Avalanche Forecast for Thursday, February 21, 2019
This information was published 02/21/2019 at 7:05 AM.
The Bottom Line
Increasing wind today will continue to create wind slabs at mid and upper elevations. These slabs will generally be on the smaller side in most areas but the possibility of a larger slab sliding under the weight of new wind slabs should remain front and center in your travel plan. A CONSIDERABLE rating best describes the situation today due to the potential for a natural avalanche to occur in steep terrain as well as the strong possibility of a skier or climber triggering a freshly formed and reactive wind slab. Upslope snow showers and fog will make it difficult to make assessments or to identify loading on slopes above you.
Summit temperatures rose above 0F yesterday and broke the two day cold snap. The current temperature at 4,000’ is close to 28F with 17F on the summit. Snow study plots accumulated around 2” of dense new snow that is already being drifted by the wind. The bulk of the snowfall seems on track to quit soon but upslope snow showers will bring up to 2” more this afternoon as an advancing cold front accompanied by howling NW wind arrives. Wind from the SW in the 55-65 mph range will increase through the day and shift to the NW, ultimately reaching 75-95 mph by nightfall.
Primary Avalanche Problem – Wind Slab
Wind from the northwest typically works very efficiently to develop wind slabs in our largest avalanche paths. Terrain which primarily faces east will contain the largest of these slabs which will be reactive today. Wind speeds late in the day and tonight will begin scouring and hammering these wind slabs into something much less reactive by tomorrow. Today will be a good day to find wind sheltered terrain at lower elevations with softer snow.
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.
Cold temperatures slow settlement in the snowpack and a look at snow grains at a crown profile in the Lower Snowfields confirmed the snow that fell Monday wasn’t very rounded yet. Stability tests in that location indicated stability was good at the time, but as is usually the case with wind slabs, it is dangerous to draw conclusions from snow stability tests when the distribution of the avalanche problem varies so widely across the terrain. Today will mark another day of active weather that will create another in a complex series of wind slabs on the old icy crust that developed February 8. Wind loading is one of the red flags that signal increased avalanche risk and is all that’s necessary to keep me away from steep wind loaded terrain.
The Sherburne and Gulf of Slides ski trails are snow covered to Pinkham Notch. Please avoid walking on these ski trails without snowshoes or skis. The holes punched in the snow can persist for days and create a hazard to others.
Snow rangers and White Mountain Avalanche Education Foundation folks will be at Flatbread Pizza from 5-8:30 tonight in North Conway. An hour long Avalanche Awareness talk starts at 7pm followed by Q & A session. Got a burning question about the new forecast zones? Come join us. Proceeds of every pizza sold will support the WMAEF and our mission!
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 02/21/2019 at 7:05 AM.
USDA Forest Service
White Mountain National Forest