Northwest Coastal

Expect storm slabs to continue to grow in size and sensitivity through the weekend. As the snow piles up, there is increased potential for larger avalanches on deep weak layers near the base of the snowpack. Conservative terrain choices are essential Sunday.

Looks like we’ll be in the storm track through the weekend followed by a brief break in the action Tuesday. The longer term models show another series of storms beginning Wednesday night.

SATURDAY NIGHT: Freezing level at valley bottom, strong to extreme southwest wind, 10 to 15 cm of snow.

SUNDAY: Overcast, freezing level at valley bottom, strong southwest wind, 8 to 10 cm of snow possible.

MONDAY: Overcast, freezing level at valley bottom, strong southwest wind, 5 to 15 cm of snow possible.

TUESDAY: Scattered cloud cover, freezing level at valley bottom, moderate south/southeast wind, no significant snowfall expected.

Control work Friday produced avalanches to size 3 on northwest , north, northeast and east facing slopes between 1900 and 2200 m. 

Widespread storm slab avalanche activity is expected throughout the storm this weekend.

During the cold snap, avalanche activity was predominantly wind slab related amid the strong outflows. Sizes dwindled to the 1-1.5 range in the latter part of the week.

Reports of deep persistent slab avalanches have been trickling in since the previous snowfall and wind event. They are associated with a November crust layer near the base of the snowpack, producing very large avalanches (size 3+) with crown depths of around 2 m. Observations are typically in alpine terrain, on lee or cross-loaded slopes. This may be a preview of what is to come as a significant new snow load and a large upswing in temperatures stress these layers further.

As of Saturday afternoon the storm had produced about 30 cm in the southern portion of the region with potential for another 10 to 20 cm by Sunday morning. Strong to extreme winds Saturday night are expected to readily form storm slabs and a natural avalanche cycle is likely. At alpine and treeline elevations, fat pockets of wind loaded snow are expected in lee features such as below ridgecrests and roll-overs.

The new snow is falling on extensively wind affected and faceted surfaces at all elevations. Typically, the longer a snow surface sits exposed before being buried, the less eager it is to bond to new snow. After the clear skies, frigid temperatures and extreme outflows last week, we suspect that there will be plenty of avalanche activity through the storm at the new snow interface.

A deep crust from mid November lurks at the base of the snowpack. A couple of recent large avalanches have run on this layer on lee and cross-loaded features high in the alpine. As this storm applies significant new snow loads to the snowpack, these deep layers will feel the stress and we may see some very large avalanches in high places.

• Avoid all avalanche terrain during periods of heavy loading from new snow and wind.
• Watch for changing conditions today, storm slabs may become increasingly reactive.
• As the storm slab problem gets trickier, the easy solution is to choose more conservative terrain.
• If triggered, storm slabs in-motion may step down to deeper layers and result in very large avalanches.