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Archived

Avalanche Forecast

Jan 22nd, 2017–Jan 23rd, 2017
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
1: Low
The avalanche danger rating below treeline will be low
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
1: Low
The avalanche danger rating below treeline will be low

Regions: Olympics.

Watch for new wind slabs forming early Sunday on exposed lee slopes at higher elevations. Allow recent storm and wind slabs further time to stabilize Sunday. Wind slabs are most likely on NE-W-SW aspects. Storm slabs should continue to slowly stabilize Sunday, but still possible in the wrong spot.

Detailed Forecast

Occasional light snow showers are expected Sunday with a period of moderate to strong S-SE winds early Sunday. The winds early Sunday may move available surface snow and build some new shallow wind slab layers, mainly below exposed ridges.  

Allow existing storm and wind slab time to continue stabilizing on Sunday. Wind slabs are most likely on NE-W-SW aspects due to recent and expected SE transport winds. Look for recent wind effects near and below ridgelines and treat wind loaded slopes with caution.  

Storm slabs should be more stubborn to trigger, but still possible in the right spot on Sunday, especially near and above treeline and can be avoided by sticking to lower angled slopes. Storm slabs may extend into the upper portion of the below treeline band, but the overall avalanche danger will be rated as low.

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. Recent observations on Thursday, Friday and helped confirm that we aren't dealing with any older layers formed during this period.  

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 the Olympics during this period.

The NPS reported 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. About a foot of snow fell at the Waterhole NRCS Snotel over a 24 hour period ending Friday morning. 

A very pleasant day Saturday allowed temperatures at Hurricane to reach the freezing mark. No showers had arrived by late Saturday afternoon.

Recent Observations

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 midday Wednesday with the avalanche likely occurring Tuesday or Tuesday night. 

Pro-observer, Matt Schonwald, was at Hurricane Ridge Friday. Matt found 10" (25 cm) of settled recent storm snow generally well bonded to the underlying rain crust and not reactive. SE winds were forming shallow wind slabs on lee slopes near treeline. The recent rainfall had penetrated well into the snowpack. At a limited number of test sites, the 12/17 PWL was found to be rounding and unreactive in snowpit test and well over a meter down. We feel comfortable removing the persistent slab from the avalanche problem list moving forward. 

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, South West, West, North West.

Elevations: Alpine, Treeline.

Likelihood: Possible

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