Avalanche Forecast for Thursday, February 28, 2019
This information was published 02/28/2019 at 7:16 AM.
The Bottom Line
Wind slabs were loaded and cross loaded into our east facing, middle, and to a much lesser extent, upper elevation terrain. Lower elevation areas also contain firm wind slabs are have a characteristic upside-down nature.The lingering but unlikely potential for a large avalanche in steep terrain should lead you to continue using safe travel techniques. In addition to the old firm wind slabs, you’ll find some small new wind slabs built from the new snow that fell overnight and early this morning. These will most likely be found piled up beneath steep terrain features rather than blown in. Natural and human-triggered avalanches are unlikely with all forecast areas having LOW avalanche danger today. The old adage “LOW avalanche danger does NOT mean no avalanche danger” definitely applies.
A slippery and hard but thin melt freeze crust that developed on Feb 27 along with the much harder and the earlier, bulletproof Feb 8 crust are near or just below the surface in places. The combination of firm snow and icy crust obscured by new snow will create fast surface conditions that will be hard to visually identify. Travel carefully to avoid a long sliding fall.
Wind slowly tapered yesterday and was accompanied by temperatures in the single digits below zero as the pressure gradient slackened. High pressure now overhead will allow for clearing skies, light wind but continued cold temperatures. The current summit temperature of -4F will rise only to around 0F today with teens at mid elevations. Our snow study plots received some new snow overnight with 1 cm at Hermit Lake and 2 cm at Gray Knob. Light snow will continue this morning before clearing out. Clear skies, warmer temperatures near 10F and continued light wind are on tap for tomorrow.
Primary Avalanche Problem – Wind Slab
Wind slabs remain a concern on larger, steeper slopes. Triggering one of these wind slabs is unlikely but not impossible. Look for hollow sounding snow, pillows of smooth snow or softer new snow piled up beneath steep features that have been sluffing the new light density snow falling this morning.
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.
After the extreme wind earlier in the week, there was very little snow on the ground left to move yesterday though high winds continued through the morning. A few folks braved the wind and cold to ferret out smooth albeit firm snow for skiing. A field report from Mt. Willard shows firm, 1F-P hard wind slab over softer fist hardness snow. Avalanches have not been reported anywhere since mid-storm Monday into Tuesday and the few field reports received confirm the firm and unreactive nature or surface slabs, though they are lying over softer snow in some sheltered terrain. The wind hammered nature of these wind slabs lead us to believe that they will be unreactive in most places but possibly stubborn to triggering in a few. Good visibility allowed allowed east side observations of the changes that record setting wind speeds brought to the terrain. The only obvious crown from avalanche cycle Monday was in the Lower Snowfields though debris in the runout of Hillmans appears to be from that cycle. Most other debris was likely scoured out as continuous sluffing occurred during the storm. The hard slabs built by extreme wind have impressive bridging power but we know that triggering even hard slabs can be done if the right spot is triggered.
The Sherburne and Gulf of Slides ski trails are snow covered to Pinkham Notch. The Little Headwall filled in during the storm though it is guarded by large and aggressive sastrugi snow in the drainage.
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 02/28/2019 at 7:16 AM.
USDA Forest Service
White Mountain National Forest