Avalanche Forecast

Archived

Dec 30th, 2017–Dec 31st, 2017

Alpine

Natural avalanches unlikely, human triggered possible.

Treeline

Natural avalanches unlikely, human triggered possible.

Below Treeline

Natural avalanches unlikely.

Alpine

Natural avalanches unlikely, human triggered possible.

Treeline

Natural avalanches unlikely.

Below Treeline

Natural avalanches unlikely.

Regions

Olympics.

Keep an eye out for signs of wind transported snow such as fresh cornices, uneven snow surfaces, and wind drifts. Avoid areas of wind deposited snow, especially above treeline where more soft snow is available for transport.

Detailed Forecast

With mild weather prevailing, the snowpack will continue to adjust and gain strength. Watch for lingering wind slabs in exposed terrain, especially above treeline. Look for signs of wind transported snow such as uneven snow surfaces, fresh cornices, and snow drifts. Each of these observations are signs that the wind has redistributed snow in the area. Identify and avoid wind loaded terrain features.

Wind slabs can be deceptively difficult to manage in the terrain. Take a moment and read our recent blog post by NWAC Pro Observer Jeremy Allyn on wind slabs.

Lots of uncertainty still exists around the potential for a Persistent Slab problem. Observations on the 12/16 crust continue to demonstrate that there is potential for avalanche on this layer. Signs of instability on the 12/16 crust will be difficult to gather, but any resulting avalanche could be very consequential. Avoid large terrain convexities on W-NW-N-NE slopes below ridgelines where the somewhat reactive 12/16 layer was found on Friday.

Despite all the new snow, early season hazards still exist at some lower elevation locales and especially around creek beds that are not filled in.

Snowpack Discussion

Several inches of new snow fell in the Olympics Friday night into Saturday morning. SW winds overnight redistributed the new snow in exposed terrain forming new shallow wind slabs. This new snow and fresh wind slabs sit atop a rain crust formed during Friday’s warm temperatures.

A surface crust from mid-December can still be found within the snowpack. 2-3 feet of snow now rest on the 12/16 crust.

Observations

NWAC pro observer, Matt Schonwald visited Hurricane Ridge on Friday, 12/29 and stressed that this is a different snowpack than the Cascades! He visited W-NW-N-NE aspects and found the 12/16 crust and 1-2 mm facets down 2.5 feet. Several PST tests, self-arrested, but propagated through most of the column along the 12/16 layer. This should indicate that a persistent slab layer could be triggered most likely on large convex slopes below the ridgelines. Matt also noted wind slab developing with weaker storm layers down about 6 inches in recent snow and at 10 inches at recent/old snow interface.

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.

Persistent Slabs

Persistent Slab avalanches are the release of a cohesive layer of snow (a slab) in the middle to upper snowpack, when the bond to an underlying persistent weak layer breaks. Persistent layers include: surface hoar, depth hoar, near-surface facets, or faceted snow. Persistent weak layers can continue to produce avalanches for days, weeks or even months, making them especially dangerous and tricky. As additional snow and wind events build a thicker slab on top of the persistent weak layer, this avalanche problem may develop into a Deep Persistent Slab.