Avalanche Advisory for Tuesday, April 17, 2018

Huntington Ravine has MODERATE and LOW avalanche danger. Central, Pinnacle, Odell, South and Escape Hatch have Moderate avalanche danger. Evaluate snow and terrain carefully to identify features of concern. All other forecast areas have Low avalanche danger. Watch for unstable snow on isolated terrain features.

 Tuckerman Ravine has MODERATE avalanche danger. All forecast areas in Tuckerman Ravine have Moderate avalanche danger.  Evaluate snow and terrain carefully to identify features of concern. The Little Headwall is once again an open stream and is not receiving an avalanche danger rating

AVALANCHE PROBLEM: Wet slab avalanches will be possible to human trigger this morning, with colder temperatures gradually refreezing the snowpack and reducing instability late today into tonight. Multiple layers near the surface may be capable of producing varying sized avalanches. First, sluffing action of ice pellets yesterday proved to create unstable and reactive, though isolated, slabs low in avalanche paths. These and similarly new slabs formed from the mix of sleet (ice pellets), snow, freezing rain, and rain may present pockets of instability in much of the terrain, though wetting from rain overnight has likely reduced sensitivity to a human trigger. Second, deeper slabs which have become increasingly wet could produce large avalanches, also possible to human trigger and unlikely to avalanche naturally, though we’re not ruling out this possibility entirely. Finally, snow falling today and tonight on increasing W wind may build to form new wind slabs by the time this advisory expires at midnight tonight.

 WEATHER: The extreme diversity of precipitation types continued yesterday, with freezing rain showing as the dominant precipitation type in hourly observations but snow, sleet, and rain also falling and affecting the upper snowpack. All told, the summit recorded 3 inches of water amounting to 6.7 inches of snow and mixed precipitation particles. Hermit Lake recorded 2 inches of water and 3 inches of snow and mixed precipitation particles. It’s currently raining lightly from Pinkham up to the base of the ravines, with light snow showers on the summit. Mixed precipitation is forecast to continue today in our terrain, with snow being the dominant precipitation type on the summit. This should gradually trend towards snow as temperatures drop today and tonight from the current 30F on the summit to 15F by tomorrow morning. Snow could total up to 2-4 inches today and 1-3 inches tomorrow. The current S wind under 30 mph on the summit should shift W soon and increase late today and tonight. Tomorrow is forecast to have slightly colder temperatures and continued light snowfall, with W and NW wind in the 40-60 mph range.

 SNOWPACK: Our upper snowpack has become increasingly wet over the past 24 hours, adding to moisture from the weekend and generally thawing the refreeze which occurred Sunday night. The result of nearly every form of precipitation yesterday is wet and dense snow at the surface. Sluffing of ice pellets to pockets low in our avalanche paths accumulated significantly and showed the ability to produce an avalanche. A human triggered avalanche near the base of Hillman’s Highway was triggered from below and from the edge of the slab in terrain possibly below 30 degrees in steepness. Wetting from rain overnight has likely reduced the ability for similar slabs to produce avalanches, as the moisture is slowly making layers more uniform, but layers exist in the new “snow”. Deeper layers could also produce an avalanche, which could be larger though generally less likely. Dropping temperatures today and tonight will ultimately refreeze and lend stability to the snowpack. Precipitation transitioning towards snow from rain today and tonight may ultimately build new wind slabs, but this is unlikely to pose an issue during daylight hours. Those braving the wet conditions today will likely find decent conditions to make turns, with the Sherburne still holding snow coverage.

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Please Remember:
• Safe travel in avalanche terrain requires training and experience. This advisory 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.
• Anticipate a changing avalanche danger when actual weather differs from the higher summits forecast. For more information contact the Forest Service Snow Rangers, the AMC at the Pinkham Notch Visitor Center, or the caretakers at Hermit Lake Shelters.
• Posted  8:20 a.m., Monday, April 17, 2018. A new advisory will be issued tomorrow.

Ryan Matz, Snow Ranger
USDA Forest Service
White Mountain National Forest
(603) 466-2713 TTY (603) 466-2856