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Avalanche Forecast

Archived

Jan 19th, 2017–Jan 20th, 2017

Alpine
Natural avalanches possible, human triggered probable.
Treeline
Natural avalanches possible, human triggered probable.
Below Treeline
Natural avalanches unlikely, human triggered possible.
Alpine
Natural avalanches unlikely, human triggered possible.
Treeline
Natural avalanches unlikely, human triggered possible.
Below Treeline
Natural avalanches unlikely, human triggered possible.

Regions

Olympics.

We are still getting a handle on the snowpack post storm, so be prepared to evaluate the snowpack carefully. Storm slabs remain possible on Friday due to potentially moderate snowfall rates Thursday afternoon and evening forming unstable storm layers. A period of moderate E-SE winds Friday may build new wind slab near and above treeline on westerly aspects. 

Detailed Forecast

Snow showers on Thursday should taper down overnight. A frontal band rotating up from the south Friday should bring generally light snow showers for the Olympics in the afternoon. Out ahead of this frontal band, there should be a period of light to moderate E-SE winds. 

A period of moderate E-SE winds Friday may build new wind slab near and above treeline on westerly aspects, especially in places with lower density snow available for transport. All aspects are indicated to cover older wind slab on more easterly aspects. Look for recent wind effects near and below ridgelines. 

Storm slabs remain possible on Friday due to potentially moderate snowfall rates Thursday afternoon and evening forming unstable storm layers. Give these layers time to settle and approach steeper slopes with caution.   

We are still getting a handle on the snowpack post storm, so be prepared to evaluate the snowpack carefully in your local area. 

The 12/17 PWL which has been largely unreactive as of late may awaken during the expected upcoming warm and wet period with the potential to produce large and destructive avalanches down to this layer.

Snowpack Discussion

Weather and Snowpack

An arctic air mass was over the Northwest with fair, cold weather last week. Surface hoar and near surface faceted crystals formed in wind and sun sheltered areas during this period. Observations over the next few days should help determine to what extent any of these weak persistent grain types might have survived the atmospheric river event and remain relevant to the forecast moving forward.

An atmospheric river moved over the Northwest Tuesday and Wednesday. Heavy rain was seen up to about 5500 feet in the Hurricane Ridge area. 3 day precipitation totals through noon Thursday were about 2.5 inches at Hurricane Ridge. An avalanche cycle likely occurred in Olympics during this period.

The NPS reported about a storm total of 15 inches of lower density snow at the Hurricane Ridge manual snow plot Thursday morning. Showers continued during the day Thursday but our precipitation gage didn't record any precipitation after 9 am, possibly due to equipment problems. 

Recent Observations

The last observation for the Hurricane area is from NWAC pro-observer Matt Schonwald on Klahane Ridge on Friday, 1/13. He found 90 cm of snow with no reactive layers over the 12/17 PWL on a south slope at 5170 feet. But the PWL gave a PST60/100 End result, so we still need to watch this layer.

NPS personnel reported debris in the Old Faithful track down to the Hurricane Ridge road at 3500 feet. Debris piled up 10-15 feet deep in this narrow slide path. Plow drivers discovered the debris mid-day Wednesday with the avalanche likely occurring Tuesday or Tuesday night. 

Problems

Wind Slabs

Wind Slab avalanches are the 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.

Storm Slabs

Storm Slab avalanches are the release of a cohesive layer (a slab) of new snow that breaks within new snow or on the old snow surface. Storm-slabs typically last between a few hours and few days (following snowfall). Storm-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.

Persistent Slabs

Persistent Slab avalanches are the release of a cohesive layer of 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 Slab.