Avalanche Forecast for Saturday, April 20, 2019
This information was published 04/20/2019 at 7:13 AM.
The Bottom Line
Wet slab avalanches that would be difficult to survive are possible today and could occur naturally, without a human trigger in some areas. It’s worth minimizing your time in and under avalanche terrain and especially runout zones like the floor of Tuckerman Ravine to lessen your exposure to this difficult to predict hazard. Heavy, wet loose sluffs are likely in steep terrain and are more predictable but could easily result in an unwanted fall. The Presidential Range has MODERATE avalanche danger today, with the Headwall of Tuckerman Ravine rated CONSIDERABLE for its historic ability to produce very large, wet slab avalanches from the Lip. Be aware that the spring hazards of undermined snow and potentially very deep melt holes resulting from flowing streams are increasing today. Melting conditions, such as we have today, also increase the likelihood of icefall.
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The Presidential Range is in the midst of the strongest warming yet this season. Temperatures have been above freezing on the Summit since Thursday morning and today will combine with the warming effect of rain. Over ½” of rain was recorded at Hermit Lake yesterday which mostly fell overnight. The mercury will continue to rise today and peak late this afternoon over 50F on the summit and nearly 60F at 3,500’. Tonight and tomorrow will be slightly cooler though still above freezing. Rain should total around one inch today, ultimately tapering off to lighter amounts tonight with a chance of showers continuing through tomorrow with cloudy skies. Gusty wind in the 40-60 mph range on the summits today is forecast to decrease tomorrow.
Primary Avalanche Problem – Wet Slab
Wet slab avalanches are possible today on all aspects and elevations. Elevations below 3,500’ that still hold snow could produce this type of avalanche, though larger, unsupported slopes with water flowing beneath or through the snowpack are most capable of producing a very large avalanche. Strong and extending warming of the snowpack, rather than field observations, is your indicator that wet slab avalanches are possible today. Respect avalanche terrain and runout zones for this low probability and high consequence hazard that is particularly difficult to predict.
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
Wet loose sluff avalanches are likely to be initiated under your skis or snowboard in steep terrain today. While relatively easy to predict and manageable if you’re expecting these heavy sluffs to occur, they can easily pull you off your feet or into a place you don’t want to go. Consider the consequence of a wet loose sluff induced fall wherever you travel today.
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.
The current warming and wetting weather is likely allowing moisture to penetrate deeper into our snowpack than it has yet this spring. Deeply buried crusts and remaining dry snow will be affected by melt water moving through the snowpack. Significant water is also flowing in flashy drainages. Rain and warm temperatures today increase the likelihood for this water flowing in the snowpack to create deep instabilities and resulting large or very large wet slab avalanches. Deep wet slab instabilities are very hard to predict. We’re challenged at pinning down accurate likelihood of deep wet slab avalanches, and particularly the timing of when they might occur. This means that you’ll have limited ability to make stability observations and assessments today. Wet loose sluff avalanches will oppositely be easy to predict and likely to occur on steep slopes. Sloppy, wet snow today should combine with rain falling today to drive down the reward for exposing yourself to the risk of wet slab avalanches.
The Sherburne and Gulf of Slides ski trails are mostly snow covered to Pinkham Notch, though this is changing by the day. Expect ice patches, opening stream crossings and the occasional bare patch.
Details on daily snowfall totals, precipitation type, total depth of snow and other information can be found on our page devoted to snow study plot data. Click here to check it out.
Recent snowpack and avalanche observations can be found here and on Instagram. Your observations help improve our forecast product. Please take a moment and submit a photo or two and a brief description of snow and avalanche information that you gather in the field.
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 04/20/2019 at 7:13 AM.
Ryan Matz, Snow Ranger
USDA Forest Service
White Mountain National Forest