Avalanche Forecast for Sunday, April 7, 2019
This information was published 04/07/2019 at 7:03 AM.
The Bottom Line
Warming will drive today’s wet avalanche problems. Weather forecasts indicate cloudy skies and fog that should limit the amount of warmth we see today to being much like yesterday. Wet loose sluffs can a pose a threat by carrying skiers or riders over a cliff or into trees. Watch for signs of unstable snow such as roller balls and sloppy, wet snow. A few locations, such as the Tuckerman Headwall, have the snow structure that could produce a wet slab. These are isolated in location, contained to places that saw wind slab development early this past week, and should be limited to later in the day when temperatures have enough time to penetrate into the snow. LOW avalanche danger today does not mean no avalanche danger. With springtime temperature fluctuations now in effect, watch for icy surfaces in the morning and late in the day that necessitate the use of crampons and an ice axe for safe travel.
Two and a half inches of snow arrived on westerly wind Friday night. Saturday saw temperatures go above freezing at elevations approximately 4500’ and below. Pea soup fog limited warming to being driven by temperatures rather than solar gain. Today’s temperatures will be similar to yesterday, if not a hair warmer. Cloudy skies are forecast along with summit fog again, though we may get breaks of sunshine in the afternoon. Wind will stay from the west and decrease from the current 50mph to around 30mph through the day. An approaching low pressure system will arrive tonight, spreading precipitation into the region. Temperatures will hover around the freezing mark, producing 4-8” of snow and sleet overnight and through the day tomorrow.
Primary Avalanche Problem – Wet Loose
The likelihood of wet avalanches will increase as the day progresses and weather factors warm the surface of our snowpack. Wet, mushy snow that becomes unsupportable and visible roller balls are indicators that the snowpack is weakening to the point of producing wet avalanches. These wet loose sluffs will be small in size but can carry a skier or rider over a cliff if not managed appropriately. Warming today should be limited to temperature increase rather than sunshine if the forecast pans out. If the sun is able to appear, watch for wet avalanches on all southerly aspects.
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
Some aspects in our mid-elevations have a layer of dry snow sandwiched between two melt/freeze layers. As the surface layer warms today, this could act as a wet slab. Watch for signs of warming including the development of wet loose sluffs that will indicate enough warming is happening to completely wet the surface melt/freeze layer.
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.
Snow that arrived Friday night formed wind slabs in the lee of wind from the WSW and W. Warm temperatures on Saturday penetrated around 8” into this snow at mid-elevations. These warmed and refrozen slabs sit on a mix of melt/freeze crust and unreactive older wind slab. As temperatures today mimic yesterday, expect the potential for wet avalanches to increase as the day progresses and the snow warms. Locations that have dry snow between melt/freeze crusts will be more prone to see wet snow act as a slab today, though this should take time to develop, if at all, due to slow rate of warming today.
The Sherburne and Gulf of Slides ski trails are snow covered to Pinkham Notch with lower elevation stream crossings starting to melt out.
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 04/07/2019 at 7:03 AM.
USDA Forest Service
White Mountain National Forest