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Avalanche Forecast

Archived

Jan 20th, 2017–Jan 21st, 2017

Alpine
Natural avalanches unlikely, human triggered possible.
Treeline
Natural avalanches unlikely, human triggered possible.
Below Treeline
Natural avalanches unlikely, human triggered possible.
Alpine
Natural avalanches possible, human triggered probable.
Treeline
Natural avalanches unlikely, human triggered possible.
Below Treeline
Natural avalanches unlikely, human triggered possible.

Regions

Mt Hood.

Allow recent and shallow storm and wind slab time to stabilize on Saturday. Wind slabs are most likely on easterly aspects above treeline but will be listed on all aspects due to variable loading patterns during the last few days. 

Detailed Forecast

Light to occasionally moderate snow showers should taper off Friday night before increasing again Saturday afternoon. Alpine winds may increase quickly Saturday afternoon out of the SW.  

Allow recent and shallow storm and wind slab time to stabilize on Saturday. Wind slabs are most likely on easterly aspects above treeline but will be listed on all aspects due to variable loading patterns during the last few days. Look for recent wind effects near and below ridgelines and treat wind loaded slopes with caution.

Watch for shallow storm slabs on all aspects and in all elevation bands Saturday if sensitive storm layers form during heavier showers either Friday night or Saturday afternoon.  

Snowpack Discussion

Weather and Snowpack

An arctic air mass was over the Northwest with fair, cold weather last week. Surface hoar and near surface faceted crystals formed in wind and sun sheltered areas during this period. Observations over the next few days should help determine to what extent any of these weak persistent grain types might have survived the atmospheric river event and remain relevant to the forecast moving forward.

An atmospheric river moved over the Northwest Tuesday and Wednesday. Heavy rain was seen up to about 7000 feet at Mt. Hood. 3 day precipitation totals through noon Thursday were about 1.5 - 2 inches of water at the Meadows and Timberline stations. Light snow showers and cooler temperatures were seen Thursday. A frontal band rotating up from the south dropped about 6 inches of snow at Meadows and Timberline through 6 pm Friday with steady snow levels seen during the day.  

Recent Observations

The Mt Hood Meadows pro-patrol on Wednesday reported that rain penetrated about 10 inches into the snow with no results during avalanche control using explosives. Visibility was low on Wednesday and it could not be determined if there had been any natural avalanches on the upper mountain.

Visibility remained poor on Thursday but Meadows pro-patrol reported wind slabs releasing on NE aspects well above treeline control work with explosives. Wind slabs became less reactive near tree-line and no avalanche activity or layers or concern were reported below treeline.  

Problems

Wind Slabs

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.

Storm Slabs

Storm Slab avalanches are the release of a cohesive layer (a slab) of new snow that breaks within new snow or on the old snow surface. Storm-slabs typically last between a few hours and few days (following snowfall). 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.