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Avalanche Forecast

Archived

Mar 13th, 2016–Mar 14th, 2016

Alpine
Natural and human triggered avalanches likely.
Treeline
Natural and human triggered avalanches likely.
Below Treeline
Natural avalanches possible, human triggered probable.
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.

Back country travel is not recommended in avalanche terrain near and above treeline at Mt Hood on Monday. Conservative decision making will be a good plan in the below treeline and the avalanche forecasts will get refined after new information becomes available on Monday.

Detailed Forecast

Up to about 10-20 inches of new snow seems likely at Mt Hood by Monday morning!

West winds aloft and a cool slightly unstable air mass should follow the front across the Northwest on Monday. This should cause orographic snow showers and up to another 10-15 inches of new snow seems likely at Mt Hood by the end of the day.

This is a lot of new snow.

New wind and storm slab seem likely to be the main avalanche problems at Mt Hood on Monday.

New wind slab may be deep and is mostly likely to be found on northwest to southeast slopes. Firmer wind transported snow and snowpack cracking can be good signs of wind slab.

New deep storm slab is likely in areas where new snow rapidly accumulates for more than several hours. This is likely along most of the west slopes Sunday night and Monday!

Cloudy cool conditions on Monday may limit the development of loose wet snow but the sun is gaining power so watch for the development of loose wet snow on solar slopes.

Conditions are a bit hard to predict following the low pressure system and front on Monday. The avalanche danger may decrease a little on Monday due to a little less wind, some stabilizing of new snow and fairly cool temperatures. This forecast will take a conservative approach to the danger levels and the avalanche forecasts will get refined after new information becomes available on Monday.

 

 

Snowpack Discussion

Weather and Snowpack

Storms have moved across the Northwest almost every day or two the past couple weeks with fluctuating but generally moderate snow levels.

A storm last Wednesday and Thursday caused a warming trend that peaked Wednesday night with very strong west winds seen throughout the Cascade range. About 8 inches of snow accumulated on a new crust at Mt Hood for the 2 days ending Thursday morning.

A front on Saturday caused west-southwest winds and another 4-6 inches of snow at Mt Hood ending Sunday morning.

A deep surface low pressure system is moving from the Olympic Peninsula to Vancouver on Sunday. A front is crossing the Cascades Sunday afternoon. This is causing stormy weather at Mt Hood with strong southwest winds and moderate to heavy snow.

The mid and lower snow pack at Mt Hood should generally be a stable mix of crusts and layers of moist and rounded snow crystals.

Recent Observations

On Wednesday morning, new extensive wind slabs were forming on lee slopes near and above treeline. Explosive control performed by Mt. Hood Meadows pro-patrol Wednesday produced widespread slab releases on wind loaded N-NE facing terrain with crown depths of 2 feet common. Storm slabs Wednesday morning changed to loose-wet avalanches by midday as temperatures rose and snow changed to rain below treeline. 

On Thursday, NWAC pro-observer Laura Green toured in the Mitchell and Heather drainages and found evidence of rain up to at least 6600 feet. Thin wind pockets were observed on lee slopes above treeline. A supportable rain crust did not make for good skiing, but on the plus side no avalanche problems were noted. Runnels were observed below 6000'.

The Mt Hood Meadows pro-patrol on Sunday reported stormy conditions with widespread wind and storm slab of 6-12 inches on all aspects in the area, easily triggered by ski cuts and running on the crust buried on Thursday.

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.