Avalog
Register
Get forecast notifications
Create an account to receive email notifications when forecasts are published.
Register
Login
Archived

Avalanche Forecast

Mar 16th, 2014–Mar 17th, 2014
Alpine
3: Considerable
The avalanche danger rating in the alpine will be considerable
Treeline
3: Considerable
The avalanche danger rating at treeline will be considerable
Below Treeline
2: Moderate
The avalanche danger rating below treeline will be moderate
Alpine
2: Moderate
The avalanche danger rating in the alpine will be moderate
Treeline
2: Moderate
The avalanche danger rating at treeline will be moderate
Below Treeline
2: Moderate
The avalanche danger rating below treeline will be moderate

Regions: Olympics.

Watch for areas of newly forming wind slab on lee easterly aspects near and above treeline.  Watch for unstable storm slabs formed Sunday.  Dangerous avalanche conditions are expected so make careful snowpack evaluations and choose more conservation terrain.  

Detailed Forecast

A cool upper trough should pass the Olympics early Monday with cool but drying northwesterly flow expected through the day Monday.  A few light snow showers early Monday should give way to partly to mostly cloudy conditions with continued strong northwest winds lasting through the day.

This weather should allow for the older wet snow to form a more solid underlying crust which may act as a sliding surface.  The strong winds should transport recent snow, mainly above treeline but look for transported snow near tree line as well.  Winds should shift to more northwesterly Monday so watch for wind slab formation on east to south facing slopes.  

Storm slabs should begin to slowly stabilize Monday but should remain a concern and expect triggered slides to remain likely on steeper open terrain where cautious route-finding and conservative decision making will be essential.  

Snowpack Discussion

A stalled moist  frontal boundary has been draped over the Olympics since Saturday afternoon.  This caused very strong winds late Saturday with moderate precipitation and a snow line near 5500 feet causing rain and wet snow conditions late Saturday near and below treeline as reported in the Hurricane Ridge area by observer Katy Reid.  Temperatures cooled overnight and through early Sunday depositing about 1 foot of new snow at Hurricane Ridge by Sunday morning with diminishing winds.  

Given the weather since Saturday, I expect to see new wind slab formed on lee terrain near and especially above treeline where most of the precipitation fell as snow during the very windy period. The bonds of new snow to a refreezing crust should be good given the slow cooling trend. 

Storm slabs should be found on all aspects and elevations, especially with daytime warming and continued snowfall Sunday afternoon.   

A NPS ranger reported 9 inches of moist new snow as of Sunday morning over a slowly refreezing crust at Hurricane Ridge.  A few more inches of snow likely accumulated during the day down to around 4000 feet with some possible wind transported snow likely above treeline where westerly winds are expected to be stronger.

While composing the forecast I just received word from a Park Ranger at Hurricane Ridge reporting a backcountry skier on Sunrise Ridge above the road Sunday afternoon was able to ski cut and release a storm slab about 15-20 ft wide that covered the uphill lane with debris. Fortunately the skier was not caught in the slide. This confirms that the daytime warming Sunday anticipated did indeed lead to warming of the surface snow layers creating unstable storm slab conditions.

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.

Elevations: Alpine, Treeline.

Likelihood: Likely

Expected Size: 1 - 1

Storm Slabs

Release of a soft cohesive layer (a slab) of new snow that breaks within the storm snow or on the old snow surface. Storm-slab problems typically last between a few hours and few days. 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.

 

You can reduce your risk from Storm Slabs by waiting a day or two after a storm before venturing into steep terrain. Storm slabs are most dangerous on slopes with terrain traps, such as timber, gullies, over cliffs, or terrain features that make it difficult for a rider to escape off the side.

 

Storm slabs usually stabilize within a few days, and release at or below the trigger point. They exist throughout the terrain, and can be avoided by waiting for the storm snow to stabilize.

Aspects: All aspects.

Elevations: All elevations.

Likelihood: Possible

Expected Size: 1 - 1