Avalanche Forecast for Wednesday, April 10, 2019
This information was published 04/10/2019 at 7:15 AM.
The Bottom Line
New wind slabs formed from drifting snow overnight may be possible to human trigger. Assess smooth areas of new snow to avoid this avalanche problem. With colder temperatures today, the wet older snow from Monday will be refreezing though the day, resulting in a hard icy surface capable of sustaining a long sliding fall, especially later in the day. An ice axe, crampons and your ability to not fall will be required once the existing snow freezes. A small avalanche could easily cause a sliding fall. Avalanche danger is MODERATE today, evaluate snow and terrain carefully.
Yesterday, the summit of Mount Washington reached a high temperature of 32F with middle and lower elevations remaining well above freezing. The temperature dropped in the afternoon enough for evening precipitation to fall as snow showers, with 1.9” recorded on the summit on WSW wind at 50 mph. The Hermit Lake Snow Plot recorded 2.75” of snow in the past 24 hours. Today, we start the day with 9F on the summit rising only to the mid teens F in the fog under cloudy skies. NW wind will diminish a bit mid-day but otherwise remain fairly steady at 45-60 mph.There is a chance of snow showers with accumulations of a trace to 2”, mainly in the morning. Tomorrow’s forecast offers more settled weather as high pressure brings clear skies, light/moderate winds and summit temperatures in the low 20s F.
Primary Avalanche Problem – Wind Slab
New wind slabs formed from overnight snow may be reactive to a human trigger. These wind slabs will be found mainly on the eastern half of the compass at elevations above 3500’ beneath and in the lee of steep terrain features.. Look for wind drifted snow as the avalanche problem today which will appear as soft, smooth areas of new snow. In locations where the slabs are thickest and reactive, they could be large enough to carry and perhaps bury a person.
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.
The roughly 3.5 inches of mixed precip types that fell on upper elevations on Monday, were ultimately wetted to the bed surface at ravine levels by several hours of freezing rain in the late afternoon & evening. Warm air Tuesday morning melting an ice crust may also have played a part in adding water to Monday’s snow. Due to southern wind direction with limited fetch, moderate wind speeds, dense, sticky snow and possibly an ice crust, drifting snow was limited resulting in an evenly distributed covering of snow. By Tuesday, this moist snow covering was found at all levels in the ravines. Overnight, temperatures dropped well below freezing at mid and upper elevations which will eventually freeze the existing snowpack solid, if it hasn’t done so already. Any instabilities in the snowpack today will be limited to new windslab formed from the 1.9” of snow recorded on the summit overnight.
The Sherburne and Gulf of Slides ski trails are snow covered to Pinkham Notch with some 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 04/10/2019 at 7:15 AM.
Jeff Fongemie, Snow Ranger
USDA Forest Service
White Mountain National Forest