Avalanche 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. 

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. 

Be suspicious of any wind loaded slope Thursday, especially one that might harbor the combination of wind slab and a persistent weak layer. 

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. 

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.  

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.  

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.