Avalanche Forecast for Monday, April 15, 2019
This information was published 04/15/2019 at 6:58 AM.
The Bottom Line
Rain and above freezing temperatures are continuing to create conditions for wet avalanches while also making the spring hazards of icefall and undermined snow relevant today. Wet loose sluff avalanches are possible to initiate under your skis or snowboard, especially in areas that were not heavily skied over the weekend. While difficult to predict, conditions allowing deeper wet slab avalanches may develop today. These are unlikely but should motivate you to minimize time spent in the runout of steep terrain, as should possible icefall in much of the same terrain. Streams flowing under the snow are actively opening new melt holes, and the possibility of breaking through a thin snow bridge should keep you on your toes in any drainage or where you hear water flowing. All forecast areas have LOW avalanche danger today. Remember the possibility of potentially unstable snow, and also manage the emerging spring hazards.
It’s a rainy day all the way to the summit of Mount Washington, where ¼” of rain was recorded last night and another ½” to ¾” is expected today. Temperatures briefly dipped below freezing on the summit early yesterday, but otherwise above freezing temperatures have been affecting our snowpack since Friday. High temperatures in the upper 40sF are expected throughout our terrain. Tonight should bring a return to wintry conditions, with precipitation transitioning to snow this evening and NW wind ramping up towards sustained 100 mph speeds through the night. Snowfall may total 1-3” tonight and another trace-2” tomorrow morning. Tomorrow’s forecast calls for temperatures below freezing in our middle and upper elevation terrain, with snowfall tapering off but clouds remaining and wind diminishing slightly.
Primary Avalanche Problem – Wet Loose
Wet loose avalanches remain easy to initiate under skis or board in steep terrain that saw minimal traffic over the weekend, and are possible throughout our middle and upper elevation forecast areas. Keep in mind the potential consequence of being knocked off your feet and pulled downhill by these heavy, wet sluffs that typically initiate below your feet but could pull you towards hazards down slope.
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.
Secondary Avalanche Problem – Wet Slab
Wet slab avalanches are unlikely but not impossible today, with rain driving the strongest warming of our upper snowpack in recent days. The chance of water pooling, flowing, or otherwise helping form a weak layer well below the snow surface on all middle and upper elevation aspects should motivate you to continue treating avalanche terrain and runout areas with respect.
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.
Warming and wetting have been the dominant weather affects on our snowpack since late last week. Snow temperatures of 32F, which it’s worth remembering is quite warm for snow, were recorded at three feet below the snow surface on Saturday and this warming has only penetrated deeper since. Rain today is driving the most rapid melt our snowpack has yet experienced this week, or really this spring. Wet avalanches during prolonged warming events are particularly tricky to forecast. While we don’t have specific weak layers of concern, it’s possible for percolating meltwater to collect and flow on any number of buried crusts and create the conditions for a wet slab avalanche. This is scenario is unlikely but should be considered if you’re in avalanche terrain today. Wet loose avalanches will be more likely and should be relatively easy to initiate in steep terrain, especially anywhere that saw minimal skier traffic over the weekend.
The Sherburne and Gulf of Slides ski trails are snow covered to Pinkham Notch, though this is changing by the day. Expect ice patches, opening stream crossings and the occasional bare patch.
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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 04/15/2019 at 6:58 AM.
Ryan Matz, Snow Ranger
USDA Forest Service
White Mountain National Forest