Avalanche Forecast

Issued: Feb 17th, 2016 10:11AM

The alpine rating is moderate, the treeline rating is moderate, and the below treeline rating is moderate. Known problems include Wind Slabs, Storm Slabs and Loose Wet.

Northwest Avalanche Center NWAC, Northwest Avalanche Center

The avalanche danger should increase Thursday afternoon and evening due to building wind and storm slab. A cooling trend Wednesday night and Thursday should limit the loose wet problem to below treeline but wet snow hazards will continue. Change your travel plans accordingly if conditions deteriorate more quickly than forecast.

Summary

Detailed Forecast

An occluded front pushing through Wednesday night should be followed by light showers on Thursday morning and a cooling trend. A surface low tracking inland near the Oregon/Washington border mid-day Thursday should provide a burst of moderate snowfall in the afternoon and evening hours for the southern Washington Cascades, followed by strong W-SW winds. Lighter snowfall is expected further areas further north during the daylight hours. 

New wind slab should build on lee slopes near and above treeline Thursday afternoon and evening. Watch for new firmer wind transported snow mainly on lee NW to SE slopes near ridges. 

Storm snow with a cooling trend Wednesday night and Thursday should generally bond well to surface snow comprising of wet grains or various crust layers. Heavier precipitation rates Thursday afternoon and evening should increase the storm slab likelihood especially for the southern Washington Cascades.  

Despite the cooling trend, loose wet avalanches will remain possible below treeline Thursday especially on steeper slopes. Watch for wet surface snow deeper than a few inches that usually precede loose wet avalanches or other triggered or natural loose wet avalanches. 

Snowpack Discussion

Weather and Snowpack

Two fair weather periods in January allowed surface hoar and near surface faceting to occur. These persistent weak layers were buried intact on Jan 3rd and 11th, but are no longer considered a persistent slab threat since several warm and wet systems have tested this layer and since it has been unreactive where it can still be identified in recent snowpit tests.

A warm and wet system late last month caused a rain crust to form in most areas and elevations along the east slopes on Jan 28th. 

Dry weather with the warmest temperatures of the winter occurred February 7-10th with temperatures climbing into the 50's in most areas east of the crest. This caused loose wet avalanches and overall snowpack consolidation with a crust as temperatures cooled late last week.

A pair of warm fronts brought a wide range of new snow amounts east of the crest from Thursday, February 11th to Saturday, February 13th with areas at higher elevations receiving 6-12 inches and with the most near Washington Pass.

Rain and and mild temperatures should have dominated along most of the lower east slopes Sunday and Monday. NWAC sites had a couple inches of mainly rainfall ending Monday morning. About 8-14 inches of new snow fell at higher elevations in the central east and northeast zones indicated by NRCS sites at Harts Pass and Lyman Lakes. Mild temperatures with spotty light rain were seen on Tuesday and Wednesday. 

The upper snowpack should consist of a mix of crusts and wet grains.

Recent Observations

The North Cascades Mountain Guides reported significant firm wind slab formation in the above tree line on Sunday. Ski cuts gave roller balls and hand tests in the recent storm snow gave easy shears in upside down storm snow. A crust buried Thursday, February 11th was identified in a snow pit at 6000 feet at 25 cm but did not indicate propagation.

Tom Curtis was in the Icicle Creek drainage Wednesday and found soft wet snow in the upper snowpack along with challenging travel conditions. A thin melt-freeze crust limited loose wet activity Wednesday. Tom also noted open creeks due to the ongoing warmth and rainfall at lower elevations. 

Problems

Wind Slabs

An icon showing 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: Alpine, Treeline.

Likelihood

Possible

Expected Size

1 - 1

Storm Slabs

An icon showing 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

Loose Wet

An icon showing Loose Wet

Release of wet unconsolidated snow or slush. These avalanches typically occur within layers of wet snow near the surface of the snowpack, but they may quickly gouge into lower snowpack layers. Like Loose Dry avalanches, they start at a point and entrain snow as they move downhill, forming a fan-shaped avalanche. They generally move slowly, but can contain enough mass to cause significant damage to trees, cars or buildings. Other names for loose-wet avalanches include point-release avalanches or sluffs. Loose Wet avalanches can trigger slab avalanches that break into deeper snow layers.

 

Travel when the snow surface is colder and stronger. Plan your trips to avoid crossing on or under very steep slopes in the afternoon. Move to colder, shadier slopes once the snow surface turns slushly. Avoid steep, sunlit slopes above terrain traps, cliffs areas and long sustained steep pitches.

 

Several loose wet avalanches, and lots of pinwheels and roller balls.

Loose wet avalanches occur where water is running through the snowpack, and release at or below the trigger point. Avoid terrain traps such as cliffs, gullies, or tree wells. Exit avalanche terrain when you see pinwheels, roller balls, a slushy surface, or during rain-on-snow events.

Elevations: Below Treeline.

Likelihood

Possible

Expected Size

1 - 1

Valid until: Feb 18th, 2016 10:11AM