This information was published 02/17/2020 at 7:08 AM.
NOT THE CURRENT FORECAST
This is an archived avalanche forecast.
The Bottom Line
Largely firm and unreactive wind slabs are widespread through northeast to south facing aspects at mid and upper elevations. Some areas of softer snow exist here as well but are small and easily managed or avoided. The icy, unbreakable crust that lies below this past week’s snowfall makes crampons useful in wind scoured areas at the tops of gullies. Avalanche danger is LOW today with human triggered and natural avalanches unlikely. Wind packed snow has created generally safe avalanche conditions.
Yesterday, the summit received 1.2” of 13% snow from a quick round of light and moderate snow showers in the morning ending at around 9am. During the time of snow shower activity, wind was from the W at 65-80 mph, and later dropped to the 30-40 mph range with blowing snow recorded every hour until 5pm. High temperature was 12 °F.
Today, high temperatures in the lower single digits, with W wind shifting to NW 50-70 mph and dropping to 40-55 mph. A slight chance of snow showers in the morning and clearing in the afternoon. The 6am hourly record from the summit reports light snow showers.
Tomorrow, 4 to 8 inches of snow is forecast and avalanche danger will increase. Snow is expected to begin in the morning with a 20-35 mph S wind. The heaviest snow falls mid-day coinciding with the S wind ramping up to 55-75 mph. Snow intensity will diminish overnight as wind shifts to the W and increases 75-95 mph. Snow showers may continue into Wednesday. At this time, models suggest precipitation from this system will be all snow, with no rain or mixed precipitation expected.
Primary Avalanche Problem – Wind Slab
Most of our prime avalanche terrain was tested over the weekend. Finger and pencil hardness wind slabs allowed boot penetration over the ankle where the snow had accumulated over the stout rain/sleet crust. Ski penetration varied from essentially none on the supportable but easily carved wind packed areas to 10-15cm in more sheltered areas of finger hardness snow. Despite moderate shears, the layers above the ice crust have been proven to be unreactive. Some pockets of softer (4F) snow exist but these areas are very isolated and easily managed or avoided. Lower elevation areas in Crawford Notch are likely well settled though no observations create uncertainty.
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.
If we had rain or a large storm in the forecast, the existing upper snowpack would be a concern. The fracture planes between the multiple layers of wind slab which developed in the past week are quite obvious in snow pits. The upside down nature of the snow is evident when booting or poking hard into the snow with a pole handle. Heat in the form of heavy rain would be the only foreseeable trigger that could move the existing wind slab in a big way. The snow on it’s way tomorrow will create its own avalanche problem and it remains to be seen if new avalanche activity will be enough to step down and pull in the existing slab. My money is on that happening in some of the larger slopes and longer gullies.
Avalanche Size. From Snow, Weather, and Avalanche Guidelines; Greene, et al
Field time yesterday in somewhat reduced visibility over in the Main Gully of Gulf of Slides revealed the disconcerting trend of folks hanging out directly in the avalanche path, in fact standing on top of old debris. We frequently see this in Tuckerman Ravine as well. Even in times of lower avalanche hazard, continue the habit of limiting time in the barrel of the gun. And please, don’t leave your partner, unprotected without a beacon in front of the loaded gun. Low avalanche danger does not mean that there is no avalanche danger . It’s really important to understand that a Low danger rating allows for D1 avalanches to occur. While relatively harmless, a D1 could still lead to a bad outcome, especially if it pushes you through trees, into rocks or buries you in a terrain trap. This also assumes that your forecasters nailed it. A D2 would be much, much worse. Repeated exposure to low probability environments eventually leads to trouble so always practice good, thoughtful travel techniques.
The Sherburne and Gulf of Slides ski trails are snow covered with good skiing 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. 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 02/17/2020 at 7:08 AM.
USDA Forest Service
White Mountain National Forest
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