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Avalanche Forecast

Archived

Dec 1st, 2016–Dec 2nd, 2016

Alpine
Natural avalanches unlikely, human triggered possible.
Treeline
Natural avalanches unlikely, human triggered possible.
Below Treeline
Natural avalanches unlikely, human triggered possible.
Alpine
Natural avalanches unlikely, human triggered possible.
Treeline
Natural avalanches unlikely, human triggered possible.
Below Treeline
Natural avalanches unlikely, human triggered possible.

Regions

Mt Hood.

New shallow wind and storm slab layers seem possible by later Friday mainly in the near and above treeline at Mt Hood due to winds, new snow and a warming trend.

Detailed Forecast

A frontal system will cross the Northwest on Friday. This will cause increasing southwest to west alpine winds, increasing rain or snow and a rise in snow levels.

Perhaps about 5-7 inches of snow seems likely at Mt Hood mainly in the near and above treeline band by later Friday with a warming trend.

This weather should build new upside down wind slab on lee slopes which should be mainly southeast to northwest slopes near and above treeline by later Friday. Watch for firm wind transported or hollow sounding snow.

New shallow storm slab is possible in areas that see rapid accumulations of new snow by later Friday. The warming trend will also aid in the development of new upside down storm slab.

Small human triggered avalanches in many areas in the near and above treeline seems possible by later Friday at Mt Hood due to the alpine winds, new snow and the warming trend.

In the lower part of the below treeline band there is still insufficient snow for avalanches but watch for early season hazards such as terrain traps, rocks and creeks.

Snowpack Discussion

Weather stations on Mt. Hood picked up 11 and 15 inches of new snow at Timberline and Mt. Hood Meadows respectively Monday morning. Another 7 to 9 inches fell during heavy showers through Monday evening. West to northwest winds remained moderate on Monday helping to transport new snow to lee easterly aspects. 

The Mt. Hood Meadows pro patrol found sensitive, but soft 6-12 inch storm slabs on most aspects Monday morning failing within the new storm snow.  Wind slabs averaged 18 inches on north aspects above treeline with a crust for a bed surface that formed during a warm-up last Friday. 

On Tuesday the Mt Hood Meadows pro patrol found shallow storm slabs near and below treeline generally unresponsive to ski cuts and explosives.  However, evidence of two larger natural wind slab avalanches was observed in White River Canyon, on east to southeast aspects, both with start zones around 7000 feet. One stepped down to deeper layers.

Another front passed over the Northwest on Wednesday. NWAC stations at Mt Hood had winds generally in the teen to 20 mph range with variable, very strong gusts. New snow on Thursday morning was around 10 inches at NWAC stations at Mt Hood.

A bit of a break has been seen on Thursday with snow showers tapering at Mt Hood.

NWAC pro observer Laura Green was at Mt Hood Meadows ski area on Thursday and reported significant wind transport but with minor small wind slab via ski area control results. But wind slab will be deeper in areas not visited by the ski area control crews.

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.