Avalanche Forecast for Tuesday, February 19, 2019
This information was published 02/19/2019 at 7:03 AM.
The Bottom Line
Low density snow yesterday has been affected by NW wind overnight which has produced relatively small new slabs that are possible to human trigger. In contrast, older wind slabs remain which are unlikely to trigger but would produce large avalanches. Continue to use safe avalanche terrain travel practices, bring your avalanche rescue gear and a good partner or two, and respect large avalanche paths and high consequence terrain. All forecast areas have MODERATE avalanche danger today.
Approximately 3 inches of new snow fell in our terrain yesterday with measured density as low as 3.7%. Temperatures dropped in the late afternoon to bottom out at -15F on the summit overnight while NW wind increased to the current 70-80 mph range with stronger gusts. Wind should hold through the day and decrease slightly by tomorrow morning. Skies will be mostly clear with no precipitation expected. A high temperature just below 0F is forecast for the summit. Tomorrow should trend warmer, less windy, and increasingly cloudy as a system bringing another shot of snow and possibly mixed precipitation arrives by evening.
Primary Avalanche Problem – Wind Slab
Yesterday’s low density snow has been easily transported by NW winds. You can expect to find new wind slab that may visually appear quite similar to the older surface slabs formed late last week. We expect scouring to these older slabs has also occurred. The new wind slab should be firm and stubborn, with softer and reactive pockets in wind sheltered terrain. These new slabs are most likely to exist in the middle to lower portions of terrain in the ravines, on the east side of the range. The large older wind slabs are becoming unreactive though still worth considering as a low probability, high consequence 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.
A melt freeze crust formed February 8th has been mostly covered by layers of wind slabs formed in the past week, with a few areas wind scoured to this crust. Stability concerns are limited to snow above this robust crust. The 12 hours of summit wind in the 70-80 mph range will have easily transported yesterday’s low density snow, scouring it from windward aspects and upper start zones. The new slabs built from this snow should be firm and supportable under skis or board, with a few softer exceptions in sheltered terrain. On the surface, this layer may look and feel similarly to the older wind slabs which will also be present at the surface. These older layers above the crust are unlikely to produce an avalanche, but a strong over weak structure has been observed in these layers and should keep the possibility of large avalanches on your radar. Realize that the newer wind slabs will be more likely to human trigger, which will make applying your snowpack observations to anywhere but the snow under your feet fairly difficult today. Low elevations with significantly less wind affect may provide the most enjoyable conditions.
The Sherburne and Gulf of Slides ski trails are snow covered to Pinkham Notch.
Please join us for an evening of avalanche awareness at Flatbread Company in North Conway this coming Thursday!
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 02/19/2019 at 7:03 AM.
Ryan Matz, Snow Ranger
USDA Forest Service
White Mountain National Forest