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Avalanche Forecast

Archived

Jan 22nd, 2017–Jan 23rd, 2017

Alpine
Natural avalanches possible, human triggered probable.
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.

Watch for new wind slabs forming early Sunday on exposed lee slopes at higher elevations. Allow recent storm and wind slabs further time to stabilize Sunday. Wind slabs are most likely on N-E aspects. Watch for new storm slabs if snow showers become heavy early Sunday. 

Detailed Forecast

A period of light snow showers is expected Sunday with a period of moderate to strong S-SE crest level winds early Sunday. The winds early Sunday may move available surface snow and build some new shallow wind slab layers, mainly below exposed ridges.  

Allow existing storm and wind slab time to continue stabilizing on Sunday. Wind slabs are most likely on east aspects above treeline, but will be listed on all aspects due to

A period of light snow showers is expected Sunday with a period of moderate to strong S-SE crest level winds early Sunday. The winds early Sunday may move available surface snow and build some new shallow wind slab layers, mainly below exposed ridges. 

 

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

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 to determine 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.

Light snow showers moved across the Mt Hood area near midday Saturday and deposited a few inches on new snow with generally light to moderate winds. 

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 was reported below treeline.

By Saturday, patrol at Meadows were seeing very limited results as much of the recent wind slab problem had begun settling. At elevations below the recent wind loading, storm snow lacked cohesion and was providing nice conditions.  

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.