Avalanche Forecast

Issued: Jan 29th, 2016 10:15AM

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

Northwest Avalanche Center NWAC, Northwest Avalanche Center

The main problem Saturday, is expected to be wind slab in the near and above treeline zones as well as storm slabs of heavy, dense snow or wet surface snow stressing weaker underlying snow at all elevations. Storm slabs may me masked by the drier, colder surface snow. 

Summary

Detailed Forecast

Cool weather with light snow showers and moderate westerly winds Saturday should allow for an overall slowly decreasing danger. Weak lower density snow that may exist above any crust present in the upper snowpack, especially near or below treeline, will slowly settle, but remain reactive to human triggers Saturday. 

The greatest avalanche problem should be sensitive wind slabs on lee slopes, mainly NW-N-SE facing slopes, especially below ridges, near and above treeline. 

Watch for newly formed cornices along ridges as well. These may be sensitive to human triggers.

Shallow storm slabs may be masked by the lower density surface snow. 

Snowpack Discussion

Weather

Two fair weather periods earlier this month allowed surface hoar and near surface faceting to occur. These persistent weak layers were buried intact on Jan 3rd and 11th and were reported throughout the Cascade east slopes.

A warm front last Thursday caused light or moderate amounts of snow which changed to freezing rain or rain east of the crest.

This was followed by cooler weather and some snow. NWAC stations along the east slopes indicate up to about 4 inches for the 2 days ending Sunday morning.

Warm air arrived Wednesday with freezing levels climbing to near 7000 feet.

A strong front moved across the region Thursday, bringing significant warm air and likely causing snow levels to fluctuate greatly before cooling arrived late Thursday. 

This recent warm and wet weather should hopefully have cleaned out the earlier January persistent layers, however, until confirmation, we will still list this as an unlikely problem.  

Another strong, but significantly cooler front, arrived Friday with strong winds and additional moderate snowfall. This latest front has likely built new wind slab conditions near and above treeline with some new storm slab as well.

Snow and Avalanche Observations

NWAC pro-observer Tom Curtis was on Iron Mountain near Blewett Pass Sunday and found the January 11th layer 35-70 cm down on NW-SE aspects in the below and near treeline bands. He found that the buried surface hoar crystals are rounding and the tests gave low quality results and did not indicate propagation.

Tom was out again on Table Mountain near Blewett Pass on Monday in the 4-6000 feet elevation range and found the January 3rd and 11th layers well defined down 70 and 50-60 cm and still giving sudden planar results with ECT test indicating propagation. He also noted crowns from natural avalanches that released on the January 11th layer late last week.

Crown of persistent slab avalanche from about January 21-22 by Tom Curtis.

A report via the NWAC Observations page for Stemilt Basin near Mission Ridge strongly indicated that persistent weak layers at about 40 and 60 cm are still present east of the crest with test columns failing on isolation and evidence of previous widespread persistent slab avalanches.

Thanks to those taking the time to submit so many high quality, pertinent observations to the NWAC Obs page here: http://www.nwac.us/observations/ 

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

Persistent Slabs

An icon showing Persistent Slabs

Release of a cohesive layer of soft to hard snow (a slab) in the middle to upper snowpack, when the bond to an underlying persistent weak layer breaks. Persistent layers include: surface hoar, depth hoar, near-surface facets, or faceted snow. Persistent weak layers can continue to produce avalanches for days, weeks or even months, making them especially dangerous and tricky. As additional snow and wind events build a thicker slab on top of the persistent weak layer, this avalanche problem may develop into a Deep Persistent Slabs.

 

The best ways to manage the risk from Persistent Slabs is to make conservative terrain choices. They can be triggered by light loads and weeks after the last storm. The slabs often propagate in surprising and unpredictable ways. This makes this problem difficult to predict and manage and requires a wide safety buffer to handle the uncertainty.

 

This Persistent Slab was triggered remotely, failed on a layer of faceted snow in the middle of the snowpack, and crossed several terrain features.

Persistent slabs can be triggered by light loads and weeks after the last storm. You can trigger them remotely and they often propagate across and beyond terrain features that would otherwise confine wind and storm slabs. Give yourself a wide safety buffer to handle the uncertainty.

Aspects: North, North East, East, West, North West.

Elevations: All elevations.

Likelihood

Unlikely

Expected Size

1 - 2

Valid until: Jan 30th, 2016 10:15AM