An icy bed surface and many obstacles exist in our snowpack which has been plagued by rain and well-above average temperatures since mid-December. Choose your timing and terrain carefully today. New snow in steep terrain today will likely not bond well, particularly as snow loads into avalanche start zones by wind action and sluffing. These wind slabs will become easier to trigger and larger as snow piles up. Though we are starting out the day with MODERATE avalanche danger this morning, areas that see wind loading will rise to CONSIDERABLE. Natural avalanches will be likely in a few areas, such as the Headwall and Lip of Tuckerman Ravine late in the day and overnight.
The low pressure system strengthening off the coast of Maine will send an arm of heavier precipitation to interior southwestern Maine and, with less certainty, into northern NH. This band of snow would add a few more inches to the 2-4” of snow already forecast for the range if it reaches this far interior. Additionally, while total QPF calls for only .25”, this storm should have a drier ratio of snow with deeper accumulations as a result. The key weather factor to keep your eye on today is the wind speed and direction. Currently wind is from the SW in the 20-35 mph, as measured on the summit, which is barely fast enough to move snow into most of our avalanche paths. Wind shifts to the NW and ramps up in velocity this afternoon and will move much of the 3cm of snow from yesterday along with new snow falling today.
Primary Avalanche Problem – Wind Slab
Wind slabs will accumulate and grow larger, more dangerous and will run further through the day. Wind slabs will be poorly bonded to the icy crust. Look for signs of wind loading, shooting cracks in the surface snow and increasing snowfall rates. Wind can build thick wind slabs quickly. Avoiding steep terrain and avalanche paths is the simplest strategy to stay out of trouble today.
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.
Today is the kind of day you really need to bring your A game into the field with you if you go looking for good skiing or climbing. There is likely to be a lot of visible refrozen and icy bed surface today despite the new snow. Scouring winds will probably make this the case tomorrow as well. It’s important to note that much of the avalanche activity this year has occurred on the frozen crusts that have developed several times this season. While we have no concerns currently about any weak layers beneath the current melt/freeze and rain crust, our wind action and new snow will create more than enough avalanche hazard. The good news is that this avalanche problem is easily avoided and obvious. The bad news is that the lean snowpack in lower elevation glades and trails may push you towards more open slopes and gullies at a higher elevation, which is of course, prime avalanche terrain. If it makes your decision any easier, remember that the icy bed surface lurks just below any new snow today and the magic combination of low angle terrain and snow deep enough to keep your skis off the icy surface will be really limited. Today and tomorrow look like a good time to let the snow pile up and bury the icy crust and obstacles in the lee terrain.
The Sherburne and Gulf of Slides ski trails are improved but rocks 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. Clickhere to check it out.
Recent snowpack and avalanche observations can be foundhere and on Instagram. Your observations help improve our forecast product. Please take a moment andsubmit 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 01/16/2020 at 7:10 AM.
USDA Forest Service
White Mountain National Forest
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