Avalanche Advisory for Monday, March 20, 2017

This advisory expires at Midnight.

Huntington Ravine will have MODERATE and LOW avalanche danger. Damnation, Yale and Central Gully will have Moderate avalanche danger. Natural avalanches are unlikely but human-triggered avalanches will be possible this afternoon. All other forecast areas have Low avalanche danger. Natural and human-triggered avalanches are unlikely. Watch for unstable snow on isolated terrain features.

Tuckerman Ravine will have MODERATE and LOW avalanche danger. Sluice, Lip, and Center Bowl will have Moderate avalanche danger. Natural avalanches are unlikely and human-triggered avalanches will be possible this afternoon. All other forecast areas have Low avalanche danger. Natural and human-triggered avalanches are unlikely.

 AVALANCHE PROBLEM: Our primary avalanche problem today will be caused by the breakdown of strength in existing wind slabs due to heating of the snow. These firm slabs have been strong enough to bridge over soft weak layers beneath and resist failure but today’s heating will be the first real test since their formation. Today’s ratings are based on slope aspect as well as the size and continuity of the slabs. Lobster Claw and Right Gully along with North Gully in Huntington have much less snow in them or have already seen heating during yesterday’s mid-day warmth. As a result, those slopes are less likely to avalanche due to a human-trigger than nearby slopes with a moderate rating. When you begin to sink in to your boot tops or when it becomes easy to make a snowball on a steep slope, it will be time to reevaluate whether you should be travelling in that terrain.

 WEATHER: It has been a cold and snowy March so far, though today’s forecast is definitely spring-like. So far this month, temperatures are averaging 10F below normal and we are just 2” of snow shy of the monthly average with 11 days remaining in the month. Current temperature at Pinkham Notch is 10F, with 23.5F at Hermit Lake elevation on the Auto Road and 17F on the summit. With the warm band in place at Ravine elevations and relatively light NW winds, we are on track to begin the snow destabilizing warm up. Summit high temperature are forecast to reach the mid-20sF with NW winds in the 20-35mph range. Wind speeds this low from that direction are generally not enough cool our slopes. This is especially true in wind sheltered areas in the Ravine which receive a fraction of the wind that the summit feels.

SNOWPACK: Our snowpack now consists primarily of one-finger to pencil hard wind slab with softer pockets on the surface in southeast facing aspects. The wind slab has weak interfaces in the upper layers along with soft layers below. Last week, the upper slabs which range in thickness from a few centimeters to a meter, demonstrated the potential to propagate a crack, as seen in this video, but resisted doing so from human-triggers due to their strength. Today’s warming may change this. Though heating today doesn’t seem to indicate fast enough melting to lubricate the ice crust, it does raise concerns for weakening the upper slabs enough to make them prone to cracking, especially at a thin spot near the edge of a slab or over a rock. Air temperatures today may reach the point where even shadier aspect warm and soften a bit so more heavily scoured features like Left Gully and Hillman’s may soften. The weather conditions affecting our snowpack today will be as dynamic as those that created it, so look for clues and anticipate softening and weakening of the wind slabs as well as refreezing of the snow as it cools down again as the sun moves across the sky.

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 8:15 a.m., Monday, March 20, 2017. A new advisory will be issued tomorrow.

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

2017-03-20