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Avalanche Forecast

Archived

Mar 22nd, 2017–Mar 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 possible, human triggered probable.
Below Treeline
Natural avalanches possible, human triggered probable.

Regions

Mt Hood.

Be aware that conditions and avalanche problems will change with elevation and aspect. Solar effects can occur rapidly at this time of year. Cornices have recently proven dangerous and unpredictable and capable of triggering very large avalanches.

Detailed Forecast

A break between weather systems should be seen on Thursday. Sun is most likely in the Olympics, the south Cascades and east of the crest but watch for solar effects in all areas.

Recent winds have been mostly S-SW so firmer wind slab should be found mainly on NW-SE slopes. But there have been winds from other directions lately such as local east winds on Monday so watch for firmer wind transported snow on all aspects, especially in areas of complex terrain.

Watch for solar effects in all areas especially in areas where there has been more than a few inches of snowfall the past couple days. Solar effects should make loose wet avalanches possible mainly on solar slopes in all the terrain bands but keep an eye out on all aspects. Watch for surface wet snow deeper than a few inches, pinwheels and increasing natural releases.

Make sure you avoid areas on ridges where there may be a cornice and slopes below cornices! Natural cornice releases and resulting slab avalanches are unpredictable.

See a blog post regarding cornices here.

Increasing alpine winds, increasing clouds, and increasing rain or snow should be seen Thursday evening as the next front begins to move into the Northwest.

Snowpack Discussion

Weather and Snowpack

The first week or so of March was very cool and snowy.  NWAC stations at Mt Hood piled up about 6-7 ft of snow.

The 2nd week of March was equally active with non-stop Pacific frontal systems pummeling the PNW. Unfortunately, these systems delivered far more rain than snow. At least two regional avalanche cycles occurred during the stretch. Significant snowpack consolidation occurred over this period due to rainfall and warmer temperatures. 

Another strong low pressure system brought about an inch of predominately rain to the NWAC Mt. Hood stations Friday 3/17. Rapid cooling Saturday morning was followed by snow showers with light new snow accumulation. Strong W-SW winds were transporting new snow above treeline by Saturday mid-day.

Sunday was cool with light winds and mostly sunny skies at Mt. Hood. Increasing clouds Monday with moderate daytime warming allowed additional slow snowpack settlement. 

Weaker fronts crossed the Northwest on Tuesday and Wednesday. This will be continuing to cause snow mainly in the above treeline areas. 2 day snowfall ending Thursday morning may be about 5 in in the above treeline at Mt Hood.

Recent Observations

Mt. Hood Meadows pro-patrol reported a switch from rain to snow at mid-mountain by noon Saturday with strong W-SW winds beginning to build fresh new wind slab above treeline. 

NWAC Observer, Laura Green was out Saturday and observed a snowscape of deep rain runnels from overnight rains. Along exposed ridges in higher elevations, wind slabs were forming along lee ridges as well as cornices.

Observations from Mt Hood Meadows Tuesday indicated light rain had maintained shallow moist to wet surface snow, but no natural or triggered loose wet avalanches were reported. Above treeline the strong winds and light snowfall began to build shallow wind slabs.

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.

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.

Cornices

Cornice Fall is the release of an overhanging mass of snow that forms as the wind moves snow over a sharp terrain feature, such as a ridge, and deposits snow on the downwind (leeward) side. Cornices range in size from small wind drifts of soft snow to large overhangs of hard snow that are 30 feet (10 meters) or taller. They can break off the terrain suddenly and pull back onto the ridge top and catch people by surprise even on the flat ground above the slope. Even small cornices can have enough mass to be destructive and deadly. Cornice Fall can entrain loose surface snow or trigger slab avalanches.