Avalanche Forecast for Wednesday, February 27, 2019
This information was published 02/27/2019 at 7:04 AM.
The Bottom Line
The snowpack on Mount Washington is no stranger to high wind events but the past two days were a little unusual. The record setting high wind speeds which would ordinarily strip and scour most of the snow from our terrain was accompanied by moderate snowfall which deposited more snow than expected, in our east facing terrain anyway. A brief window of visibility and moderating wind speeds yesterday revealed sizable, smooth wind slabs deposited almost all the way to the tops of gullies (except the Northern gullies). A brief view of Boott Spur confirmed the notion that wind slabs developed during what could have been a scouring event. Expect wind slabs to exist lower in start zones in terrain to the lee of a west wind. MODERATE avalanche danger exists with human-triggered avalanches possible on recently wind loaded slopes. Lower elevation areas may contain softer and more reactive wind slabs.
No new snow in the past 24 hours following the 25cm that was recorded at Hermit Lake through this storm and subsequent lake effect snowfall. Streamers of moisture from the Great Lakes added to our more typical up-slope enhanced snow and the initial storm on Monday to deposit one and a half inches of SWE on the summit. Yesterday, precipitation wound down while wind continued from the WNW in the 70-90 mph range. Today, NW wind will diminish to 30-45 mph today with ambient air temperature rising to around -5F after a cold start. Current temperature is -18F, up from -27F earlier this morning, on the summit.
Primary Avalanche Problem – Wind Slab
Wind slabs built during the recent snowfall and wind event are gaining strength slowly due to the cold temperatures. These wind slabs are likely to be stubborn to the point of being unreactive in the typical mid and upper elevation avalanche paths like Tucks and the Gulf of Slides but possibly more reactive than stubborn in the sheltered locations at lower elevations like the slides on Mount Webster. As with any firm wind slab, triggering often occurs at a thin spot in the slab and propagates through the thicker parts. The thin spot can be at the bottom edge of the slab such as we’ve seen near the floor of Tuckerman Ravine below the Headwall or over a submerged boulder.
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.
Among the surprises for field observers yesterday was a 3’ crown line on the north side of Raymond’s Cataract. The Lower Snowfields also contained a refilled crown line beneath the rock slab below Dead End gully. Past high wind and snow events have a history of building wind slabs in Lower Snowfields and it seems likely that you could find a similar loading pattern low on gullies and snowfields in Gulf of Slides and anywhere else in the range that has a large flat expanse (fetch) of snow available upwind of a steep open slope or gully. The other surprising field observation yesterday was the lack of old ice crust exposed. The February ice crust been exposed by wind in a number of places in east facing terrain despite 85cm (33”) of snowfall (at Hermit Lake) since its development. The new snow associated with this wind event refilled the terrain nicely, at least on the east side. We hope to gather observations from the west side to see if mid elevation gullies, which have been lacking in snow, have gained any coverage. Please submit photos of west side or any other interesting avalanche activity or developed avalanche paths that you may encounter in the field. You can upload photos and videos here or use #mwacobs or @mwacenter on Instagram.
The Sherburne and Gulf of Slides ski trails are snow covered to Pinkham Notch.
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 02/27/2019 at 7:04 AM.
USDA Forest Service
White Mountain National Forest