Avalanche Advisory for Thursday, December 21, 2017

This advisory expires at Midnight.

Huntington Ravine has Moderate and Low avalanche danger. Central, Pinnacle, and Odell gullies have Moderate avalanche danger. Natural avalanches are unlikely and human triggered avalanches are possible. All other areas have Low avalanche danger. Natural avalanches and human triggered avalanches are unlikely.

Tuckerman Ravine has Considerable, Moderate and Low avalanche danger. Lip, Center Bowl, and Chute have Considerable avalanche danger. Natural avalanches are possible and human triggered avalanches are likely. Careful snowpack evaluation and cautious route-finding are essential. Right Gully, Sluice, Left Gully, and Hillman’s Highway have Moderate avalanche danger. Natural avalanches are unlikely and human triggered avalanches are possible. Lobster Claw, Lower Snowfields and Little Headwall have Low avalanche danger due to limited snowpack development. Natural and human triggered avalanches are unlikely.

AVALANCHE PROBLEM: Wind slab remains the primary avalanche problem. The 10” of snow that has fallen in the past two days has been moved into our terrain and created a mix of stability ranging from fair to good. Some of these slabs are thick and would result in an avalanche large enough to carry and bury you. Beware of smooth pillows of wind slab beneath steep features and in the most sheltered lee terrain. Mixed in the more recent wind slabs are layers of heavily rimed snow grains which tend to be slow to bond and stabilize. Wind loading is continuing this morning and is a player in areas with elevated danger.

WEATHER: The temperature this morning on the summit has rebounded a bit to -4F from a low of -8F. Temperatures will warm under clear skies with good visibility and reach around 5F. Wind will diminish through the day to the 35-50 mph range from the NW on the summit. Winds in the 60-75 mph range, predominately from the WNW in the past 36 hours, moved the last 3 days snowfall into the Ravines. Today’s winds are unlikely to find more snow to transport into our start zones. More precipitation is on the way for the weekend so stay tuned.

SNOWPACK: The successive Canadian clippers this week delivered 19” of snow (2.23” water) to the higher terrain since Sunday. Combined with strong winds, this snow built wind slabs in many areas throughout our terrain. Field work in Huntington Ravine on Tuesday revealed that more recent new snow built thick slabs beneath steep features due to sluffing with softer and more reactive layers near the surface and in well protected lee areas. Deeper down in the snowpack, the slabs built by the 16” storm on the 11th/12th have proven to be unreactive, despite sitting on a melt freeze crust from warming that occurred on the 4th/5th of December. The relative stability is likely due to a period of freezing rain on the 19th combined with the stabilizing influence of time along with the bridging power of strong slabs. Our snowfall this week has been light to moderate without the howling winds that quickly build unstable slabs which fail under their own weight. Crown lines and debris exist in Central Gully indicating that a natural avalanche occurred there. That said, move carefully in the terrain today since the fresher wind slabs could still be triggered on foot or ski.

Due to this week’s snow filling things in and expanding the start zone on the Lion Head Summer trail, the Lion Head Winter Route will likely be open by the weekend. Coverage on the John Sherburne Ski Trail continues to improve but there are still rocks barely submerged by new snow.

Please Remember:
• Safe travel in avalanche terrain requires training and experience. This bulletin 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., Thursday, December 21, 2017. A new Advisory will be issued tomorrow.

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