This information was published 01/11/2020 at 7:17 AM.
NOT THE CURRENT FORECAST
This is an archived avalanche forecast.
The Bottom Line
Rain and warm air makes human triggered avalanches possible today giving us a MODERATE danger rating. This avalanche danger will continue through the weekend and only decrease when our snowpack refreezes, possibly Sunday night. Today is a great day to avoid steep terrain, and also areas under steep terrain where debris from an avalanche sweeps through. The floor of Tuckerman Ravine and the bottom of Gully #1 in the Gulf of Slides are examples of this.
Warm air brings concern for falling ice anywhere there are ice cliffs, particularly hanging ice at Frankenstein Cliffs in Crawford’s Notch.
Overnight temperatures in the Presidential Range were above freezing, with the summit of Mt Washington rising above freezing at approximately 2am. Warming will continue possibly reaching 40 F on the summit today. Light rain this morning will become steady rain this afternoon with .25” of rain expected before dark. Rain will continue overnight into tomorrow delivering as much as 1.8” of rain for the weekend total. Wind today will be from the west at 80-100 mph with gusts to 140 mph.
After multiple days of snowfall, west winds loading and building wind slabs, rain today will infiltrate and consequently weaken the snowpack. Exacerbating this problem, these slabs are resting on an icy bed surface that has been proven to be a contributor to naturally occurring avalanches earlier in the week. This avalanche problem exists not only high in steep terrain, but at the bottom of ravine gullies where sluffing snow piles up.
An additional concern is the known and unknown water courses that run under the snowpack when we see significant melting and rain. The Lip area of the headwall has proven itself to fail catastrophically during extreme warm rain events.
It’s worth mentioning that predicting wet snow instability involves a higher than normal amount of uncertainty. It would be wise to factor that into your decision making if you venture into the mountains today.
What is a Wet Slab Avalanche?
Wet Slab avalanches are the release of a cohesive layer of snow (a slab) that is generally moist or wet when the flow of liquid water weakens the bond between the slab and the surface below (snow or ground). They often occur during prolonged warming events and/or rain-on-snow events. Wet Slabs can be very unpredictable and destructive.
Rain-on-snow events present challenges to avalanche forecasters. The models we use to determine sensitivity, distribution and likelihood become less effective tools due to the fact that water infiltration is non-linear, meaning water will work itself into and through the snowpack at inconsistent rates. We do know that water will weaken the snowpack, increasing the chance for an avalanche. We know that natural avalanches in soft or unsupported snow (if they happen at all) will tend to occur soon after the rain begins, though the increased risk remains until the snowpack has drained or refreezes. The real difficulty lies in estimating just how much of an effect the rain has on any weaknesses within the snowpack.
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. Clickhere to check it out.
Recent snowpack and avalanche observations can be foundhere and on Instagram. Your observations help improve our forecast product. Please take a moment andsubmit 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 01/11/2020 at 7:17 AM.
USDA Forest Service
White Mountain National Forest
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