Avalanche Forecast

Issued: Feb 19th, 2017 11:48PM

The alpine rating is moderate, the treeline rating is moderate, 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 along the Cascade west slopes will go up another notch on Monday. Evaluate snow and terrain carefully in this area on Monday.

Summary

Detailed Forecast

Weakening low pressure systems will pass near Cape Flattery on Sunday and near Astoria on Monday. This will generally cause a trend of increasing alpine winds and increasing snow especially in the south Cascades by Monday. Temperatures should not change greatly through Monday. Expect up to about 10-20 inches of snow along the Cascade west slopes on Sunday and Monday with the most in the south part.

This will cause the avalanche danger along the Cascade west slopes to go up another notch on Monday especially in the south part.

The size and extent of wind slab should generally increase on Monday especially in the southwest zone. Watch for firmer wind transported snow on potential lee slopes. Significant winds lately and on Monday will have been mostly south to southwest so north to southeast aspects will be indicated but keep an eye on all aspects.

The size and extent of storm slab should also generally increase on Monday especially in the southwest zone. Storm slab is most likely in areas that receive more than a few inches of rapidly accumulating snowfall.

Loose wet avalanches will not be indicated as an avalanche problem but watch for loose wet surface snow if you find yourself on solar slopes during cloud or sun breaks.

Snowpack Discussion

Weather and Snowpack

Hopefully the last atmospheric river of the season arrived on Valentines Day 2/14 causing heavy rain, avalanches, crusts and consolidation through Thursday along the Cascade west slopes. Freezing rain also occurred along the lower slopes of the Cascade Passes. A little snow was seen along the Cascade west slopes at the tail end of the storm.

A short period of fair weather on Friday 2/17 caused another surface crust in many areas.

Some snow was seen with light winds on Saturday. NWAC stations along the Cascade west slopes had about 2-7 inches ending Sunday morning.

Temperatures haven't changed much on Friday to Sunday.

Recent Observations

North

On Thursday 2/16, the Mt Baker pro patrol reported a very large naturally triggered slab at the east end of Shuksan Arm that released sometime during the rain event. The initial estimates of the crown height are 20 feet!

Central

The Alpental pro-patrol did explosives control on Sunday to be sure the slopes were safe for the rando race and had little results. But cloud or sun breaks and a brief warm up caused some triggered size 1-2 loose wet avalanches in the below treeline that were sliding on the crust from Friday 2/17.

Skiers triggered this apparent small storm slab on a southeast facing slope at about 4200 feet in the Alpental Valley on Sunday.

Small triggered slab in the Alpental Valley on Sunday. Photo by Shane Robinson.

South

NWAC pro-observer Ian Nicholson was in the Paradise area on Saturday and observed no avalanches or wind transport though visibility was limited. He generally found 10 cm on new snow on the 2/17 crust and the 2/14 crust at 23 cm down from the surface. Test results on the 2/14 crust were moderate to hard and did not indicate propagation.

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

Valid until: Feb 20th, 2017 11:48PM