Avalanche Advisory for Friday, December 19, 2014

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This advisory expires tonight at 12:00 midnight.

Tuckerman Ravine has CONSIDERABLE and MODERATE avalanche danger. The Sluice, Lip, Center Bowl, Chute and Left Gully have Considerable avalanche danger. Natural avalanches are possible and human triggered avalanches are likely. Careful snowpack evaluation, cautious route-finding and conservative decision making are essential. All other forecast areas have Moderate avalanche danger. Natural avalanches are unlikely and human triggered avalanches are possible.

Huntington Ravine has CONSIDERABLE and MODERATE avalanche danger. Central, Pinnacle, and Odell Gullies have Considerable avalanche danger. Natural avalanches are possible and human triggered avalanches are likely. Careful snowpack evaluation, cautious route-finding and conservative decision making are essential. All other forecast areas have Moderate avalanche danger. Natural avalanches are unlikely and human triggered avalanches are possible.

AVALANCHE PROBLEM: Wind slab is our primary avalanche concern again today. New snow and increasing wind speeds yesterday created hard wind slab in many areas. Where these wind slabs exist, they are likely to be hard, fairly thick and capable of propagating a crack on a slope above you. In other areas more sheltered from the strongest NW winds, softer Storm Slabs mixed with the wind slabs may also present a problem. Travel very carefully to avoid these unstable slabs as well as the paths and terrain traps beneath them.

WEATHER: Visibility today, though improving, will be hit or miss as remnants of this last low pressure system clear out. Temperatures will rise to around 20F (-7C) with light to moderate wind from the north in the 25-35 mph (40-55 kph) range. Gusts, potentially to 45 mph (75 kph), will keep things cool but should allow for a decent summit day. Don’t forget that we are approaching the shortest day of the year with around 9 hours of daylight with which to get things done. Sunset is at 4:07 pm.
SNOWPACK: Yesterday snowfall rates continued to impress us adding 6+” (15+cm) by noon, totaling 16.6″ (42cm) for the storm event.  Wind speed increased from the NW during the early morning hours on Thursday and ramped up over several hours moving snow from the alpine zone, mixed with new snowfall, into aspects with an eastern component.  Undoubtedly some crossloading occurred on slopes perpendicular to the prevailing NW direction.  We believe new slab density increased as wind speeds climbed through daylight peaking around 6pm creating an unstable snowpack with dense over lighter slabs.  High volumes of snow were transported into most forecast areas creating a peak instability more than likely close to the twilight hours. Velocities dropped all night to a current of 18mph.  As winds fell back down below about 50mph around 10pm, the vast majority of loading shut down.  This information has us less focused on natural avalanche activity, but based on the cold temperatures and short duration for natural settlement humans have a possible to likely chance of triggering slab avalanches. Expect slab densities and instability to change depending on whether or not slopes were exposed to high NW winds. You may start ascending on heavily wind scouring locations down low and then move into much lighter and unstable slabs as you approach protected terrain features.

Both Ravines are still encapsulated in clouds so we are unable to verify where avalanche activity occurred or what forecast areas received more snow than others. However, historically the locations posted at Considerable today receive the most volume, harbor higher slab instabilities, and require more caution from weather events similar to the last 48 hours.  We expect clouds to begin clearing soon allowing us to get into the terrain for new data collection that will get posted in the Weekend Update late in the afternoon.  In the mean time expect cold slabs to be unstable and hold the ability to propagate a fracture.  With bluebird days approaching with light winds watch yourself and the heuristic traps we can fall into when we are basking in what appears to be an “unbelievably awesome” day. Anticipate lingering instability in the high mountains this weekend.

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 or the Harvard Cabin.
  • Posted 7:50 a.m. December 19, 2014. A new advisory will be issued tomorrow.

Frank Carus/Christopher Joosen, Snow Rangers
USDA Forest Service
White Mountain National Forest
(603) 466-2713 TTY (603) 466-2856

2014-12-19