Avalanche Forecast for Saturday, January 12, 2019
This information was published 01/12/2019 at 7:01 AM.
The Bottom Line
Human triggered avalanches remain possible in the wind slabs formed late this week, but they have become stubborn and firm in most areas. You should continue to respect these hard slabs which can be tricky to assess due to their stubborn nature. The Presidential range and a majority of our forecast areas have MODERATE avalanche danger today. The Left Side and Boott Spur areas of Tuckerman Ravine as well as the Northern Gullies of Huntington Ravine have LOW avalanche danger. These Low rated areas may have pockets of unstable snow on isolated terrain features, so remember that Low does not mean no avalanche danger.
Sustained NW wind from 70-90 mph with gusts to near 100 has been the dominant weather factor affecting our snowpack in the past 36 hours. No significant precipitation was recorded over the same period. The current summit temperature of -20F will rise towards 0F by this evening, with wind slackening to under 50 mph. Partial and variable cloud cover is forecast with summit fog generally decreasing. Wind should continually decrease to 20 mph by tomorrow evening while remaining from the NW. Expect mostly clear skies and slightly warmer temperatures tomorrow.
Primary Avalanche Problem – Wind Slab
Wind slabs built from the recent extreme wind speeds are variable across our terrain and mostly located found on the eastern half of the compass rose. We generally expect these relatively new slabs to be quite firm and stubborn to a human trigger. Even across similar aspects and elevations, expect a great deal of spatial variability today. Avalanche activity, upwind fetch zones, and degree of scouring all vary across the Presidential range and have created this spatial variability.
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.
Continuous wind loading both during and after the recent storm produced a number of natural avalanches. Our terrain illustrated the effect of what 20” of snow in a large upwind fetch area can do, with a widespread natural avalanche cycle in Tuckerman Ravine. Huntington Ravine, with it’s much smaller fetch zone, experienced little if any natural avalanche activity and appears much more scoured. It’s notable that these ravines of similar aspect and elevation can vary significantly due to upwind fetch. We have yet to make observations in other east facing terrain, though we expect similar variability in recent avalanche activity and wind affect due to upwind fetch variability. We expect remaining wind slabs to be stubborn and potentially even unreactive, but spatial variability will challenge your ability to extrapolate stability observations across terrain. If you see evidence of avalanches from the recent storm or have any pertinent snowpack information, please use our observations submission option.
The Sherburne and Gulf of Slides ski trails are snow covered to Pinkham Notch.
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 01/12/2019 at 7:01 AM.
Ryan Matz, Snow Ranger
USDA Forest Service
White Mountain National Forest