Avalanche Forecast
Regions: Mt Hood.
Avoid backcountry travel on slopes with previous firm wind slab where you could trigger the slab from an area where the wind slab is shallower. Watch for new wind slab that is expected to form during the day on Sunday. New storm slab is likely in areas where there is more than several inches of rapidly accumulating snowfall.
Detailed Forecast
A warm front will move south to north over the Northwest on Sunday. S-SE winds should increase on Sunday and shift to S-SW in the south Cascades and Mt Hood Sunday afternoon. There could be a foot or more of new snow for the Saturday night to the end of daylight hours Sunday at Mt Hood with a warming trend.
Watch for new wind slab mainly on northwest to southeast slopes in the near and above treeline on Sunday. Previous wind slab will linger on other slope aspects in all the terrain bands and will have been slow to heal due to the cold temperatures.Watch for firmer wind transported snow.
New storm slab is likely at Mt Hood by later Sunday where there is more than several inches of rapidly accumulating snowfall. The slight warming trend will aid in developing new storm slab.
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Snowpack Discussion
Weather and Snowpack
A cold dry Arctic air mass was over the Northwest last week. The main weather event last week was a day or so of nuclear east winds on Wednesday that went on a powder wrecking rampage, scouring windward slopes, and re-distributing it to lee slopes or other areas.
It sounds like the winds may have eliminated much of the faceted surface snow and surface hoar that may have formed out there in the near and above treeline during the cold weather. But these weak surface crystals may still be found in sheltered areas below treeline and will need to be avoided where buried by snowfall especially above terrain traps.
Recent Observations
No reports were received on Wednesday and fierce winds that kept the ski areas from running most of their lifts also likely prevented anyone from venturing into the near or above treeline on Mt. Hood.Â
NWAC pro-observer Laura Green was at the Mt Hood Meadows Ski Area on Thursday following some of the strongest winds experienced on Mt Hood for a very long time. She found a mix of wind sculpted conditions, ranging from deep hard slabs, likely 1-2 meters or more in depth as well as scoured slopes to crust layers, sastrugi, and a thin melt-freeze sun crust on many south aspects. These deep slabs are unlikely to be triggered by a solo traveler, however, more shallow wind slabs may be a different story. Settled and stable powder was found on wind sheltered terrain, where skiing conditions remained very nice.
Laura was out again on Friday in the Mitchell/Iron Creek area in the 4200-6000 foot range and found hard slab drifts from the high wind event of 1/4 even below treeline, and in reverse of what is usually present due to the east winds. She reported lots of variation in surfaces - wind hardened snow, sastrugi, bumpy ice, settled powder and tree debris. But no signs of instability were observed.
Avalanche 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: Alpine, Treeline.
Likelihood: Likely
Expected Size: 1 - 1
Storm Slabs
Release of a soft cohesive layer (a slab) of new snow that breaks within the storm snow or on the old snow surface. Storm-slab problems typically last between a few hours and few days. Storm-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.
You can reduce your risk from Storm Slabs by waiting a day or two after a storm before venturing into steep terrain. Storm slabs are most dangerous on slopes with terrain traps, such as timber, gullies, over cliffs, or terrain features that make it difficult for a rider to escape off the side.
Storm slabs usually stabilize within a few days, and release at or below the trigger point. They exist throughout the terrain, and can be avoided by waiting for the storm snow to stabilize.
Aspects: All aspects.
Elevations: All elevations.
Likelihood: Likely
Expected Size: 1 - 1