Avalanche Forecast
Regions: Cascades - East.
Avoid steeper terrain features with firmer wind transported snow. Be prepared to step back from your plans if the next system arrives stronger and sooner than expected. The extent of a persistent slab is still uncertain, use conservative terrain selection to maintain a margin of safety.
Detailed Forecast
The next system should begin to bring light snow mainly the south Cascades Sunday afternoon. Alpine winds should stay fairly light with no great change in snow levels or cool temperatures.
Keep an eye out for previous or newly forming wind slabs on Sunday. Winds will have potentially loaded non-traditional aspects. If you see signs of winds transporting snow including plumes, drifts, and uneven snow surfaces, then avoid slopes where the wind loading is occurring.
Be prepared to step back from your plans if the next system arrives stronger and sooner than expected.
Also despite all this new snow, early season hazards still exist at lower elevations and especially around creek beds that are not filled in.
 Lots of uncertainty exists around the distribution and sensitivity of the 12/15 layer. Limited observations on this layer make it difficult to forecast. When uncertainty goes up, terrain selection must go down to maintain wide margins of safety. Time and patience will allow this layer to gain strength.Â
Snowpack Discussion
A few inches of light snow fell across the east slopes of the Washington Cascades in the 24 hours ending Saturday morning. Up to about 3 feet of recent snow sits atop the 12/15 crust/persistent weak layer. Overall recent snow has been gaining strength leading to a recent stabilizing trend.
Ridge top winds Tuesday to Thursday redistributed snow in exposed and wind prone areas forming wind slabs on a variety of aspects. Limited information about the location and sensitivity of these winds slabs has been received.Â
A large amount of uncertainty exists around the 12/15 crust/persistent weak layer. This layer has generally been observed at elevations below 6000 feet. Snowpack tests in a few locations continue to demonstrate results and the potential for an avalanche to propagate. The limited amount of incoming information makes it difficult to paint a clear picture of this potential avalanche problem.
Another rain crust, a few inches above the 12/15 crust, is seen in some pits in some areas below treeline east of the crest.
Snowdepth decreases substantially the further east from the Cascade crest. In many areas below treeline, there is not enough snow to present an avalanche danger.Â
Observations
North
Many observations are coming from the NCMG through the week.
On Saturday in the Washington Pass area recent snow was well bonded and small steep slopes did not give results in slope tests. There was widespread new surface hoar.
On Friday at the Washington Pass Hairpin they found 45-65 cm of storm snow on the 12/15 crust, with an ECTP21 result in 2 cm buried surface hoar on the 12/15 crust. However, the buried surface hoar was not seen in pits above 5300 feet.
On Thursday on Delancey Ridge they noted blowing snow, moderate CT results in storm snow and a ECTP result between crust layers at about 48 cm.
Central
On Thursday, professionals on Dirtyface Peak observed the 12/15 layer/PWL interface reactive in PST and ECT tests at 5000 ft on an east aspect. The interface was 2.5 ft down. Wind transport was occurring near and above treeline and actively loading lee slopes. Recent storm instabilities were healing.Â
South
No recent observationsÂ
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: All aspects.
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.
Aspects: All aspects.
Elevations: All elevations.
Likelihood: Unlikely
Expected Size: 1 - 1