Avalanche Forecast for Sunday, March 17, 2019
This information was published 03/17/2019 at 7:02 AM.
The Bottom Line
Wind slabs sitting on top of a melt/freeze crust make up today’s avalanche hazard. These should be easy to identify by their smooth, white appearance when contrasted next to the dirty, textured, exposed melt/freeze surface. This melt/freeze surface presents a long, sliding fall hazard that makes bringing crampons, an ice axe, and the ability to use them effectively mandatory for travel today. Avalanche danger will be LOW at all aspects and elevations. Keep in mind the consequences of triggering even a small a avalanche would be a long, sliding fall with limited ability to self-arrest; be sure to factor this into decision-making today.
After a warm second half of the work week, temperatures began their plunge Friday night, falling from 32F midnight on Friday to 0F 24 hours later. Wind from the west blew in the 70 mph range Saturday, dropping slightly after dark to the 50 mph range. Snow showers delivered 2.3” of snow to the summit and 1.5” to Hermit Lake at 3800’. Today, light snow this morning may accumulate up to another inch (likely less) with a current W wind of 70 mph decreasing to 40-60 mph midday. Temperatures will increase slightly this afternoon, but expect a cold day overall. Monday should see skies clearing with temperatures staying around 0F.
Primary Avalanche Problem – Wind Slab
Newly formed, isolated wind slabs can be found on slopes with an easterly aspect at mid and upper elevations. These will be stubborn to a trigger and small in size. Wind speeds yesterday were capable of scouring some locations, so expect a good amount of spatial variability of today’s avalanche problem. With good visibility, especially later in the morning or afternoon, it should be easy to identify and avoid 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.
New snow that fell yesterday contained periods of graupel as well as experienced breaks in snowfall and slight dips in wind speeds. While some bonding at the interface of new and old snow may have taken place, interfaces in the new snow due to changing conditions as the snow arrived will likely be the culprit of an avalanche today should someone get a crack to propagate. The old snow is still in the process of refreezing after the thaw this past week. At mid-elevations yesterday, trails were supportable, but any off trail travel involved postholing. The continuing freeze today will lessen the amount of postholing, but also increase the hazard of long, sliding falls. Crampons and ice axes will be necessary for moving safely in avalanche terrain and flotation may be required to get to avalanche terrain. Despite the snowpack settling almost 7” at our snowplots during the thaw, no open holes were observed above 3500’ and stream crossings remain frozen.
The Sherburne and Gulf of Slides ski trails are snow covered to Pinkham Notch.
Considering an avalanche airbag pack? Take advantage of our current auction for the Black Diamond Saga 40 airbag pack, ending at 8:00 pm this Monday. Increase your margin of safety AND support your avalanche center!
Don’t miss our last avalanche awareness presentation of the season, free to attend, this Tuesday evening at Plymouth State University.
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 03/17/2019 at 7:02 AM.
USDA Forest Service
White Mountain National Forest