Avalanche Forecast
Regions: Cascades - East.
Watch for recently formed storm and wind slab layers, especially if you venture to higher elevations. Due to lack of observations be prepared to perform your own stability evaluations. Don't overlook early season terrain hazards, such as poorly covered rocks or vegetation and creeks, particularly at lower elevations.
Detailed Forecast
On Wednesday, a shortwave ridge moves across the Pacific Northwest with a temporary decrease in winds and precipitation. This weather should generally allow new or recent storm and wind slab layers to begin stabilizing.
Watch for storm slabs in areas that received more than a few hours of moderate or heavy snowfall intensities, likely near Washington Pass.
Watch for firmer wind-transported snow on lee slopes near ridges, generally northwest through southeast aspects, but watch for it on other aspects in areas of more complex terrain.
Storm or wind slab avalanches may entrain recent snow down to the firmer refreezing snow from last week.
Clearing on Sunday night may have created the conditions for near-surface faceting or surface hoar. If present, storm and wind slab layers may be more reactive than anticipated, particularly in wind-sheltered areas.
Although they will not be listed as avalanche problems, loose dry avalanches are possible Wednesday, particularly on steep slopes. Watch for potential loose wet avalanches on steep, solar slopes mainly in the South Cascades if there are significant mid-day sun breaks.
Watch for the numerous travel hazards such as open creeks, barely buried rocks and trees, creating poor and challenging travel conditions, especially below treeline.Â
The avalanche danger should generally decrease for all elevation bands on Wednesday.
Remember that closed ski areas without avalanche mitigation are equivalent to back-country terrain! Â
Snowpack Discussion
The great start to the 2017/18 PNW Winter hit the pause button last week throughout the Cascades during this incredibly active period last week with an extended period of warm and wet weather. Several cycles of wet snow and glide avalanches occurred, especially on steep unsupported slopes and rock faces.Â
Over the weekend, the weather system that brought rain to the rest of the Cascades brought several inches of snow Saturday night through Sunday at Washington Pass. Mixed precipitation or a brief period of rain may have occurred Sunday afternoon as temperatures warmed aloft. Â
Little to no new snowfall was seen for most of the central-east and southeast Cascades over the weekend due to warmer temperatures and a period of rain. Snow depths are limited in these areas.
A front and a splitting upper trough crossed the US west coast Sunday and Sunday night. Along the Cascade East slopes, temperatures generally dropped from near or above freezing to the 10's and 20's. New snow amounts were T-3" on Monday morning with the most at Harts and Washington passes. The only wind observations from NWAC stations along the Cascade east slopes are from Washington Pass where winds shifted from SW 20's to WSW 10's.
Lingering light snow showers were seen mainly along the Cascade west slopes on Monday, with little or no accumulation along the east slopes, at continued cooler temperatures and lighter winds.
On Tuesday, a front brought a slight bump in snow levels into the 3000 ft range. By Wednesday morning 24 hour snow totals are expected to be in the 5-12" range, with the with the highest storm totals expected in the northeast Cascades.
Observations
See the recent observations tab for reports of several wet snow avalanches near the WA Pass area as a result of the warming and rain last week.
Avalanche Problems
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: Alpine, Treeline.
Likelihood: Possible
Expected Size: 1 - 1
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