Avalanche Forecast
Issued: Mar 22nd, 2018 11:00AM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Wind Slabs.
NWAC,
Northwest Avalanche Center
The avalanche danger will gradually decrease Friday. Wind Slabs formed Thursday will slowly heal and stabilize. However, you will be able to trigger a Wind Slab avalanche on steep lee slopes, mainly below ridges or convexities on steep slopes. Fresh cornices have formed along ridges and if dropped may trigger a slide on slopes below.
Summary
Detailed Forecast
New Wind Slabs have formed on lee slopes near and above treeline in the Mt Hood area. These Wind Slabs will slowly heal and stabilize Friday but you will still be able to trigger a slab avalanche on steep slopes that have received wind deposited snow. Use visual clues to assess and avoid wind loaded features. Snow drifts, fresh cornices, and uneven snow surfaces with cracking can all indicate wind slabs exist on nearby slopes.
Strong winds have stripped much of the new snow down to older hard surfaces on many exposed slopes.
The new snow began as rain Thursday and gradually cooled below treeline forming a good bond to the old snow surface. Lighter winds and shallower new snow will make avalanches less likely at lower elevations Friday.
Snowpack Discussion
As of Thursday afternoon, about 4-6" of new snow had accumulated in the Mt Hood area. The new snow was easily transported by moderate to strong southerly winds, building sensitive Wind Slabs on lee slopes near and above treeline.
Several previous days of sunny, spring-like weather created a strong melt-freeze crust on sun exposed slopes and a weaker, thinner crust on shaded slopes, now below the 6 plus inches of new snow.
Several melt-freeze cycles the weekend of March 10-11 helped to consolidate the snowpack.
While we are tracking several layers deeper in the snowpack, there are currently no significant layers of concern.
Observations
On Thursday, MHM pro patrol reported explosive triggered slab avalanche releases on steep lee slopes, mainly above treeline. These were fresh wind slabs formed from transport of the new snow and ranged on average from 8" up to 2' in isolated areas.
On Wednesday, MHM pro patrol reported a thin crust on non-polar slopes with 3-4” of old cold snow on shaded slopes and the afternoon precipitation dissolved into existing snow surfaces.
On Tuesday, MHM pro patrol reported shaded terrain holding soft snow while the sun was softening snow surfaces on non-polar aspects. Very minor loose wet activity was reported below treeline. He noted that the NWAC Cascade Express station at MHM was reading higher than observed temperatures on Tuesday due to its proximity to a dark roof surface.
Problems
Wind Slabs
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 Slabs form in specific areas, and are confined to lee and cross-loaded terrain features. They can be avoided by sticking to sheltered or wind-scoured areas..
Wind Slab avalanche. Winds blew from left to right. The area above the ridge has been scoured, and the snow drifted into a wind slab on the slope below.
Wind slabs can take up to a week to stabilize. They are confined to lee and cross-loaded terrain features and can be avoided by sticking to sheltered or wind scoured areas.
Aspects: All aspects.
Elevations: All elevations.
Likelihood
Expected Size
Valid until: Mar 23rd, 2018 11:00AM
