Avalanche Forecast

Issued: Dec 22nd, 2013 10:00AM

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

Generally stormy weather and quite a change in conditions should be seen on Monday.

Summary

Detailed Forecast

Quite a change is in store on Monday.

A strong cold front will cross the Northwest on Monday. This will cause strong west crest level winds, moderate to heavy rain changing to snow heaviest near and west of the crest, and cooling and lowering snow levels. These conditions should generally spread from the Olympics and north Cascades Monday early morning to the south Cascades Monday late morning to Mt Hood by Monday afternoon.

During the day on Monday this weather should build some new wind slab in the alpine and some shallower wind slab in near treeline areas on lee slopes. Remember that it can be difficult to determine where wind slab lies but it often has a chalky texture and may be triggered from thin areas near the edge of the slab. Storm slab seems more likely in mostly calmer below treeline areas especially where snowfall rates exceed about an inch per hour.

A high danger is not in the forecast mainly due to the expected cooling trend. But a considerable danger still means dangerous avalanche conditions.

In areas with light wind, where rain changes to snow, and with lower rates of loading, the cooling trend should help bond new snow to old snow and help build right side up conditions with lower density snow nearer the surface. This can mean some good skiing in those types of areas. Underlying wet snow should also begin to refreeze and strengthen.

Shallow snow cover and shallow new snow will continue to limit the avalanche danger at lower elevations.

Remember this is a regional forecast and you must make your own on site evaluations to see if they are different than forecast.

Snowpack Discussion

Warm wet weather the weekend of 14 and 15 December created another crust layer in most areas. This seems to have capped and stabilized underlying hoar frost and faceted snow and made those layers generally unreactive.

Cold weather, light amounts of low density snow and some surface hoar formation were generally seen mid-week last  week.

This set the stage for warmer wet weather that was seen Friday and over the weekend. A warm front moved across the Northwest Friday. This generally caused strong southwest winds, increasing moderate to heavy snow and greatly warming temperatures. Another warm front moved across the Northwest on Saturday night and Sunday. This generally caused strong northwest winds and wet snow or rain at further warming temperatures. NWAC sites near and west of the crest for the 48 hours ending Sunday morning had ended up mostly with about 4-17 inches of very wet heavy snow. The warming should have created some unstable upside down wind or storm slab conditions at alpine elevations and wet loose conditions near or below treeline.

Reports are a bit sparse for the weekend probably primarily due to the poor ski conditions. A couple of most pertinent observations follow. The Hurricane ranger reported shallow consolidated snow, little new snow and a lower danger in that area on Saturday and Sunday. The Mt Baker ski patrol reported fairly extensive ski triggered wind slabs Friday and Saturday. An NWAC observer just east of Stevens Pass on Saturday reported that the recent snow gave some cracking and small wet loose avalanches. The Mt Hood Meadows ski patrol Saturday and Sunday reports a stable freezing rain crust in the alpine and near tree line but it is unknown if this condition extends very far around the mountain.

Shallow snow cover has been limiting the avalanche danger at the lowest elevations.

 

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.

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.

Elevations: Treeline, Below Treeline.

Likelihood

Possible

Expected Size

1 - 1

Valid until: Dec 23rd, 2013 10:00AM