Avalanche Forecast for Monday, March 11, 2019
This information was published 03/11/2019 at 7:02 AM.
The Bottom Line
Wind drifted snow will dominate today and grow wind slabs quickly on slopes with an easterly aspect. These wind slabs should be reactive to a trigger and may entrain a large amount of snow due to the connected nature of avalanche paths on the east side of the Presidential Range. You will also be able to find newly formed wind slabs on slopes with a northerly aspect, though growth of these today will be limited to cross-loading and may become stubborn due to high wind speeds. Slopes that contain wind drifted snow will have CONSIDERABLE avalanche danger due to the likelihood of an avalanche and the size it could produce. Remember that wind can quickly turn inches of snowfall into feet of wind slab, particularly where the upwind fetch is largest.
A 12-hour period of steady snowfall yesterday that started late morning left around 4” on the east side of the Range, though less on the northwest side due to the location of the storm. During this time wind from the south decreased from 65 mph to the 40 mph range. Just as snow stopped, wind shifted to blow from the west and has increased to 75 mph where it should remain for the forecast period. Current temperatures on the summit are 14F and 23F at 3800’ which should gradually drop about 10F over the day. Conditions are ripe for upslope snow showers through the day and could deliver up to 3” by dusk. Snow showers may continue overnight into Tuesday, delivering another inch of two on steady high wind speeds from the NW.
Primary Avalanche Problem – Wind Slab
Wind slab that began building yesterday will continue to grow today with more snow and wind. These will likely be reactive to a human trigger. After northerly aspects saw direct loading yesterday, the shift in wind overnight will primarily load easterly aspects today. Those with a large fetch, such as the Tuckerman Headwall and main gullies in Gulf of Slides, are the areas that could produce a large avalanche, particularly in the unlikely but not impossible event of an avalanche stepping down into wind slab that formed last week.
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.
Wind slab that formed early last week with a clear weak layer between the overlying slab and bed surface remained largely unreactive except for a small skier-triggered avalanche on Friday. Wind slab that built yesterday and will continue to grow today rests on top of this older wind slab in locations that see largest loading today. Though outside our forecast area, a human-triggered avalanche yesterday in Lincoln’s Throat demonstrated how quickly wind slabs can grow, particularly at the base of ice bulges where sluffing can contribute quickly to the mass of growing slabs. Wind speeds today will border on the verge of scouring in some locations, but with wind direction making a 90 degree swing last night, be prepared to find today’s avalanche problem around micro-features in many places of our terrain.
The Sherburne and Gulf of Slides ski trails are snow covered to Pinkham Notch.
Join us next Tuesday evening, March 19th, for an Avalanche Awareness presentation at Plymouth State University! Details on our events page.
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 03/11/2019 at 7:02 AM.
USDA Forest Service
White Mountain National Forest