Avalanche Forecast for Wednesday, March 18, 2020
This information was published 03/18/2020 at 7:06 AM.
The Bottom Line
New, wind transported snow brings the possibility of human triggered avalanches today. A smooth and icy snow surface exists beneath the fresh wind slabs which continue to grow this morning. Carefully assess steep terrain for these wind slabs as well as a sliding fall hazard, depending on aspect. Avalanches in a few areas of Tuckerman Ravine, Gulf of Slides and other areas with a large fetch could be large enough to bury a person. Even a small avalanche today could be a big problem due to the icy, hard surface beneath the new snow. Avalanche danger today is MODERATE. Evaluate snow and terrain carefully.
Yesterday morning, snow showers began with moderate southwest winds in the 30-40 mph range. Around mid-day, wind shifted to the west and west-northwest while increasing in speed, reaching the 70-80 mph range. Around 2” of new snow fell through the past 24 hours. Temperatures remained well below freezing at mid and upper elevations.
Today, skies will clear this morning with wind from the west slowly dropping off this afternoon when clouds build again and temperatures rise. Expect wind from the west to diminish to the lower side of the 45-60 mph range by this afternoon. Temperatures will rise to the high teens on the summit, 20’s F at Ravine elevations.
Tomorrow, a warm front arrives, bringing some more snow initially before transitioning to rain showers by evening. Expect poor visibility due to summit fog with warm temperatures in the high 20’s. Wind will be light from the south in the 20 mph range during daylight hours.
Primary Avalanche Problem – Wind Slab
Snow that is softer than ice in steep terrain is also the avalanche problem so consider the consequences of triggering a slide. Wind slabs will appear smooth and white in contrast to the gray, icy surfaces nearby. It’s easy to underestimate the potential size of an avalanche when only two inches of snow has fallen, but our terrain is configured in such a way that allows large amounts of snow to move with a westerly wind. Wind slabs are likely to be stubborn, but don’t rule out reactive slabs that can carry a crack and fail in steep terrain. Look for lower consequence and lower angles slopes to test before committing. Fresh wind slabs take some time to heal and bond.
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.
Wind transport is likely to continue through the morning though clearing skies and improved visibility and light should allow for some observations of the latest wind slab avalanche problem. Using the terrain configuration to your advantage today will be a challenge since only areas with new snow and wind slab will be at all attractive for skiing. Lower elevation glades and ski trails did not receive much new snow but may be a decent alternative if warmed by the sun this morning or increased ambient temperatures this afternoon.
The Sherburne and Gulf of Slides ski trails are snow covered to Pinkham Notch though hard and icy due to the melt-freeze cycles over the past week or so.
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 03/18/2020 at 7:06 AM.
Frank Carus, Lead Snow Ranger
USDA Forest Service
White Mountain National Forest