Avalanche Advisory for Tuesday, February 20, 2018

Huntington Ravine has Considerable and Moderate avalanche danger. Central and Pinnacle Gullies have Considerable avalanche danger. Careful snowpack evaluation, cautious route finding, and conservative decision making are essential. All other forecast areas have Moderate avalanche danger. Evaluate snow and terrain carefully to identify features of concern.

Tuckerman Ravine has Considerable and Moderate avalanche danger. Sluice, Lip, Center Bowl, Chute, and Hillman’s Highway have Considerable avalanche danger. Careful snowpack evaluation, cautious route finding, and conservative decision making are essential. All other forecast areas have Moderate avalanche danger. Evaluate snow and terrain carefully to identify features of concern. The Little Headwall is the exception with a Low rating and potential for snow undermined by running water to be a larger concern.

AVALANCHE PROBLEM: Wet Slab is our primary avalanche problem with small slabs in many areas and large slabs in specific areas decreasing in stability through the day. Rain and melt water should wet recently formed wind slabs and percolate to the robust crust beneath them, lubricating this bed surface for avalanches to run on. Moderate rated areas hold significantly less new wind slab and are instead dominated by an old melt/freeze crust snow surface that is again melting. Wet slab avalanches are unlikely in anything but the recently formed wind slab. Peak instability of wet slabs is difficult to predict, but we expect human triggered avalanches to become likely and natural avalanches possible at some point later today. Loose Wet avalanches will be a rising concern as well today and could occur in much of our terrain. If in our terrain today, be on the lookout for overhead hazards in the form of icefall, rockfall, and of course the wet slab avalanches mentioned above. Also watch for weak snow undermined by water running beneath it which you could break through.

 WEATHER: Temperature could push to the upper 40’s F in our terrain today as around 0.5” of rain falls. In total, this warm and wet system looks to be bringing approximately 0.75” of rain. Showers today are forecast to generally taper off tonight and into tomorrow with a chance at clear skies by the end of Wednesday. Our typical westerly wind won’t be absent, blowing in the 60 mph neighborhood for much of the day and increasing tonight. Below freezing temperatures are likely to return by Thursday morning.

SNOWPACK: Our mixed snow surface conditions of wetting wind slab and older melt/freeze will react in distinctly different ways to the rain and warming temperatures today. Wind slab overlying the crust, today’s primary avalanche problem, will become wet and could avalanche on the crust which will become lubricated by percolating rain and melt water. These surface slabs have been formed since Saturday and are largest in the Sluice, Lip, Center Bowl, Chute, and right fork of Hillman’s Highway in Tuckerman Ravine as well as Central Gully in Huntington Ravine. This slab is relatively soft and should increase in sensitivity to a trigger through the day. Our upper start zones and significant portions of our other terrain experienced more scouring over the past few days and hold much more exposed melt/freeze crust from earlier last week. These areas in which the crust is currently softening will offer a safer and likely more enjoyable option for skiers and climbers. Keep in mind that wet loose avalanches will be possible in this snow as the crust becomes fully melted.

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 7:50 a.m., Tuesday, February 20, 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

2018-2-20