Avalanche Advisory for Wednesday, March 7, 2018

Huntington and Tuckerman Ravines have MODERATE avalanche danger. All forecast areas have Moderate avalanche danger. Heightened avalanche conditions on specific terrain features. Evaluate snow and terrain carefully to identify features of concern. The Little Headwall does not receive a rating due to lack of snow.

 AVALANCHE PROBLEM: We’re starting the day with pockets of wind slab formed over the past week as our primary avalanche problem, but wind slab and storm slab will develop towards the end of this forecast period which ends at midnight tonight. The pockets of wind slab are the same problem we’ve been referencing all week and have been generally easy to visually identify in contrast with a grey older surface. Though they’ve likely gained some stability, your ability to visually locate these pockets of wind slab will be challenged by new snow and reduced visibility today. The old, icy, and grey in appearance snow surface may prove challenging for incoming new snow to stick to. By late today or tonight we expect the new snow to develop into reactive slabs on this slick bed surface, particularly lower in our avalanche paths where sluffing from higher terrain may deposit snow. Also remember that this old, icy snow surface will easily allow a long sliding fall that would be difficult to arrest.

 WEATHER: Relatively stable and mild weather over the past two days is giving way to the winter storm you’ve surely heard about. Light snowfall is forecast to begin this morning and build in intensity through the day, becoming heavy tonight. 1-3” of new snow should fall before dark today, with 8-12” forecast for tonight and another 3-6” tomorrow. On the summit, SE wind under 30 mph will shift E tonight and peak around 50 mph before slackening slightly and shifting NE by tomorrow morning. Temperatures should remain in the teens F through tomorrow and the storm continues.

SNOWPACK: The old and icy snow surface we continue to talk about will be a key player as avalanche conditions develop from the incoming storm, with no deeper instabilities of concern. Slab building on a slick bed surface will likely occur in some areas while the old surface may remain exposed in others. We’re unsure at this time how well new snow will stick to the old surface for a variety of reasons. Slower wind speeds than might normally accompany a winter storm on Mount Washington could result in minimal scouring, but snow today will likely be light and easily transported by wind. Sluffing will also be a significant factor in slab development, as new snow may sluff off of the steepest portions of slide paths before slabs can build in these areas. These variables affecting slab development and distribution in our terrain will determine the number and size of avalanches we may see, with either many small avalanches or fewer large avalanches being potential outcomes of this storm cycle. Regardless, newly forming cohesive slabs should be reactive on the slick old snow serving as a bed surface.

The Harvard Cabin will be open all nights this week thanks to a fill-in caretaker.

• 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 8:05 a.m., Wednesday, March 7, 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