Avalanche Forecast for Monday, March 4, 2019
This information was published 03/04/2019 at 7:12 AM.
The Bottom Line
Avalanche danger will increase rapidly today as wind shifts to blow from the west while increasing in speed. This will transport the new snow of last night and this morning, building new wind slabs that will grow in size quickly and may become likely to human trigger. Windward terrain may become scoured and lack an avalanche problem, while wind loaded terrain is likely to reach CONSIDERABLE avalanche danger by late afternoon or evening. This danger rating is largely driven by a weather forecast which calls for shifting winds, which means that your observations of where wind slabs are actively being created is crucial information today.
Snowfall last night totaled 2-3 inches and it’s snowing lightly across most of our terrain this morning. Another inch or so of snow is forecast today as precipitation winds down and summit temperature holds around 10F. Southerly wind overnight peaked at 30 mph, with stronger gusts, and has since decreased. Light and variable wind early today will gradually increase and blow from a westerly direction. Summit wind speeds should be in the 30-45 mph range by this evening and continue to increase after dark. Tomorrow is forecast to be about 10 degrees colder, with W wind around 60 mph and light snowfall possible late in the day.
Primary Avalanche Problem – Wind Slab
New wind slabs will build on the eastern half of the compass rose as wind shifts W and increases today. We expect these new slabs to be touchy to a human trigger and to grow in size quickly. It’s a great day to remember that wind transport can easily build slabs multiple feet thick from just a few inches of snow on the ground. Terrain with a large upwind fetch zone, like Tuckerman Ravine and the Gulf of Slides, has the greatest potential for rapid development of unstable slabs today. Windward terrain should ultimately be scoured and could lack an 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.
The hard and unreactive surface wind slabs of late last week are slowly being covered by reactive newer slabs from moderate wind and several recent snowfall events. Areas with the greatest upwind fetch for westerly wind, like the headwall of Tuckerman Ravine, had the greatest new wind slab development and also saw human triggered avalanche activity yesterday. A majority of our terrain yesterday was not loaded with enough new snow to have an avalanche problem, but the day served as an excellent reminder that our extreme terrain can build slabs and produce avalanches very quickly. This will be the story for later today, as increasing wind shifts west and loads easterly terrain, this time with twice as much snow available for wind transport. Today’s relatively dense snow is sitting on lighter snow from yesterday, and new wind slabs forming today should be more dense still and provide a cohesive over weak structure. Many areas will lack an avalanche problem this morning, particularly terrain on the western half of the compass rose where wind slabs are unlikely to develop at all today. Between this aspect driven variability, softer old slabs at lower elevations, and other typical mid-winter snowpack variability, it’s as important as ever to make good observations to guide your terrain choices today.
The Sherburne and Gulf of Slides ski trails are snow covered to Pinkham Notch.
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 03/04/2019 at 7:12 AM.
Ryan Matz, Snow Ranger
USDA Forest Service
White Mountain National Forest