Avalanche Forecast for Monday, April 20, 2020
This information was published 04/20/2020 at 6:50 AM.
The Bottom Line
Recent snowfall was affected by NW wind creating wind slabs in east facing ravines. The chance for continued loading exists today due to elevated wind speeds and available snow overnight. In addition, wet loose avalanches may become possible if there is enough warming effect from solar gain and ambient air temperatures. The possibility of small to large wind slabs and small wet loose avalanches warrants a MODERATE danger rating today.
Yesterday, summit temperatures peaked at 30F and the sky remained clear most of the day. Wind was calm at 30-50 mph from the W. After daylight hours 0.8” of new snow was recorded on the summit
Today, a trace to 1” of snow is possible in the morning. As snow showers taper off the skies should clear for the afternoon. Summit temperatures will be in the upper 20s F and wind will begin the day from the NW between 60-80 mph. Around midday the wind speed will begin to drastically decrease towards single digit mph and remain from the NW.
Tomorrow a discrepancy between MWObs and NWS exists at this time regarding the amount of snowfall we will get with possibly up to 6” forecast Regardless, fresh snowfall is on the way and will be accompanied by a SW wind between 30-50 mph. Day time temperatures will reach low 20sF at the summit.
Primary Avalanche Problem – Wind Slab
New snowfall over the last 24 hrs has resulted in 1.4” at the summit with blowing snow being recorded at 0700. Watch for wind slabs near terrain features that can create pockets of wind drifted snow such as rock outcroppings, rock buttresses and convex slopes. If you do trigger a wind slab it will most likely not be enough snow to bury you but could put you into unwanted terrain or cause you to fall on the old snow surface resulting in a long sliding fall. Slabs may be difficult to trigger but can become more reactive if warming weakens the strength of the slab.
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
Warm ambient temperatures and solar effects will influence the top layers of the snowpack today creating conditions for wet loose avalanche hazards. Summit temperatures are forecast to be in the mid 20sF and cloud cover will become reduced as the day continues. Snow in the ravines may get warm enough to cause a point release avalanche on steep slopes. Some areas may warm faster than others, locations of note are areas where heat transfer occurs more rapidly such as near rocks or buttresses and south facing slopes.
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.
It may feel like spring in the valley as grass is turning green in yards, but it is still winter in the mountains. Over the past week we have been nickel and dimed to gain near 7” of snowfall at the summit. More snow is in the forecast for this upcoming week as well. Currently a myriad of conditions exist in the Presidential Range. Sunny calm days down low have not correlated up high at all times. Wet snow that is difficult to travel in exists at lower elevations while areas of old surface that require crampons and an ice axe lie next to wind slabs at higher elevations.
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 04/20/2020 at 6:50 AM.
USDA Forest Service
White Mountain National Forest