Avalanche Forecast

Issued: Mar 13th, 2016 12:11PM

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

Northwest Avalanche Center NWAC, Northwest Avalanche Center

Conditions are a bit hard to predict following the low pressure system and front on Monday and back country travel is not recommended in avalanche terrain above treeline on Monday. Conservative decision making will be a good plan in the near and below treeline and the avalanche forecasts will get refined after new information becomes available on Monday.

Summary

Detailed Forecast

Up to about 7-15 inches of new snow seems likely along the west slopes by Monday morning.

West winds aloft and a cool slightly unstable air mass should follow the front across the Northwest on Monday. This should cause orographic snow showers and up to another 6-12 inches of snow seems likely along the west slopes by the end of the day.

This is a lot of new snow.

New wind and storm slab seem likely to be the main avalanche problems along the west slopes on Monday.

New wind slab may be deep and is mostly likely to be found on northwest to southeast slopes. Firmer wind transported snow and snowpack cracking can be good signs of wind slab.

New deep storm slab is likely in areas where new snow rapidly accumulates for more than several hours. This is likely along most of the west slopes Sunday night and Monday!

Cloudy cool conditions on Monday may limit the development of loose wet snow but the sun is gaining power so watch for the development of loose wet snow on solar slopes.

Conditions are a bit hard to predict following the low pressure system and front on Monday. The avalanche danger may decrease a little on Monday due to a little less wind, some stabilizing of new snow and fairly cool temperatures. This forecast will take a conservative approach to the danger levels and the avalanche forecasts will get refined after new information becomes available on Monday.

 

 

Snowpack Discussion

Weather and Snowpack

Storms have moved across the Northwest almost every day or two the past couple weeks with fluctuating but generally moderate snow levels.

A storm last Wednesday and Thursday caused a warming trend that peaked Wednesday night with very strong west winds seen throughout the Cascade range. About 4-14 inches of snow accumulated on a new crust in most areas along the west slopes through Thursday morning. Additional accumulations were light Thursday except above 5000 feet in the Mt. Baker area where another foot of snow likely fell.

A front on Saturday caused west-southwest winds and another 1-10 inches of snow with the most at Baker, Crystal and Paradise ending Sunday morning.

A deep surface low pressure system is moving from the Olympic Peninsula to Vancouver on Sunday. A front is crossing the Cascades Sunday afternoon. This is causing stormy weather along the west slopes with strong shifting southeast to southwest winds and moderate to heavy snow in most areas.

The mid and lower snow pack along the west slopes should generally be a stable mix of crusts and layers of moist and rounded snow crystals.

Recent Observations

The Stevens Pass and Alpental pro-patrols reported an extensive natural cycle Wednesday night. As the sun poked out and temperatures rose Thursday, large loose wet avalanches, both natural and skier triggered, were reported at Stevens.

A report for the Alpental Valley for Saturday via the NWAC Observations also noted a large avalanche had run from Chair Peak likely on Thursday.

A ranger on Saturday above Paradise reported 16 cm of recent F snow on a crust from Wednesday, with little effect from wind, no results in pit tests and ski cuts, and good snow conditions.

NWAC ambassador Jeff Hambleton was near Mt Baker on Sunday and reported a 3 foot natural slab avalanche on a north slope at about 6000 feet on Mazama Peak. He also reported there were several ski triggered slab avalanches on the north side of Shuksan Arm which would be in the 4500-5000 foot range but details are lacking.

The Crystal pro-patrol on Sunday reported mostly small but widespread ski triggered 4-12 inch wind slabs on north to west slopes near the ridges due to strong southeast winds. A private party via email also reported sensitive wind slab of 6-12 inches on varied aspects at Crystal Mountain on Sunday.

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

Very Likely

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

Very Likely

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.

Aspects: South East, South, South West.

Elevations: All elevations.

Likelihood

Possible

Expected Size

1 - 1

Valid until: Mar 14th, 2016 12:11PM