This information was published 03/15/2019 at 7:03 AM.
NOT THE CURRENT FORECAST
This is an archived avalanche forecast.
The Bottom Line
Warming will continue today, increasing the likelihood of a human-triggered wet slab. Roller balls and natural wet avalanche activity were both observed yesterday. While terrain with an easterly aspect has the most developed slide paths and could produce larger avalanches, the widespread warming on all aspects we are experiencing today pushes the danger rating for the day to CONSIDERABLE. All elevations and aspects could produce wet avalanche activity today. Uncertainty always comes with wet avalanches, so today’s mitigation may be best done with avoidance of the terrain. Warm weather and rain also increase the chance of rock and ice fall.
As of noon today, all elevations will be have been above freezing for 24 hours, with lower elevations surpassing that mark early this morning. Some direct sunshine yesterday encouraged warming on southerly aspects, but the ambient temperature was the key yesterday and will continue to be so today. The summits will crest into the lower 40s F this afternoon . Rain should fall today, with the bulk of the forecast 0.2” coming early afternoon. Temperatures will fall below freezing tonight with snow in the forecast tomorrow. Upslope snow showers could bring several inches through the day. Tomorrow will bring hazards to avalanche terrain, though much different than today’s wet issues.
Primary Avalanche Problem – Wet Slab
Continued temperatures above freezing will allow meltwater to percolate through the upper part of our snowpack. When this free water encounters an ice crust, the interface between the bed surface and overlying slab will become lubricated. This lubrication, in combination with the loss of strength in the overlying slabs due to continued warmth, has the potential to create wet slab avalanches. Thin areas of the snowpack (think extreme slopes that shed snow more readily, near rock buttresses, and lower elevations in general), particularly slopes with a degree of northerly aspect which have seen much less direct sunshine, are places that are easier for a skier or climber to trigger wet slabs.
What is a Wet Slab Avalanche?
Wet Slab avalanches are the release of a cohesive layer of snow (a slab) that is generally moist or wet when the flow of liquid water weakens the bond between the slab and the surface below (snow or ground). They often occur during prolonged warming events and/or rain-on-snow events. Wet Slabs can be very unpredictable and destructive.
Secondary Avalanche Problem – Wet Loose
As the snowpack continues to see temperatures above freezing, the risk of wet loose avalanches continues. While these are often small and usually do not contain enough snow to bury a person, it can be very hard to escape once caught in this slow-moving snow that acts more like wet cement. Sluff management for skiers today will be crucial to avoiding this likely problem. Being carried over a cliff or ice bulge is often the real hazard associated with this sort of avalanche problem. Also, a loose wet avalanche could produce the required mass to trigger a lurking wet slab that skiers or climbers have not found yet.
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.
Yesterday afternoon, the penetration of the warmth into the snowpack reached around 5”. While some slopes did see direct sun, the depth of warming seemed consistent on all aspects due to the ambient air temperature. Our layered snowpack contains several ice crusts that have been producers of large avalanches this year. Digging to these crusts in several locations, well-developed facets can be found. While not widespread enough to create a persistent problem, these may lurk in shallow areas of the snowpack and offer trigger points for wet avalanches today. We are currently in the midst of our first big warm-up of late winter. Seasoned travelers often avoid avalanche terrain during this annual event as the first warm-up has a history of producing avalanche events that make people scratch their head or head to the history books to find if that has happened before.
The Sherburne and Gulf of Slides ski trails are snow-covered to Pinkham Notch. Don’t miss our last avalanche awareness presentation of the season, this coming Tuesday evening at Plymouth State University. Details for this free event are on our website 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 03/15/2019 at 7:03 AM.
USDA Forest Service
White Mountain National Forest
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