Avalanche Forecast for Friday, February 22, 2019
This information was published 02/22/2019 at 7:16 AM.
The Bottom Line
Recent new snowfall and high winds have created wind slabs in predominantly east facing terrain. Wind slabs that formed in high winds late last night will be firm and large in some areas but likely to be stubborn to a human trigger. That said, around half of the total 4-5” of snow fell after peak wind speeds which occurred late last night. This wind deposited snow is likely to be reactive to triggering. An avalanche triggered in that snow could step down into the more stubborn wind slabs. Due to the potential to trigger these wind slabs, along with the lingering possibility of a natural avalanche, you’ll find CONSIDERABLE avalanche hazard where you find this wind deposited snow. On higher south facing terrain, like the northern gullies in Huntington Ravine, which are rated MODERATE, you’ll find less snow due to the scouring action of the high winds and reduced avalanche danger as a result.
In the past day and a half, the summit recorded 5” of new snow with 4” at Hermit Lake and Gray Knob including periods of graupel observed. During this snowfall, moderate (35-50mph) S and SW wind increased in velocity while shifting west (55-75 mph) then WNW and ramping up further (70-85mph) late last night. Peak hourly gusts from the WNW last night reached 110 and 111 mph for several hours before midnight. Wind is currently moderating and is forecast to remain in the 45-60 mph range on the summit. Wind is currently at the lower end of that range. Temperatures yesterday were in the 20’s at mid-elevations and just above freezing at lower elevations. Summit temperature will be steady at 10F with no new snow in the forecast until Saturday night and Sunday. Skies should clear today with summit fog breaking up through the day.
Primary Avalanche Problem – Wind Slab
Wind speeds like those last night typically strip snow from our fetch and upper start zones, depositing it into lower start zones. As a result, areas like the terrain beneath the ice in the Headwall of Tuckerman Ravine and mid-gully in the Gulf of Slides are likely to hold large wind slabs. Bear in mind that even in firm, finger to pencil hardness snow, you can still trigger an avalanche at a thin or weak spot in the slab, in an area perforated by bushes and rocks, or on a convexity. Probe around with your pole handle or ice axe on the firm snow. You will likely feel less resistance which indicates weaker, softer snow beneath.
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.
Relatively warm temperatures yesterday were conducive to settlement of older wind slabs that still contained unsintered, unrounded grains above the Feb 8th ice crust. Sharp particles were observed Wednesday in the layers of wind slab just above the ice crust in the human-triggered avalanche that occurred in the Lower Snowfields on Tuesday. As mentioned a number of times in the past month, we have a very well developed snowpack, particularly in the eastern half of the range which has seen enhanced upslope snowfall during nor’easters, with associated avalanche activity. Slopes and gullies are large and connected, with the smooth February 8th ice crust present in the snowpack throughout our terrain. The ice crust serves as a slippery and smooth bed surface and is challenging if not impossible to arrest a fall on where it is exposed. The 4-5” of snow that fell in the past 36 hours or so is likely to have created wind slabs two to four feet thick and even thicker in places across mid slope start zones low in Chute and the Headwall. Slabs have been triggered from well beneath these start zones in the past so be sure to be mindful of terrain factors while assessing the snowpack, travel one at a time across suspect areas, and keep your guard up until further settlement and bonding occurs.
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 02/22/2019 at 7:16 AM.
USDA Forest Service
White Mountain National Forest