This information was published 01/31/2019 at 6:55 AM.
NOT THE CURRENT FORECAST
This is an archived avalanche forecast.
The Bottom Line
Frigid temperatures and wind will make for challenging travel while you sort through deep drifts, wind scoured ice and stubborn but potentially large wind slabs that exist in our forecast area. Wind has had time to build these wind slabs from the 14” of snow which has fallen in the past 36-48 hours. These slabs may be above a weak layer of soft snow and many will be resting on an icy bed surface. It will be possible for a climber or skier to trigger an avalanche today in steep, predominantly east facing, mid-elevation avalanche terrain which has MODERATE avalanche danger. While natural avalanches are unlikely today, it is not impossible as wind continues to move some snow and stress the slabs on steep slopes. Lower elevation areas may have some easily avoided areas of wind slab and have LOW avalanche danger. The icy snow surface that is exposed or barely covered creates the potential for a long sliding fall were new snow has been scoured away.
Two to three inches more snow fell yesterday and overnight. That brings the 48 hour total at east side snow study plots, as well as Gray Knob to 14”, with only about half that amount captured in the precipitation can on the summit. Wind early yesterday shifted from the SE before becoming variable in the 35-45 mph range. It then shifted SW through the NW before settling into a W and WNW direction. West wind overnight increased to the 60-70 mph range. Expect cold temperatures and wind in the 50-70 mph range today. The current low temperature is -27 F and will remain in the -20’s through today and overnight. High pressure will build in tonight with west wind approaching 100 mph, driving out snow showers that may deliver another inch of snow this afternoon.
Primary Avalanche Problem – Wind Slab
Recent snowfall and strong wind has created wind slabs in mostly east facing terrain as well as beneath steep terrain features. Crossloading of slopes and gullies is almost always a factor so be looking for accumulated snow behind natural drift fences like alpine trees and shrubs or behind small terrain features. Piles of sluff debris, such as develops beneath the first pitch of Pinnacle Gully and across the Headwall of Tuckerman, are the type of wind slabs that grow well beyond the amount of snow accumulated on the ground and produce larger than expected avalanches.
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 timing of recent snowfall, wind speed and direction make it likely that we passed through peak instability late yesterday or overnight. Reactive to touchy wind slabs were present during our field time early yesterday afternoon in Huntington Ravine though wind speeds had only reached into the 50 mph range on the summit for a few hours at that time. Wind speeds have ramped up quite a bit since that time and have likely blown most, but not all, available snow from the fetch. The icy crust that you may find in lower elevation or wind protected areas may be breakable with near-crust faceting that does not exist in most of our larger, mid-elevation avalanche terrain where the snow is either a knife hard mass to the ground or has the kind of bridging power that reduces deeper instability concerns to near zero for the time being.
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 01/31/2019 at 6:55 AM.
USDA Forest Service
White Mountain National Forest
Sign up to get the daily MWAC avalanche forecast in your inbox