Avalanche Forecast

Issued: Feb 17th, 2018 12:10PM

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

NWAC, Northwest Avalanche Center

Cascades - North East.

Sunday will be a dangerous day to engage avalanche terrain as large human triggered avalanches are likely. Recent storm and wind slab instabilities will be there waiting, and if triggered, may release down to deeper crusts. Due to shifting winds, avoid steep wind loaded terrain in all elevation bands and aspects. Stay safe by choosing lower angled slopes, avoid traveling below paths connected to large start zones found higher in the terrain and by allowing new storm slab instabilities time to stabilize.

Summary

Detailed Forecast

Sunday will be a dangerous day to engage avalanche terrain as large human triggered avalanches are likely. Cold temperatures, less wind and additional light to locally moderate snow showers will allow the risk of natural avalanches to slowly lower Sunday, but recent storm and wind slab instabilities will be there waiting, and if triggered, may release down to deeper crusts.

Northerly winds will continue to transport recent snowfall onto lee slopes near and above treeline while recent wind slabs may be found on open slopes below treeline. Due to shifting winds, avoid steep wind loaded terrain in all elevation bands. Stay safe by choosing lower angled slopes, avoid traveling below paths connected to large start zones found higher in the terrain and by allowing new storm slab instabilities time to stabilize.

Snowpack Discussion

The east slopes of the Cascades from the Hwy 2 corridor to the northeast Cascades saw up to 15 inches of snow Saturday, before the snow level rose up to at least 4000' for the central-east zone Saturday afternoon; Blewett Pass mixed with rain while Mazama stayed all snow. Dirty Face and Mission Ridge stations reported very strong winds (Mission had several gusts over 100 mph) and although the Washington Pass station winds were not very strong, strong gusts were observed by professionals near Hart's Pass Saturday afternoon.

In the Washington Pass area, approximately 10-20 inches of snow sit on the 2/5 crust. The crust likely extends up to around 6800’. In some areas, the 2/5 crust sits over weaker snow (near surface facets).

Moderate winds created wind slabs near ridge crest on Friday. A large natural wind slab avalanche was observed Friday in the Washington Pass area (see observation, below). A previous round of NW winds transported snow on Wednesday (2/14) and created small, isolated wind slabs on easterly ridge crests in the northeast Cascades.

A relatively well consolidated snowpack exists below the 2/5 crust. While we are tracking several older crust layers there are currently no other layers of concern in the snowpack.

In the last week of January, observers reported isolated buried surface hoar on top of the 1/16 crust. This persistent weak layer was found or thought to be the cause of several avalanches. A recent observation (2/11) from the Mission Ridge area confirmed this layer is still present in some locations. This layer will be worth tracking after Saturday's storm. Snow profiles and snowpack tests are the best means to confirm the presence of this layer.

Observations

North

A professional in the Hart's Pass area found deteriorating avalanche conditions Saturday as human and skier triggered slabs within the storm snow were reported by the afternoon. Winds became very strong and gusty in the afternoon along with the heavier snowfall.

North Cascades Heli was in the Washington Pass area on Friday. They witnessed a large natural wind slab on very steep northeast-facing terrain at 7500 ft, hanging above a roll-over.

Central

NWAC Pro Observer Matt Promomo traveled in the Blewett Pass area Thursday. Matt found 1-3 inches of weak snow above the early February crust that had become faceted. No deeper persistent weak layers were identified in the lower snowpack. Winds had created variable snow surfaces above 5500 feet and snow depths averaged about 3 feet. S through W aspects have little snow cover in exposed areas.

Problems

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: All aspects.

Elevations: All elevations.

Likelihood

Likely

Expected Size

1 - 1

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.