Avalanche Forecast
Regions: Olympics.
Dangerous avalanche conditions exist due to recent snowfall, extreme winds, and weak old snow. Avoid slopes over 35 degrees where over a foot of snow sits on weak old layers. Stay far away from steep slopes and out from under any terrain where avalanches could come from above.
Detailed Forecast
You can trigger dangerous Persistent Slab avalanches that are big enough to kill you. Persistent Slabs can surprise even very experienced travelers and can break over terrain features. Put a wide buffer of terrain between where you travel and any slopes 35 degrees and steeper. Use extra caution in areas where avalanches could come down from above.  Avoid areas where the wind has recently drifted thick slabs over older weak layers. Wind sculpted features, cracks in the snow, pillows, cornices, and variable height of recent snow are all indicators of wind loading. Persistent Slab Avalanches are not common in the Cascades. Avalanches may be surprisingly large and run farther than you expect. Make conservative terrain choices until we know more about these avalanches.
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Snowpack Discussion
About 1 foot of new snow has fallen on weak old surfaces in the Hurricane Ridge area. Near surface facets and surface hoar were observed on all aspects near treeline prior to this storm. We have many reports of similar Persistent Slab structure in other parts of the Cascades. Surprising and widely propagating avalanches have been reported from Snoqualmie Pass. In exposed terrain, winds have formed drifts on lee slopes. Below the weak, facets in the upper snowpack there are no other layers of concern. Warm wet weather from the first week of February has created a relatively uniform and consolidated lower snowpack.
Observations
On Friday, National Park Rangers triggered a large avalanche that crossed from a northeast to east aspect near treeline. The avalanche was 70 feet wide and cracks propagated beyond the crown. On Thursday, NWAC observer Matt Schonwald and Rangers were on Mt Angeles. They observed 4 inches of weak sugar-like snow on the surface (near surface facets) on all aspects and elevations traveled. They were able to trigger a small isolated wind slab on an E aspect near treeline.
Avalanche Problems
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: Possible
Expected Size: 1 - 1