Avalanche Forecast
Regions: Olympics.
A decreasing avalanche danger will be seen on Saturday. Watch for new signs of wind transported snow on lee slopes on Saturday. Avoid large terrain convexities on W-NW-N-NE slopes below ridgelines where the somewhat reactive 12/16 may be present until we know more about it.
Detailed Forecast
A change to moderate to strong WSW winds and cooling should be seen Friday night following the front. Hurricane should get about 2-4 inches of new snow with a cooling trend by Saturday morning.
Overall this will lead to a decreasing avalanche danger on Saturday. The expected modest new snow and the cooling trend should favor up right side up snow.
With the recent SSE winds and the change to WSW winds Friday night new wind slab is possible on about W-SE aspects. Avoid lee slopes with these aspects especially if you see signs of firmer wind transported snow.
The persistent slab at Hurricane seen by Matt is a low probability but high consequence avalanche problem. Avoid large terrain convexities on W-NW-N-NE slopes below ridgelines where the somewhat reactive 12/16 was found on Friday, until we know more about it.
Also 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
We are at the tail end of a couple of days of very active changeable complex weather. On Thursday strong westerly flow aloft carried a very moist occluded front across the NW. On Friday a moderately deepening low pressure system is crossing the Cape Flattery area and a strong cold front will cross the Northwest Friday evening.
By Friday morning there was 3 inches of new snow that fell with an overall slight warming trend at Hurricane. Winds on Friday have been generally SSE 20's gust 40's on Friday with and it looks like the snow level rose enough for a change to rain at Hurricane.
Regular though ususally light snow also accumulated at Hurricane as very weak weather disturbances crossed the Northwest in about the past week.Â
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 at 75-80 cm. 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 a large convex slopes below the ridgelines. Matt also noted wind slab developing with weaker storm layers down about 17 cm in recent snow and at 25 cm at recent/old snow interface.
Avalanche Problems
Wind Slabs
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.
Wind Slabs form in specific areas, and are confined to lee and cross-loaded terrain features. They can be avoided by sticking to sheltered or wind-scoured areas..
Wind Slab avalanche. Winds blew from left to right. The area above the ridge has been scoured, and the snow drifted into a wind slab on the slope below.
Wind slabs can take up to a week to stabilize. They are confined to lee and cross-loaded terrain features and can be avoided by sticking to sheltered or wind scoured areas.
Aspects: North, North East, East, South East, West, North West.
Elevations: Alpine, Treeline.
Likelihood: Possible
Expected Size: 1 - 1
Persistent Slabs
Release of a cohesive layer of soft to hard 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 Slabs.
The best ways to manage the risk from Persistent Slabs is to make conservative terrain choices. They can be triggered by light loads and weeks after the last storm. The slabs often propagate in surprising and unpredictable ways. This makes this problem difficult to predict and manage and requires a wide safety buffer to handle the uncertainty.
This Persistent Slab was triggered remotely, failed on a layer of faceted snow in the middle of the snowpack, and crossed several terrain features.
Persistent slabs can be triggered by light loads and weeks after the last storm. You can trigger them remotely and they often propagate across and beyond terrain features that would otherwise confine wind and storm slabs. Give yourself a wide safety buffer to handle the uncertainty.
Elevations: Treeline, Below Treeline.
Likelihood: Unlikely
Expected Size: 1 - 2