Avalanche Forecast for Thursday, February 14, 2019
This information was published 02/14/2019 at 7:06 AM.
The Bottom Line
Wind from the west will continue at speeds sufficient for creating wind slabs in the eastern half of the compass rose this morning. Storm slabs also exist on all aspects where 15” of snow has fallen in the past 36 hours. All avalanche terrain, especially at mid and upper elevations, has CONSIDERABLE avalanche danger due to the likelihood of human-triggered avalanches. Both of these avalanche problem types are capable of producing an avalanche that is easily large enough to bury a person. If you plan to ski in avalanche terrain, keep it low angle on smaller, wind sheltered slopes. Minor wind effect on the snow could bring down a wind slab from above.
Continuous snow showers yesterday dropped another 5” of new snow at mid and upper elevations. Add this new, lower density snow (11%) to the 10” that fell Tuesday night for a total of around 15” in the alpine. The critical detail in the weather history is the lower than expected and variable wind speeds over the past 24 hours. Wind velocity averaged only in the 45 mph range from the west which failed to move much of the dense and heavy new snow. This morning’s higher wind speeds in the 70 mph range will be diminishing to the 35-50 mph range but shifting to the NW. Temperatures dropped quite a bit over night to around -2F but will rebound to 10F on the summit and around 20F at 2,000’.
Primary Avalanche Problem – Storm Slab
Fifteen inches of snow has fallen in the past 36 hours. The snow started dry, became wetter and then dried out again. The wetter layer showed signs of dangerous slab forming properties yesterday. This new snow is sitting on top of a refrozen, icy crust which makes an ideal sliding surfaces that could produce full extent avalanches in gullies and larger slopes. Turn around if you see cracks shooting from your ski tips or slab failures at kickturns.
What is a Storm Slab Avalanche?
Storm Slab avalanches are the release of a cohesive layer (a slab) of new snow that breaks within new snow or on the old snow surface. Storm-slabs typically last between a few hours and few days (following snowfall). Storm-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.
Secondary Avalanche Problem – Wind Slab
Today’s wind slab problem will be harder to avoid than our typical wind slabs. Wind speeds weren’t high enough to scour snow down to old surface or create the stubborn wind slabs we’re more familiar with here. Hollow snow piled beneath steep terrain features and at the skyline may fail naturally, but skiing across hollow wind drifted snow on a slope over 30 degrees will likely produce an avalanche large enough to carry and bury you.
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.
Visibility was very limited yesterday and we received no observations from the field. On one hand, this is good news because anyone in steep avalanche terrain would have likely reported being involved in an avalanche. Small test slopes below treeline showed fair stability but some wind at the ridge top indicated that stability would have been reduced there. Those of us that easily fall in love with steep, smooth snow will run the risk of being blinded by Cupid’s arrow today. Please remember to make an alternative plan this morning that will allow you be satisfied by skiing lower angled, protected slopes. More snow is on the way Friday and will likely keep avalanche danger elevated once again.
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 02/14/2019 at 7:06 AM.
USDA Forest Service
White Mountain National Forest