Avalanche Forecast for Tuesday, December 18, 2018
This information was published 12/18/2018 at 7:02 AM.
The Bottom Line
A few inches of new snow and sustained NW wind in the 60-80 mph range have made new wind slabs the primary avalanche concern for today. It’s worth remembering that wind can build slabs much thicker than our recent small snow totals. Wind will continue to affect snow on the ground through this forecast period, and limited visibility will make identifying features of concern a challenge. The Headwall area of Tuckerman Ravine has CONSIDERABLE avalanche danger with natural avalanches that threaten the floor of the ravine remaining possible. All other forecast areas have MODERATE avalanche danger. Natural avalanches are unlikely and human triggered avalanches are possible. Evaluate snow and terrain carefully.
Sustained NW wind around 60 mph, with several hours over 70 mph, since yesterday evening has affected our recent snowfall. Storm snow totals as of this morning are 2.5” at the Hermit Lake snow plot, 3.5” at the Grey Knob snow plot, and just over 3” on the summit. Expect NW wind to continue through today and potentially increase from the current 60 mph on the summit to just shy of the century mark. Snowfall is tapering off, though we may receive another trace to 2”. Temperatures have dropped to below 0F on the higher summits and should only increase a few degrees today. Cloud cover is likely to reduce visibility though ultimately give way to mostly clear skies by tomorrow. Forecasts indicate that temperatures should rise slightly tomorrow, with decreasing wind and no precipitation.
Primary Avalanche Problem – Wind Slab
New wind slab that will vary in size and likelihood of triggering is our primary avalanche problem. It’s likely you’ll find relatively large areas of firm and stubborn wind slab, with smaller pockets that are softer and more reactive. This avalanche problem should be confined to the eastern half of the compass rose in alpine terrain, with scouring limiting concerns on many westerly aspects and minimal new snow at lower elevations.
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.
Sustained NW wind which continues today is the primary driver of conditions in our upper snowpack, transporting the snowfall of the past 36 hours as well as older snow on the ground. A lack of visibility has not allowed visual observations to determine distribution of both wind deposition and scouring. We expect that lee areas hold relatively firm and stubborn new wind slabs of varying size. Areas more exposed to wind, which includes both westerly aspects and some upper start zones on easterly aspects, are likely scoured of soft snow. Pockets of softer and more reactive new wind slab should also exist as a result of cross loading and wind edy affects. Beneath any of these new slabs and also present at the surface where scouring has occurred will be a mixed bag including firm old wind slab and the Dec. 3rd crust. It’s worth noting that facets have been observed around this crust, though we still don’t expect them to be a player in much of our terrain due to lack of propagation potential in the slabs above.
Ski trails in the Pinkham Notch area remain snow covered.
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 12/18/2018 at 7:02 AM.
Ryan Matz, Snow Ranger
USDA Forest Service
White Mountain National Forest