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Avalanche Forecast

Archived

Mar 23rd, 2015–Mar 24th, 2015

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

Possible overnight stormy conditions Monday night at Mt Hood make avalanche problems more likely on Tuesday than in the Washington Cascades.

Detailed Forecast

Light snow showers should decrease at Mt Hood Tuesday with fairly low snow levels.

Beware that the possible overnight stormy conditions may bring more snow and increase the likelihood of the avalanche problems at Mt Hood on Tuesday.

New likely wind slab should be expected on lee slopes mainly ATL or NTL at Mt Hood on Tuesday. Use extra caution near slope convexities where storm or wind slab avalanches are more likely to be triggered.

New likely storm slab should also be found in areas that receive more than an inch of snow an hour for more than several hours Monday night where there is less wind.

We are past the equinox and the sun is rapidly getting stronger and new snow will be susceptible to sun effects. Watch for likely wet snow deeper than a few inches or snowballing or natural loose wet avalanches by Tuesday midday on solar slopes. This is also more likely at Mt Hood if there is the expected heavier snowfall Monday night.

Remember the NWAC forecast applies to elevations up to the Cascade crest. A greater danger is often found above the crest on the upper volcanoes.

Less recent snow and ample terrain anchors should limit the avalanche danger below treeline. Many areas below treeline do not have enough snow to cause an avalanche danger at Mt Hood.

Snowpack Discussion

About 4-6 inches of snow was seen above about 6000 feet Saturday as reported by the Meadows ski patrol. The winds and new snow did build some stiff wind slab on northeast slopes up to 1 foot in places. But these stiff slabs were unreactive to ski cutting as the new snow fell on old wet snow and formed a good bond and thus were resistant to triggers.

No additional avalanche activity was reported Sunday by Meadows patrol.

A weak upper short wave and unstable air mass is moving over the Northwest on Monday. This is favoring the volcanoes including Mt Hood for snow showers. The Meadows patrol on Monday reported winds and rapid loading ATL. Ski cuts were beginning to cause 4-6 inch wind slab avalanches.

Another rapidly moving shortwave will carry a surface low across the north Oregon Cascades Monday night. This should bring possible overnight stormy conditions at Mt Hood.

The mid and lower snowpack at Mt Hood consists of layers of stable consolidated rounded grains or melt forms and crusts from multiple warm periods this winter. The snowpack at low elevations remains meager to non-existent.

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.

Loose Wet

Loose Wet avalanches are the release of wet unconsolidated snow or slush. These avalanches typically occur within layers of wet snow near the surface of the snowpack, but they may quickly gouge into lower snowpack layers. Like Loose Dry Avalanches, they start at a point and entrain snow as they move downhill, forming a fan-shaped avalanche. Other names for loose-wet avalanches include point-release avalanches or sluffs. Loose Wet avalanches can trigger slab avalanches that break into deeper snow layers.