Avalanche Advisory for Sunday, April 22, 2018

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

 Tuckerman Ravine has CONSIDERABLE and MODERATE avalanche danger. Sluice, Lip, Center Bowl, and Chute have Considerable avalanche danger. Careful snowpack evaluation, cautious route finding, and conservative decision making are essential. Lobster Claw, Right Gully, Left Gully, Hillman’s Highway, and the Lower Snowfields have Moderate avalanche danger.  Evaluate snow and terrain carefully to identify features of concern. The Little Headwall is the exception with Low avalanche danger and areas of open water.

AVALANCHE PROBLEM: Wind slabs that formed since Friday are our primary avalanche problem. Of most concern today will be sun exposed areas holding largest slabs, like Sluice and Lip, where warming will push likelihood for human triggered avalanches toward likely and keep natural avalanches possible. Right Gully and Lobster Claw in Tuckerman and Yale Gully in Huntington should see similar affects though less capable of producing a truly large avalanche. We expect the somewhat firm wind slabs present in most of the terrain to be stubborn to a human trigger but capable of producing medium to large sized avalanches. This sets up a relatively low probability, high consequence day in which it’s plausible for the 5th or 10th skier, snowboarder, or climber rather than the 1st on a particular slope to trigger a large avalanche. Likelihood of triggering an avalanche will be slightly lower today than yesterday, but potential size of avalanches has not decreased, continuing to make the floor of Tuckerman Ravine an inappropriate place to linger.

 WEATHER: Temperatures below 20F on the summit and just above freezing at Hermit Lake yesterday paired with clearing skies and summit wind around 50 mph from the NW for a semi-pleasant day. Today is forecast to be approximately 10 degrees warmer with continued NW wind that should hold between 30 and 45 mph on the summit. Warmer temperatures and clear, sunny skies will make it feel spring like today and more so tomorrow, though the snowpack remains dynamic and more winter-like.

 SNOWPACK: Moisture from mixed precipitation earlier this week refroze and created a hard crust as a bed surface for the wind slabs which have formed since Friday. A few areas of this refrozen crust exist at the surface, but most of our terrain has been smoothed over by wind deposited snow. This means that the wind slab varies greatly in thickness from several inches to several feet, but it looks very similar from the surface. Layers do exist within this new wind slab, with a stiffer layer on the surface remaining reactive, but we would expect avalanches today to ultimately entrain all snow above the refrozen crust. Bonding in this upper snowpack has improved but not to the point of calling anything stable. Further, we expect stability to decrease with warming today, particularly on sunny aspects. A breakable sun crust from yesterday exists on some but not all southerly slopes, with inconsistent cloud cover suspected to be the culprit in variability of this crust. While sun today should soften this crust and affect stability, don’t expect an instant transition to corn snow from this wintry snowpack. Respect avalanche danger today by choosing terrain appropriate to your preparedness. Though avalanches may be less likely than yesterday, they could still be large. Bring your beacon, shovel, and probe along with an ability to use them as you carefully choose terrain. The Sherburne should ski well today and serve as an excellent consolation prize if today is not your day to travel in avalanche terrain.

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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  7:40 a.m., Sunday, April 22, 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