Avalanche Forecast for Saturday, February 29, 2020
This information was published 02/29/2020 at 7:05 AM.
The Bottom Line
Human triggered avalanches are possible today where west wind will combine with forecast snow to form new wind slabs. Watch for signs of wind transported snow loading slopes in the lee of the wind, especially in steep terrain and cross loaded gullies. New wind slabs formed today will feel softer under-foot than the existing wind slabs in our terrain formed from Thursday’s storm. Evaluate snow and terrain carefully. Avalanche danger is MODERATE today.
Yesterday 1.9” (SWE 0.21”) of snow was recorded on the Mt Washington summit while W wind remained steady at 60-80 mph, and decreasing overnight. Temperatures hovered around 0F.
Today, snow showers may bring a trace to 2” of new snow on a W wind 25-40 mph shifting NW in the afternoon. The chance for snow showers will be greatest mid-day, mainly before the wind shifts NW signaling drier air moving in. Temperatures on the summit will remain in the single digits below 0F. Light snow showers remain a possibility overnight.
Tomorrow, a trace to 1” of additional snow as snow showers is possible with NW wind 40-50 mph.
Primary Avalanche Problem – Wind Slab
Large wind slabs formed from Thursday’s 9.5” (17%) snow are likely to be stubborn but remain possible to trigger, particularly where they exist outside the more active avalanche paths. Wind slabs forming today from new snow and moderate wind speeds will be more reactive to a human trigger and may grow to a foot thick or more if we see the upper end of the forecast total. Since new snow today will be limited to upslope snow showers, middle and upper elevations are the main concern. If you find thick drifts of soft snow today, you’ve found the avalanche problem.
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.
Thursday’s storm is proving to be an interesting one for forecasters. First, the storm was responsible for a number of natural avalanches in our terrain including Hillman’s, Duchess, Empress, Left Gully, South Gully and Escape Hatch. Escape Hatch may have stepped down deeper into the existing snowpack, possibly failing on a facet layer near an ice crust formed during one of many warm-ups that we’ve had this winter.
Second, the 9.5” of 17% snow has stabilized quickly, likely due to the moisture content of the snow, followed by cold temps and wind. Field time yesterday showed the snowpack to be firm, pencil to 1F and stubborn. A human triggering these wind slabs would be unlikely in the majority of our terrain, though unfortunate since they are quite thick.
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 02/29/2020 at 7:05 AM.
USDA Forest Service
White Mountain National Forest