Avalanche Forecast

Issued: Feb 27th, 2018 12:39PM

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

Northwest Avalanche Center NWAC, Northwest Avalanche Center

You are increasingly likely to trigger a wind slab Wednesday as slabs build and become more reactive. In some areas it is possible to trigger a dangerous Persistent Slab avalanche. Stay away from large open slopes steeper than 35 degrees that are suspect for these avalanches.

Summary

Detailed Forecast

The avalanche danger will increase on Wednesday as increasing moderate to occasionally strong winds transport fresh and recent snow onto a variety of aspects forming sensitive shallow wind slabs near and above treeline. You will be likely to trigger an avalanche by later in the day, so watch as conditions change around you. Blowing snow is a good sign to seek wind-sheltered terrain where better and safer snow conditions are likely to be found.

Older avalanches are becoming more difficult to trigger as the extensive storm and wind slabs deposited last weekend gradually settle and stabilize. Continue to watch for areas that received significant wind loaded snow, such as leeward slopes below ridges. You may also find them in unique areas like cross-loaded terrain features mid-slope and in open areas below treeline. Avoid steep slopes where the wind deposited thick pillows of snow or obvious wind features. 

Two persistent slab problems may exist within the upper snowpack. Significant uncertainty remains on the east slopes of the Cascades surrounding these layers as recent snow and elevated avalanche danger in recent days has prevented widespread travel in many areas. Slopes that previously lacked cohesive snow over the facets, may now have developed slabs. The upper layer of concern surrounds a sun crust while the lower layer consists of weak sugary facets that can be found 2-3 feet below the snow surface and above a firm crust. These are both within the ideal depth for human triggered avalanches. Two skier triggered Persistent Slabs have occurred last week in the Washington Pass area. Continue to stay off of large open slopes where you could trigger a Persistent Slab avalanche. Collapsing and cracking are obvious signs that you could trigger a Persistent Slab. Use extra caution if you observe these signs of instability. 

Snowpack Discussion

Up to 2 inches of new snow fell on the East Slopes of the Cascades on Tuesday. Moderate and gusty crest-level winds transported recent and fresh snow forming new fresh wind slabs on Tuesday.

Up to 1.5 feet of snow fell with strong wind across the East Slopes Thursday through Sunday. Winds Slabs capped the existing snowpack on leeward slopes. Small isolated wind slab also formed Thursday and Friday.

A faceted sun crust identified near the Cascade Crest may extend to the Cascade East Slopes and provide a bed surface on steep solar aspects. 

Prior to this storm about 1 to 2 feet of generally soft snow has been reported over an old (2/5) crust from early February. Weak sugary facets can be found above this crust in some locations. This persistent weak layer has failed in two reported skier triggered avalanches over the last week.

While several layers exist in the snowpack, there are no significant layers of concern below the 2/5 crust.

Observations

North

On Tuesday, NCMG observed small localized cracking near ridgecrest and poor visibility conditions. Moderate, gusty winds were transporting snow.

On Monday, the crown of a large avalanche was visible on Scaffold Ridge (Twisp River) in the North Cascades. Interestingly the red line in the photo marked the initial crown width that released Sunday and the remainder of the slab released sometime later Monday morning.

Large natural slab avalanche visible on Scaffold Ridge in the North Cascades, starting zone about 7300'. Image, Matt Firth  

Avalanche professionals in the Washington Pass area Thursday through Saturday reported several small skier triggered wind slabs. Collapsing was reported on Saturday.

The most recent persistent slab avalanche occurred on Wednesday near Harts Pass. The avalanche was about two feet deep and 90 feet wide. It occurred on a NE aspect at 6000’.

Central

Mission Ridge Ski Patrol Reported many small natural avalanches in wind drifted areas on Sunday.

Observers reported triggering small avalanches on Blewett Pass and in the Wenatchee Mountains over the weekend.

NWAC field staff traveled in the Icicle Creek area Wednesday on NE facing slopes up to about 6000 feet. The persistent layer we are tracking was found consistently buried about 2 ft from the surface, though the distribution of this layer was intermittent. In many areas the snow above the facet/crust layer was low cohesion powder, lacking the ability to propagate a slab avalanche. Shallow fresh wind slabs were noted near and above treeline.

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: All aspects.

Elevations: Alpine, Treeline.

Likelihood

Likely

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: All aspects.

Elevations: All elevations.

Likelihood

Possible

Expected Size

1 - 1

Valid until: Feb 28th, 2018 12:39PM