Avalanche Forecast for Wednesday, March 13, 2019
This information was published 03/13/2019 at 7:03 AM.
The Bottom Line
Firm wind slabs in our mid elevation slide paths and softer wind slabs in our lower elevation paths will be subjected to relatively rapid warming today if temperatures continue to rise as forecast. Rapid warming of the snowpack is a sign of increasing risk of avalanches, especially if it is the first warming to which the snow has been subjected. Expect an increasing risk of triggering an avalanche in a slab weakened by sun, warm temperatures and high thin cloud cover. Human triggered avalanches are possible today so reduce your exposure by travelling one at a time and limit time spent in avalanche paths. Human triggered avalanches remain possible on shady slopes as well due to the recent formation of these wind slabs. It may feel like spring in the valleys but our snowpack says it’s still avalanche season. Avalanche danger is MODERATE today; evaluate snow and terrain carefully.
Two inches of snow fell on the summit through mid-day yesterday bringing the total for the past 48 hours to just over six inches. Strong WNW wind in the 60 mph range continued into the evening before calming slowly through the night to the current 25 mph. The summit is currently 18F with 12F at Crawford Notch. Light wind from the NW and warming temperatures are on tap today with generally fog free conditions on the summit. A warm but weak system will begin pushing high clouds into the area that may lead to overcast conditions later this afternoon before delivering a couple inches of snow to the high country this evening. Temperatures today are forecast to rise into the twenties. Light wind and south facing slopes at Ravine level and below are likely to rise above freezing.
Primary Avalanche Problem – Wind Slab
Our snow structure is a patchwork of wind slabs whose size and thickness are largely determined by their orientation to downwind fetch and degree to which they were exposed to pounding winds. Weak layers of softer snow may have slowly gained strength in the past 24 hours but today expect reduced stability as sun and warm temperatures weaken bonds in surface snow and send free water into the snow pack. Older wind slabs scattered around the range are bordering on the definition of persistent slabs.
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.
Secondary Avalanche Problem – Wet Loose
Areas of softer snow will be affected more quickly by sun and warm temperatures. Wet loose sluff may occur on or below steep areas, especially on or below cliffs that absorb sunshine. This type of avalanche is a warning sign for the more dangerous wet slab avalanche type. Wet loose avalanches or sluffs can add load to and trigger a slab.
What is a Wet Loose Avalanche?
Wet Loose avalanches are the release of wet unconsolidated snow or slush. These avalanches typically occur within layers of wet snow near the surface of the snowpack, but they may quickly gouge into lower snowpack layers. Like Loose Dry Avalanches, they start at a point and entrain snow as they move downhill, forming a fan-shaped avalanche. Other names for loose-wet avalanches include point-release avalanches or sluffs. Loose Wet avalanches can trigger slab avalanches that break into deeper snow layers.
Snow will soften today but not in the confidence building way that you’d want for classic spring skiing. Signs that this process is leading to instability include surface snow that becomes wet enough to wet your glove when you make a snowball, trees and steep slopes shedding balls of snow that roll down a slope, and increased penetration of your boots into the snow. Very firm slabs without a weak layer beneath (which you cannot drive your pole handle into) will provide safer travel and will benefit from today’s solar gain. There is 235 cm of snow at the stake at Hermit Lake!
The Gulf of Slides and John Sherburne 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 03/13/2019 at 7:03 AM.
USDA Forest Service
White Mountain National Forest