Avalanche Forecast
Regions: Cascades - North West.
The pattern makes for a tricky forecast Sunday. Dangerous avalanche conditions are expected to develop Sunday afternoon. Back country travel is not recommended in the above treeline Sunday afternoon.
Detailed Forecast
A deep low pressure system should move over about Cape Flattery and across south Vancouver Island Sunday morning and afternoon. This should bring stormy weather and heavy snow to the Cascades Sunday. This is a little faster than previously expected. A vigorous, strong front should rapidly cross the area and cause rapidly shifting winds Sunday.
The main avalanche problem Sunday should be increasing strong winds and increasing moderate to heavy snow rapidly transporting recent or new snow and building new wind slab on lee slopes. This is very likely to be northwest to southeast slopes. New storm slab should also be likely where there is rapidly accumulating new snowfall.
The pattern makes for a tricky forecast Sunday. Dangerous avalanche conditions are expected to develop Sunday afternoon. Back country travel is not recommended in the above treeline Sunday afternoon.
Snowpack Discussion
Weather and Snowpack
The nonstop active weather pattern continues to push a storm system through the PNW almost every day or two with fluctuating but generally moderate snow levels.
The last storm impacted the area Wednesday through Thursday with a warming trend that peaked Wednesday night and very strong west winds seen throughout the Cascade range. Generally 1 to 3 inches of water accumulated along the west slopes in the 36 hours ending 4 pm Thursday. Outside the Cascade Passes, rain reached up to 5000' feet in the north and 6500 feet in the south with snow levels only rising to about 4000 feet at Stevens and 4500 feet at Snoqualmie Pass Wednesday night. About 4-14 inches of snow accumulated at NWAC stations through Thursday morning. Additional accumulations were light except above 5000 feet in the Mt. Baker area Thursday where another foot of snow likely fell.
A front on Saturday is causing west-southwest winds and there should be up to a few inches of new snow at most sites by Sunday morning. Â
The mid and lower snow pack along the west slopes should generally be a stable mix of crusts and layers of moist and rounded snow crystals.
Recent Observations
The Stevens Pass and Alpental pro-patrols reported an extensive natural cycle Wednesday night. Control results were sensitive and widespread Thursday morning involving new storm snow. As the sun poked out and temperatures rose Thursday, large loose wet avalanches, both natural and skier triggered, were reported at Stevens with smaller loose wet noted in the Alpental area. The Crystal pro-patrol generally had 1-2 foot slab releases during control work Thursday with large and sensitive cornices along ridgelines.Â
In areas that experienced rain Wednesday night, mainly outside the Passes and in the below and near treeline bands, a breakable crust made for tough skiing Thursday.
The Alpental pro-patrol on Saturday reported little if any results from cornice control, with bad b-c ski conditions and possible loose wet conditions on the lower mountain.
A ranger on Saturday above Paradise reported 16 cm of recent F snow on a crust from Wednesday, with little effect from wind, no results in pit tests and ski cuts, and good snow conditions.
Avalanche 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: North, North East, East, South East, North West.
Elevations: Alpine, Treeline.
Likelihood: Very 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.
Aspects: All aspects.
Elevations: All elevations.
Likelihood: Likely
Expected Size: 1 - 1