Avalanche Forecast for Tuesday, February 18, 2020
This information was published 02/18/2020 at 7:11 AM.
The Bottom Line
Avalanche danger will increase today as snowfall and wind create new wind slabs in our terrain. Human triggered avalanches will become more likely through the day as new snow is transported by the south wind. Natural avalanches will become possible later in the day when the combination of heavy snowfall and wind loading is expected.
Avalanche danger will rise to CONSIDERABLE at mid and upper elevation forecast zones. Anticipate increased avalanche danger due to natural avalanches as wind shifts west and loads east facing terrain tonight.
Yesterday, the MWObs recorded .7” of new snow from morning snow showers. Temperatures dropped through the day to -2F before rising again with the approaching low pressure system. West wind shifted to the S overnight.
Today, the summit hourly WX records report Light Snow began at 4am. Snow will continue through the day and overnight, with an increased intensity from about 1pm to 7pm. 3 to 6” of snow (.5” of SWE) is possible by 7pm. South wind at 25-40 mph will increase to 60-80 mph. Temperatures will remain in the mid teens F. Tonight, wind shifts W and increases to 70-90 with gusts up to 110 mph.
Tomorrow, snow showers continue with a trace to 2” possible. West wind shifts NW at 80-90 MPH with gusts up to 120. Temperatures fall to the lower teens below 0F.
Primary Avalanche Problem – Wind Slab
New wind slabs developing through the day that will vary in size and likelihood of triggering is our primary avalanche concern. It’s likely you’ll find the largest, most sensitive slabs on north facing slopes. Scouring will limit concerns on south facing aspects if the wind remains true to the forecast. Lower elevations may see cross loading and developing small wind slabs that may be sensitive to triggering as snow accumulates, particularly if it grows wet and heavier. Generally this avalanche problem will lack size to be a significant problem at lower elevations.
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.
New snow today falling on a south wind is driving the hazard rating today, and the timing of the new snow will determine what you find for developing wind slabs in the terrain. The MWObs, is calling for 3-6” while the NWS predicts 6” or more by dark. Both are in agreement that the bulk of the snow will fall in the afternoon hours and that the wind will remain south. Today will be a good day to keep an eye on wind loading and increasing avalanche danger should factor into your travel plans. For skiers, you may find great turns in a couple of inches of soft new snow on your first run, and a true avalanche problem that’s challenging to manage on your second or third run. Watch for changing conditions, and be prepared to back off to limit your risk today.
The Sherburne and Gulf of Slides ski trails are snow covered with good skiing to Pinkham Notch.
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 02/18/2020 at 7:11 AM.
USDA Forest Service
White Mountain National Forest