, skier-induced sluff may entrain this snow, possibly greatly increasing the load on a wind slab and acting as the trigger for a weak spot. Avalanche danger is MODERATE today.
Much of the Presidential Range saw temperatures crest the freezing mark yesterday. Mild wind by local standards and partial cloud cover persisted through the day. A band of warm air at our mid-elevations has kept temperatures around 4000’ above freezing last night. Lower elevations will likely transition to above freezing today. Precipitation is forecast to start around sunset. A mix of snow, sleet, and freezing rain is forecast, with low confidence about where the rain/snow line will be. Expect heavy precipitation tonight that should taper off tomorrow morning.
Primary Avalanche Problem – Wind Slab
Wind slab bridging strength has been weakened by temperatures hovering around the freezing mark. Warming has penetrated some of the slab, but not far enough or long enough to increase stability. Continued warming today will lead to further weakening of this slab. Spatial variability of the depth of the slab makes this assessment much more difficult. Preplanning with your group to recognize the most likely places to trigger these wind slabs should aid in making safe travel easier and smoother.
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.
How long does a wind slab with an upside down structure have to persist before it changes into a persistent avalanche problem? Can this even happen or does it remain a wind slab that we continue to talk about LPHC (low probability, high consequence) avalanches being unlikely, but not impossible? We’re starting to feel this way about the current avalanche problem. While we don’t have a Bruce Tremper approved answer, the discussion surrounding the problem itself is the same. The melt/freeze crust that formed January 13 is a widespread bed surface. Wind slabs are sitting on this that exhibit an upside down structure, fair strength, and the potential to propagate. Classic Mount Washington wind created bridging strength in this slab that has left us with no recent avalanche activity. It’s a wind slab, but certainly displaying characteristics of a persistent slab though it is neither widespread nor producing any avalanches. Either way, current weather is weakening the slab that is providing bridging strength over a defined bed surface and weak layer. While we debate the name, the problem remains the same.
The Sherburne and Gulf of Slides ski trails are snow covered to Pinkham Notch.
Details on daily snowfall totals, precipitation type, total depth of snow and other information can be found on our page devoted to snow study plot data. Click here to check it out.
Recent snowpack and avalanche observations can be found here and on Instagram. Your observations help improve our forecast product. Please take a moment and submit a photo or two and a brief description of snow and avalanche information that you gather in the field.
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 01/25/2020 at 6:59 AM.
USDA Forest Service
White Mountain National Forest