Avalanche Forecast for Thursday, February 27, 2020
This information was published 02/27/2020 at 6:54 AM.
The Bottom Line
A winter storm will bring 8-14” of new snow with extreme, shifting winds to the mountains. Avalanches could occur without a human trigger, starting this afternoon, particularly in places like Tuckerman Ravine and Gulf of Slides. Avalanche conditions could develop later this morning in west facing terrain and mid-elevations like Crawford Notch due to a strong east wind and heavy snowfall. Avalanche danger will be CONSIDERABLE today. Dangerous avalanche conditions will develop today.
Yesterday, around an inch of new snow fell through the day on very light wind. The summit temperature reached 23F and dropped to 19F overnight. Generally calm and foggy throughout the day.
Today, a storm system will pass over the area and gain strength as it reaches the coast. Temperatures will fall and the snow that began around 5am at middle and upper elevations will intensify through the day. Winds are currently blowing well into loading speed, (90 mph, gusting to 129 mph), even considering the heavy, wet snow, from the E and ESE. Wind will shift quickly early this afternoon, blowing from the south for a couple of hours before shifting to the west. Wind will remain in the prime wind loading range of 50-70 mph through the day. If forecasts play out accurately, 6” snow should be available by mid-day prior to the time the wind shifts west, with 8-14” total in the forecast for today and tonight.
Tomorrow’s weather will bring a return to our standard pattern of cold temperatures and strong to extreme wind from the west. The storm system is leaving a lot of moisture in its wake with even more available from the unfrozen Great Lakes. Another 4” should fall Thursday night with another 4” possible by Saturday. Wind from the west at 50-70 mph will dump this snow into eastern facing terrain.
Primary Avalanche Problem – Wind Slab
Wind slabs will develop on multiple aspects and beneath steep terrain as snow accumulates today. Wet snow will form cohesive slabs quickly that will grow in size and become increasingly sensitive through the day. Though snow will start out wet and dry as temperatures cool, don’t expect the snow to be “right-side up” since wind effect will counter that occurrence. Natural avalanches will become possible by mid-day with avalanches potentially becoming dangerously large by afternoon after the wind shifts west.
What is a Windslab Avalanche?
Wind Slab avalanches are the release of a cohesive layer of snow (a slab) formed by the wind. Wind typically transports snow from the upwind sides of terrain features and deposits snow on the downwind side. Wind slabs are often smooth and rounded and sometimes sound hollow, and can range from soft to hard. Wind slabs that form over a persistent weak layer (surface hoar, depth hoar, or near-surface facets) may be termed Persistent Slabs or may develop into Persistent Slabs.
Secondary Avalanche Problem – Storm Slab
Wind sheltered areas will develop storm slabs as snow accumulates through the day. Be wary of steep wind sheltered terrain and check small, steep test slopes for signs of slab formation. Heavy snow can pack a punch so don’t underestimate the ability of a 6-8” slab to push you someplace you don’t want to go.
What is a Storm Slab Avalanche?
Storm Slab avalanches are the release of a cohesive layer (a slab) of new snow that breaks within new snow or on the old snow surface. Storm-slabs typically last between a few hours and few days (following snowfall). Storm-slabs that form over a persistent weak layer (surface hoar, depth hoar, or near-surface facets) may be termed Persistent Slabs or may develop into Persistent Slabs.
The powder hounds among us may be tempted to cheat the avalanche problems today to find the goods. The high wind and low visibility will hopefully lead you to the trees or resorts. Not only is wind currently hammering near treeline, the visibility will make it challenging to use visual cues to assess risk. Swirling localized winds and rapidly accumulating snow will change things rapidly today. If you do attempt to thread the needle today, keep the angle low (30 degrees or less) and avoid avalanche paths and their runouts. Timing of the wind shift and the length of time the wind blows from each cardinal point will change conditions significantly. Banking on hourly wind forecasts is a significant gamble today and probably a bet worth passing up. Seems like a good day for lift served skiing or tours in the woods.
The Sherburne and Gulf of Slides ski trails are snow covered to Pinkham Notch.
Details on daily snowfall totals, precipitation type, total depth of snow and other information can be found on our page devoted to snow study plot data. Click here to check it out.
Recent snowpack and avalanche observations can be found here and on Instagram. Your observations help improve our forecast product. Please take a moment and submit a photo or two and a brief description of snow and avalanche information that you gather in the field.
Safe travel in avalanche terrain requires training and experience. This forecast 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.
Avalanche danger may change when actual weather differs from the higher summits forecast.
For more information contact the US Forest Service Snow Rangers, AMC visitor services staff at the Pinkham Notch Visitor Center, or the caretakers at Hermit Lake Shelters or seasonally at the Harvard Cabin (generally December 1 through March 31). The Mount Washington Ski Patrol is also available on spring weekends.
Posted 02/27/2020 at 6:54 AM.
Frank Carus, Lead Snow Ranger
USDA Forest Service
White Mountain National Forest