Avalanche Forecast
Issued: Mar 13th, 2016 12:11PM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Wind Slabs, Storm Slabs and Loose Wet.
NWAC,
Northwest Avalanche Center
Conditions are a bit hard to predict following the low pressure system and front on Monday. Conservative decision making will be a good plane until the avalanche forecasts get refined after new information becomes available on Monday.
Summary
Detailed Forecast
Up to about 5-10 inches of new snow seems likely along the east slopes by Monday morning.
West winds aloft and a cool slightly unstable air mass should follow the front across the Northwest on Monday. This should cause orographic snow showers mainly along the west slopes but up to perhaps another 5 inches or so seems possible along the east slopes by the end of the day.
New wind and storm slab seem likely to be the main avalanche problems along the west slopes on Monday.
New wind slab is mostly likely to be found on northwest to southeast slopes. Firmer wind transported snow and snowpack cracking can be good signs of wind slab.
New storm slab is likely in areas where new snow rapidly accumulates for more than several hours.
Partly sunny weather is likely areas along the east slopes on Monday. The sun is gaining power so watch for the development of loose wet snow on solar slopes.
Conditions are a bit hard to predict following the low pressure system and front on Monday. The avalanche danger may decrease a little on Monday due to a little less wind, some stabilizing of new snow and fairly cool temperatures. This forecast will take a conservative approach to the danger levels and the avalanche forecasts will get refined after new information becomes available on Monday.
The persistent slab avalanche problem will not be listed in the northeast zone due to an abundance of other avalanche problems. This problem is most likely to be found on non-solar aspects in the above and near treeline band and stretching into the upper portion of the below treeline band. This interface is likely getting harder for a human to trigger but if triggered is capable of producing large avalanches. Be aware that at depths approaching the 1 m mark the extended column test becomes a less reliable indicator of propagation across a column. Deeper tests like the propagation saw test or deep tap test may help but layer identification and terrain selection are your best friends for managing a persistent slab danger.
Snowpack Discussion
Weather and Snowpack
During a period of fair weather in late February, widespread surface hoar formed in the northeast zone mainly surviving outside of steeper solar aspects and wind affected terrain. A weak front buried the surface hoar layer in the Washington Pass area about 2/27 and to a lesser extent the central-east zone.
Storms have moved across the Northwest almost every day or two the past couple weeks with fluctuating but generally moderate snow levels.
A storm last Wednesday and Thursday caused a warming trend that peaked Wednesday night with very strong west winds seen throughout the Cascade range. About 6-16 inches of snow accumulated through Thursday morning with the higher tallies of over a 1 foot seen at Holden, the NWAC station at Washington Pass and the NRCS sitess at Lyman Lake and Hart's Pass.
A front on Saturday caused west-southwest winds and light amounts or up to a few inches of snow along the east slopes ending Sunday morning.
A deep surface low pressure system is moving from the Olympic Peninsula to Vancouver on Sunday. A front is crossing the Cascades Sunday afternoon. This is causing stormy weather in the Cascades with strong shifting southeast to southwest winds and moderate snow along the east slopes.
We are no longer tracking any layers of concern formed earlier this winter in the mid or lower snowpack due to lack of recent activity at these interfaces and confirming field observations.
Recent Observations
Strong winds on March 3rd caused widespread natural wind slab avalanches in the NE zone. While many were contained to the recent storm snow, one larger slide on a N-NE aspect at Windy Pass likely released down to the 2/27 PWL. The last known human triggered avalanche on this layer was also in the Washington Pass area on March 2nd.
Mission Ridge pro-patrol reported a very active day of control on Thursday, March 10th with 1-4 foot slides produced during control work on lee slopes below ridges. Wind slabs were sensitive and ran far. Widespread cornice growth was also noted along ridge lines.
The North Cascade Guides report for the Washington Pass area on Friday indicates 20-50 cm of HST on lee slopes. Several pits had a mix of homogeneous snow or crusts in the top meter of snow. Tests gave no results or RP results including on the 2/27 interface. Crowns or debris with sizes to 2.5 was noted from an avalanche cycle on Wednesday to Thursday.
The Guides report for Washington Pass on Saturday noted snow plumes off nearby peaks indicating wind transport and more evidence of the avalanche cycle on Wednesday and Thursday.
The Guides at Washington Pass on Sunday reported large cornices with some releases and large debris. Wind transport was steady through the day but low visibility prevented further observations.
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
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
Loose Wet
Release of wet unconsolidated snow or slush. These avalanches typically occur within layers of wet snow near the surface of the snowpack, but they may quickly gouge into lower snowpack layers. Like Loose Dry avalanches, they start at a point and entrain snow as they move downhill, forming a fan-shaped avalanche. They generally move slowly, but can contain enough mass to cause significant damage to trees, cars or buildings. Other names for loose-wet avalanches include point-release avalanches or sluffs. Loose Wet avalanches can trigger slab avalanches that break into deeper snow layers.
Travel when the snow surface is colder and stronger. Plan your trips to avoid crossing on or under very steep slopes in the afternoon. Move to colder, shadier slopes once the snow surface turns slushly. Avoid steep, sunlit slopes above terrain traps, cliffs areas and long sustained steep pitches.
Several loose wet avalanches, and lots of pinwheels and roller balls.
Loose wet avalanches occur where water is running through the snowpack, and release at or below the trigger point. Avoid terrain traps such as cliffs, gullies, or tree wells. Exit avalanche terrain when you see pinwheels, roller balls, a slushy surface, or during rain-on-snow events.
Aspects: South East, South, South West.
Elevations: All elevations.
Likelihood
Expected Size
Valid until: Mar 14th, 2016 12:11PM
