This information was published 01/17/2020 at 6:55 AM.
NOT THE CURRENT FORECAST
This is an archived avalanche forecast.
The Bottom Line
Large wind slabs exist on east-facing, avalanche prone slopes that may release naturally today due to continued wind loading. These same wind slabs are likely to avalanche from human triggers. Red flags such as wind loading and even recent avalanche activity should be obvious today. An avalanche today has the potential to extend runouts into trees; standing on the floor of a ravine today will put you in avalanche terrain. Cautious route-finding on your approach should help you avoid lingering in the path of an avalanche. The possibility of natural avalanches, the likelihood of human-triggered avalanches, and the large size of potential avalanches has created CONSIDERABLE avalanche danger for today.
On Thursday, the summit of Mount Washington recorded 6.1” of 8% snow. Hermit Lake (3800’) received 8” of 9% snow. Snowfall was observed all through the day with some of the heaviest bouts falling early morning on a SW wind that hovered around 20mph. As the afternoon progressed, wind shifted to the NW and increased to above 70mph, where it has since stayed, gusting above 100mph. Currently, it is -14F on the summit with below zero temperatures everywhere above 1500’. This will moderate as the day progresses with the summit reaching zero F by evening. Wind will stay from the NW and drop from the current wind speed of 90mph to 70mph by the end of today. Saturday will start clear and cold with significant snowfall expected to start after dark.
Primary Avalanche Problem – Wind Slab
All snow at mid and upper elevations has been affected by the wind, creating wind slabs on slopes in the lee of a NW wind. Considering the overnight wind speed, these will likely present as hard slabs on open slopes with perhaps softer pockets tucked behind or under terrain features. Today’s wind slabs are sitting on a much softer (weak) layer of snow that fell early yesterday morning. While today’s hard slab may be stubborn to a trigger, the potential size of an avalanche today (D2: enough to bury or injure a person) as well as the possibility of natural avalanches due to continued wind loading should be enough factor heavily into your decision-making.
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.
After yesterday’s storm, you may be tempted to block out the rain event from last weekend. Unfortunately, that refrozen surface still plays significantly into decision-making today as it will either be exposed and present long-sliding fall opportunities or buried and act as the bed surface for an avalanche event. The rain crust that developed in mid-December acted as the bed surface for several avalanche cycles and we expect the mid-January crust to do the same. High wind speeds will have scoured many slopes on the windward (west) side of the range and deposited it on slopes with an easterly aspect, creating wind slabs much thicker than the 6” recorded on the summit. Those willing to brave the wind and cold today to find turns should have luck near treed and protected areas on western aspects or low-angled slopes on the east side. Our current thin snowpack is offering less than optimal options for skiing. Use your head today and beware of scarcity leading you into thinking you need to dig a pit on a 35-degree slope. Today’s weather is screaming caution and conservative decision-making.
The Sherburne and Gulf of Slides ski trails are improved but rocks, ice, and water bars will continue to be thinly covered through the day. Turn carefully and gently.
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 01/17/2020 at 6:55 AM.
USDA Forest Service
White Mountain National Forest
Sign up to get the daily MWAC avalanche forecast in your inbox