Avalanche Forecast
Issued: Mar 7th, 2017 11:42AM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Wind Slabs and Storm Slabs.
NWAC,
Northwest Avalanche Center
There has been a lot of snow the past few days and you cannot use the backcountry in your usual manner. Your usual slopes and routes may not be safe from avalanches. Travel in avalanche terrain is not recommended on Wednesday.
Summary
Detailed Forecast
Strong southwest flow carried a front across the Northwest on Tuesday. This should bring another 10-20 inches of snow to most of the Cascade west slopes by Wednesday morning with SW-W alpine winds and warming temperatures. Warming temperatures favor denser surface layers and the formation of wind and storm slab avalanches.
There has been a lot of snow the past few days and you cannot use the backcountry in your usual manner. Your usual slopes and routes may not be safe from avalanches. Plan to stay on low angle slopes away from avalanche terrain. There is more uncertainty than usual due to lack of backcountry field reports and the heavy snowfall. Avalanches are likely to be larger and run farther and be more dangerous than usual.
Deep wind slab should be watched for on all aspects but should be found mainly on NW to SE aspects due to recent SW to W winds. Watch for firmer wind transported snow on varied aspects especially in areas of complex terrain.
Most areas will have seen rapidly accumulating snowfall by Wednesday. Previous deep storm slab may not have stabilized. The warming trend will increase the likelihood of new storm slab layers.
We may have had enough snow for avalanches to step to or run on the Valentine's Day crust. Such avalanches would be large and very dangerous.
Cornices won't be listed as an avalanche problem but avoid travel on ridges near where cornices may have formed and avoid steep slopes below cornices that may fail at any time.
There has been enough snow lately that snow immersion accidents seem possible. Travel with a partner and keep them in sight at all times.
Snowpack Discussion
Weather and Snowpack
The most recent warm, wet storm arrived on Valentines Day forming a strong rain crust, now buried several feet or more in most areas along the west slope Cascade zones.
Strong southwest flow carried a strong front across the Northwest on Friday evening. Along the Cascade west slopes this caused strong southwest alpine winds, heavy, moist, dense new snow above about 3-4000 ft and wet snow or rain below about 3-4000 ft.
NWAC stations along the Cascade west slopes have had about 2-6 ft of snow in the past 5 days with the most at Mt Baker!
A region wide avalanche cycle was seen late Friday and Saturday, especially at Mt Baker, with some more avalanches on Sunday.
Recent Observations
North
Widespread 1-2 ft storm slabs and larger, 3-5 foot wind slabs were seen in the backcountry near Mt Baker on Saturday. A close call occurred on Mt Herman when a large wind slab was triggered, and partially burying two and fully burying one in a separate party at the base of the slide path after completing a run. It was a fortunate outcome with no significant injuries.
Central
NWAC observer Dallas Glass was in the Alpental Valley Saturday and reported that previous moist to wet snow was settling and less reactive. The earlier storm slab problem was rapidly improving in this zone.
Wind slab, 1-2 feet, ski triggered Saturday 3/4, while ascending the ridge on Arrowhead Peak, east of Stevens Pass, 5500 ft elevation, SW aspect, no skier involvement. Photo: K. Johnson
Dallas was back in the western Alpental Valley Sunday, from 3000-5500 ft and noted a recent natural cycle occurred, likely early Sunday during heavy snow shower intensities. Numerous 6-8 inch storm slabs released in steep terrain near and below treeline. There is significant low density surface snow available for transport, so watch for increased winds over the next few days.
Several more triggered avalanches at Stevens and Snoqualmie were reported via the NWAC Observations page on Sunday.
A report from Stevens Pass above 4500 feet on Monday indicated knee to waist ski penetration and snow immersion potential.
The Alpental patrol reported building storm slab, becoming more sensitive to ski triggering, up to 1 ft deep by Tuesday afternoon.
South
NWAC observer Jeremy Allyn was in the Crystal Mountain backcountry Sunday and reported a favorable profile of the recent storm snow on N and W aspects. There was about 90 cm of snow on the Valentines crust. Hand and ski tests and test cornice drops did not give results and wind effects were minimal in this area.
Here is a nice photo example of the cornice features, captured Sunday in similar terrain, near Crown Point by our good friend Seth Waterfall, thanks.
Sensitive cornice and wind slab on Crown Point near Crystal Mountain on Sunday 3/5. Photo: Seth Waterfall
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: Alpine, Treeline.
Likelihood
Expected Size
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
Expected Size
Valid until: Mar 8th, 2017 11:42AM
