Avalanche Forecast for Saturday, April 6, 2019
This information was published 04/06/2019 at 7:02 AM.
The Bottom Line
New snow overnight and this morning is combining with westerly wind to build new wind slabs that are possible for you to trigger. We expect these possible avalanches to be small in most of our terrain, but continued snowfall this morning means that you should make careful observations of the size and location of this avalanche problem. Larger wind slabs may develop, and even a small avalanche in high consequence terrain is a big deal. As the day progresses, watch for warming and rainfall to make wet avalanches possible as well. All forecast areas have MODERATE avalanche danger. You may be able to avoid today’s avalanche problem by travelling on old melt freeze crust that is distinctly grey in appearance. The presence of this icy crust at or near the surface means that anyone considering travel on steep snow slopes today should bring crampons, an ice axe, and a strong ability to use them to prevent a long sliding fall.
Snow accumulation overnight totaled 1-2 inches at the Summit and our 4000’ elevation snow plots. Light snow with some mixed frozen precipitation types continues this morning and may total up to another 2”. We should see a brief transition to rain in our middle and lower elevation terrain as precipitation tapers off late this morning. The forecast rain/snow line seems to be roughly 4000’-5000’ in elevation, meaning that the Ravines are on the cusp of receiving some rain. Regardless of precipitation type, above freezing temperatures in middle and low elevation terrain may combine with partial clearing to make warming a key weather factor today. Wind from the NW should hold near 50 mph on the summit today and tonight before diminishing slightly tomorrow. Forecasts for tomorrow call for no precipitation, partial cloud cover, and temperatures near or above freezing.
Primary Avalanche Problem – Wind Slab
Wind slabs began to form on W wind overnight and continue to develop this morning as wind shifts NW. We expect that these wind slabs will be small, but size will ultimately be determined by amount of additional snowfall this morning. Warming may further increase the likelihood of human triggered slab avalanches through the day. You’re most likely to find this reactive new layer on the eastern half of the compass rose. Wind slabs formed earlier in the week seem well bonded to the old melt freeze crust and more stable, but will be visually similar to the new wind slabs which may challenge your observations.
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
Between possible rain, above freezing temperatures, and some sun peeking through the clouds, we expect warming to make wet avalanches increasingly possible through the day at middle elevations. Wet loose sluff avalanches will be mostly confined to the new snow. Look for a moistening snow surface and roller ball activity as clues that wet avalanches are possible.
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.
Prior to last night’s snow, our snow surfaces were a mix of old, robust melt freeze crust and thin, unreactive wind slabs formed nearly a week ago. Until we have a significant warming event, we have no stability concerns below this melt freeze crust. New wind slabs which continue to develop this morning are the primary stability concern. This same new snow may warm later today to make wet avalanches a problem to manage. Continued snowfall and potential rain this morning will ultimately determine the size, sensitivity, and type of avalanche problem that you’re dealing with today. Pay close attention to how today’s weather plays out and make careful observations of the developing avalanche problems.
The Sherburne and Gulf of Slides ski trails are snow covered to Pinkham Notch, with a few areas of open water at low elevations.
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 04/06/2019 at 7:02 AM.
Ryan Matz, Snow Ranger
USDA Forest Service
White Mountain National Forest