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Avalanche Forecast

Archived

Mar 20th, 2017–Mar 21st, 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.

Above treeline, dangerous avalanche conditions should develop through the day with storm or wind slabs becoming touchy by afternoon. At lower elevations, shallow wet snow conditions should maintain the possibility of mostly small loose-wet avalanches. Avoid overhead hazard, such as the recently formed large cornices.

Detailed Forecast

A weak disturbance will spread increasing light rain and snow at gradually rising freezing levels overnight Monday. Only light amounts of precipitation are expected overnight. This should begin to wet and weaken shallow surface snow layers.

A stronger band of moisture Tuesday should bring periods of moderate rain and snow at gradually rising freezing levels. Winds should be moderate along the ridges above treeline Tuesday.

These conditions should cause dangerous avalanche conditions through the day, building fresh wind slabs at higher terrain along ridges, mainly on NW-NE-E facing terrain. The wind slab problem should become increasingly sensitive to human trigger, especially by Tuesday afternoon. Watch for firmer wind transported snow on all aspects, especially in areas of complex terrain. 

At lower elevations, light rain should maintain shallow wet snow conditions and make small loose-wet snow avalanches possible.

Avoid areas with terrain traps where a small loose wet avalanche could have unintended consequences. 

It is always a good plan to travel well back from ridges, suspected of cornice formation, or on steep slopes below cornices.

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. 

After a short respite from the active weather pattern on Thursday, another strong low pressure system brought about an inch of predominately rain to the NWAC Mt. Hood stations Friday night and Saturday morning. Rapid cooling late Saturday morning was followed by snow showers with light new snow accumulation. Strong W-SW winds were transporting new snow above treeline by mid-day Saturday. 

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. 

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.  

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.