Avalanche Forecast

Issued: Feb 19th, 2016 10:26AM

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

Northwest Avalanche Center NWAC, Northwest Avalanche Center

The avalanche danger should be greatest early Saturday following the storm Friday. Watch for wind loaded terrain and avoid steep slopes suspected of wind deposits. Conditions should improve as the snow pack slowly stabilizes through Saturday afternoon.

Summary

Detailed Forecast

Continued periods of moderate snow at cool temperatures, Friday night, along with strong winds.

This weather should build wind slabs on lee slopes near and above treeline. Watch for firmer wind transported snow, mainly on the lee slopes facing N to SE, below ridges. 

Heavier precipitation rates Friday night should increase the storm slab possibility by early Saturday. 

Showers should end quickly early Saturday with partial clearing and diminishing winds. Cooler temperatures should continue Saturday. 

Watch for greater depths of storm snow and avoid wind loaded terrain and steep rollover features. 

Storm snow problems should improve through the day Saturday as upper snow layers settle and stabilize.

Snowpack Discussion

Weather and Snowpack

Dry weather with the warmest temperatures of the winter occurred February 7-10th with mild temperatures and high freezing levels, leaving behind, well settled older snow and a strong melt-freeze crust.

The warm weather transitioned to rain at high freezing levels over the Mt Baker area that gradually cooled by 2/12, depositing increased storm snow amounts ranging from a few inches to 18 at Mt Baker. 

Rain and mild temperatures dominated along the west slopes Sunday and Monday. NWAC sites in the near and below treeline recorded 2-6+ inches of mostly rain over the 2 days ending early Tuesday morning. The Paradise station recorded over 5 inches of precipitation (mostly rain) in the 24 hours ending Monday 4am!   

Snow showers at cooler temperatures arrived Thursday 2/18 followed by a strong front Friday. These storms have deposited storm snow amounts ranging from about 1-2 feet by Friday afternoon.

The new snow has been wind affected at higher exposed elevations, but in general is well bonded to the old wet but draining snowpack. 

The upper snowpack consists of a mix of crusts and wet grains below the recent storm snow with no deeper instabilities of note.

Recent Observations

NWAC pro-observer Lee Lazzara  was out in the Mt Baker backcountry Friday, 2/19 as the front arrived. Strong winds were building wind slab layers on exposed lee slopes near and above treeline. Storm snow was generally well bonded to the old wet and draining upper snowpack. No avalanches were noted and steep slopes were tested in wind protected areas. 

NPS ranger above Paradise Thursday morning, 2/18, reported new storm snow of 4-6" bonding well to moist crust with no avalanches of note, other than localized small dry-loose on steep slopes.

NWAC observer, Ian, in the Alpental Thursday 2/18 indicated similarly to Paradise: shallow new snow over moist thin crust, well bonded and no avalanches or indicative tests noted. 

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

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

Possible

Expected Size

1 - 1

Valid until: Feb 20th, 2016 10:26AM