Avalanche Forecast
Regions: Olympics.
There is a high degree of uncertainty in the current snowpack structure and likely widely varying conditions. Choose conservative and low consequence terrain and evaluate the snowpack carefully Thursday. Persistent weak layers may lurk below the recent storm snow and may be sensitive to human triggering. Observe local wind loading patterns and allow for wind slab even into the below treeline band.
Detailed Forecast
A weak front will move across the Olympics Thursday, depositing generally light amounts of new snow, but arrive with a period of moderate ridge top winds. This may build some new shallow slab layers on some lee slopes at higher elevations near ridges.
Wind slab should be the primary problem Thursday with the possible new persistent slab problem a close second problem.Â
We know in at least one area, Stevens Pass, that recently buried surface hoar from 12/17 was buried intact and sensitive in snowpit tests. Strengthening crusts in many areas near and below treeline may limit how sensitive this layer will be to human triggering. For elevations that did not see rain, wind loaded slopes could present the dangerous combination of a deeper slab overlying a persistent weak layer. We need more information about the sensitivity and spatial distribution of the 12/17 PWL throughout the west slopes, but based on limited observations and weather station data, persistent slabs are more likely an issue from Stevens Pass and north.Â
Be suspicious of any wind loaded slope Thursday, especially one that might harbor the combination of wind slab and a persistent weak layer.Â
With a high degree of uncertainty as to the extent of, or sensitivity to trigger of persistent slabs, choose conservative and low consequence terrain and evaluate the snowpack carefully Thursday before committing to terrain of consequence. Persistent weak layers may lurk below the most recent storm snow and may be sensitive to human triggering. Observe local wind loading patterns and allow for wind slab even into the below treeline band.Â
Snowpack Discussion
Weather and Snowpack
Clear and cold weather from Wednesday, December 14th to Friday, December 17th allowed widespread surface hoar and near surface faceted snow to develop in the Cascades and Olympics. Thin sun crusts formed on steeper solar slopes during sunny periods. In many areas, these persistent grain types were buried intact by December 18th during a period of light snow and light winds.
Strong westerly flow directed two Pacific frontal systems into the PNW Sunday night and again Monday night with generally up to an inch of water accumulating at the Hurricane Ridge station through early Tuesday morning. Storm snow totals varied quite a bit due to a mix of precipitation types during the storm cycle. Many areas in the Cascades experienced a natural avalanche cycle involving either shallow, loose wet or storm slabs Monday night or early Tuesday morning.  A strengthening rain crust was noted in many areas by later in the day Tuesday with the arrival of colder air. Â
Westerly winds were especially strong with the 2nd system late Monday night and into Tuesday with gusty winds mixing down into usually more wind sheltered terrain.
Cool, dry weather with sunshine Wednesday helped settle recent storm snow and possibly create a thin sun crust on steep directly south facing slopes. Â
Recent Observations
NWAC pro-observer, Matt Schonwald was at Hurricane on Friday and found a mostly right side up snowpack. The December 8th layer was found at a depth of 30 cm but gave moderate to hard compression tests and did not show signs of propagation. Isolated small wind slab was found on NE slopes above 5000 feet. He noted widespread large surface hoar and near surface faceted snow.
Surface hoar at Hurricane Ridge on December 16th. Photo by Matt Schonwald.
An observation for Hurricane for Saturday via the NWAC Observations page indicated surface hoar and near surface faceted snow in sheltered a spot sheltered from the sun. But perhaps not as widespread as seen by Matt Schonwald on Friday.
The road to Hurricane Ridge is closed midweek and no observations have been received since the weekend.Â
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, North West.
Elevations: All elevations.
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: Possible
Expected Size: 1 - 1