Avalanche Advisory for Tuesday, March 7, 2017

This advisory expires at Midnight.

Huntington Ravine and Tuckerman Ravine have LOW avalanche danger. Natural and human-triggered avalanches are unlikely in all forecast areas. Watch for unstable snow on isolated terrain features. The Little Headwall is no longer forecast this season as it is now an open river.

AVALANCHE PROBLEM: The avalanche problem today will change as the day progresses, from wind slab this morning to wet slab tonight. Two inches (2”) of snow overnight combined with up to another inch this morning on 30 mph SW winds will deposit isolated pockets of wind slab near the start zones of our forecast areas. These will be larger in gullies with N or E aspects like Central, Odell, South, Left and Hillman’s Highway, but as these displayed the largest amounts of scouring during this past weekend, the contrast between this newly deposited snow and the old hard surface will make safe travel possible.  As precipitation changes to rain this evening, these newly formed wind slabs as well as the wind slab that formed over the weekend will turn to wet slab. As the rain adds weight to the slabs, stability will decrease, with the greatest period of instability occurring after this advisory expires at midnight.

WEATHER: After a bluebird day yesterday with light winds from the NW, snow showers began last night at the same time winds shifted to the SW. It is likely light precipitation will be intermittent through the day. As temperatures warm, snow will switch to mixed precipitation and eventually to rain around dark with 0.25” of rain forecasted. Winds will stay from the SW and start to increase around the same time rain arrives. Rain should stay light and steady overnight.

SNOWPACK: The thaw around the start of the month created a very stable snowpack. Five inches (5”) of snow fell on top of this snowpack at the end of last week and into the weekend. In Huntington, strong NW winds carried this all down into the trees. In Tuckerman, this 5” formed firm (pencil hard) wind slab in isolated areas, the largest being in the Lip. Cold temperatures over Saturday and Sunday allowed facet growth at the interface of the old surface and this wind slab. Field tests indicated reactiveness, but with wide variability and uneven distribution of the snow combined with the firmness of the slab, initiating a crack that is capable of propagation is unlikely. The two inches of snow that fell overnight is heavy with some areas at lower elevations already seeing mixed precipitation, meaning lighter winds today will struggle to transport this snow. Old surface is identifiable in many areas due to the high contrast of dirty old surface to new snow and indicates that winds are not forming widespread slabs. When rain arrives tonight, the wind slab from the weekend as well new snow from last night will turn wet, decreasing the stability of the snow.

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 Harvard Cabin.
  • Posted  8:20 a.m., Tuesday, March 7, 2017. A new advisory will be issued tomorrow.

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