This information was published 03/05/2020 at 6:55 AM.
NOT THE CURRENT FORECAST
This is an archived avalanche forecast.
The Bottom Line
Extreme winds yesterday stripped the 4” of new snow from windward slopes and deposited it into sheltered east facing terrain. These areas will contain the primary avalanche problem today and are likely to also provide the most attractive skiing surface. Be sure to consider the consequences of triggering a hard, wind slab avalanche and travel carefully, committing only one person at a time to an avalanche path. Avalanche danger is MODERATE today with natural avalanches unlikely and human-triggered avalanches possible. A long sliding fall hazard exists on steep terrain scoured to icy or firm surfaces.
Yesterday, west winds howled near 100 mph for most of the daylight hours as 4” of snow fell at higher and colder elevations. Lower on the mountain, snow was dense and less abundant, with only 1.5” of 14% density snow in the study plot at Hermit Lake. Overnight, the wind subsided, but only by a little, and shifted to the west-northwest as light snow showers continued. Temperatures in the past 24 hours fell from a maximum of 29F to the current reading of 10F on the summit.
Today, gusty WNW winds will continue this morning as high pressure settles in. Snow shower activity should end early with skies clearing and wind dying down this afternoon to a more tolerable 45-60mph on the summits.
Tomorrow, southwest flow returns and with it comes slightly warmer temperatures, high clouds and a slight chance of snow showers in the afternoon as another low passes well to our east. Wind will die down significantly.
Primary Avalanche Problem – Wind Slab
The passing low dropped more than enough snow to build wind slabs in easterly terrain. These wind slabs will likely be unreactive in most areas though the possibility of a large avalanche in steep, lee terrain should encourage you to assess the thickness and structure of the firm snow underfoot. Even stubborn, hard slabs can be triggered, typically from a thin spot in the slab. West side terrain at mid and upper elevations is likely stripped of new snow and lower elevations should be largely devoid of new snow over a melt/freeze crust.
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.
Raging winds and terrible visibility leaves us with no observations from higher terrain yesterday. Yesterday was the windiest day so far in what has been a relatively calm season with few days seeing winds near or over the century mark. The sustained winds moved any loose snow on the ground and built the hard slabs so characteristic of Mount Washington’s snowpack. You’ll find the snow to be edgeable in many places but beware of the ice crusts that are likely to be exposed and lurking nearby. It’s important to note that the deeper ice crusts have deep, weak layers of sugary facets. This facet layer broke and allowed a recent avalanche in Escape Hatch to step down and turn a large, dangerous avalanche into a slightly larger, more dangerous one during a wind loading event. This faceted layer is likely to have been swept out of the majority of our larger and more frequently running avalanche paths but it may still exist, and be near enough to the surface, to be a contributor to an avalanche from a lower starting zone, in an infrequently running path. Wind speeds like we had yesterday tend to push snow lower and build thick slabs in some of these lower start zones. If you head out today, probe frequently with your pole handle and ponder the paradox that you may actually be less likely to trigger an avalanche skiing the thickest part of a new wind slab. A safer bet would be to let these wind slabs heal for another day.
The Sherburne and Gulf of Slides ski trails are snow covered to Pinkham Notch with snow quality degrading lower on the mountain due to the recent thaw.
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. Clickhere to check it out.
Recent snowpack and avalanche observations can be foundhere and on Instagram. Your observations help improve our forecast product. Please take a moment andsubmit 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 03/05/2020 at 6:55 AM.
Frank Carus, Lead Snow Ranger
USDA Forest Service
White Mountain National Forest
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