Avalanche Forecast
Issued: Jan 25th, 2018 10:30PM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Wind Slabs, Storm Slabs and Loose Dry.
NWAC,
Northwest Avalanche Center
Dangerous avalanche conditions will persist on Friday mainly above treeline due to additional light snowfall, an uptick in wind transported snow, and cold temperatures preserving recent storm instabilities. Give safe margins near and below growing cornices. Loose dry avalanches will run fast in steep terrain with plenty of loose snow available, avoid steep lines with terrain traps. Persistent weak layers may be found in isolated areas in the northeast and central-east zones. Greater margins of safety are required due to the uncertainty surrounding the distribution of these layers, especially when considering consequential terrain.
Summary
Detailed Forecast
Dangerous avalanche conditions will persist on Friday mainly above treeline due to additional light snowfall, an uptick in wind transported snow near and above treeline, and cold temperatures preserving recent storm instabilities. Due to lower density surface snow available for transport and moderate W-SW winds forecast, wind slabs may develop below treeline or further downslope than you might expect Friday. Avoid travel on slopes if you encounter wind stiffened surface snow, especially on steeper exposed terrain and steep roll-overs. Give safe margins near and below growing cornices.
Loose dry avalanches will run fast in steep terrain with plenty of loose snow available, avoid steep lines with terrain traps.
Persistent weak layers may be found in isolated areas in the northeast and central-east zones. Greater margins of safety are required due to the uncertainty surrounding the distribution of these layers, especially when considering consequential terrain.
There's plenty of good snow out there, so maintain a conservative approach to terrain selection.
Snowpack Discussion
An extended storm cycle continues, sharing (almost) equally with the east slope locations! Total storm snow of 2 to 3.5 ft is typical, and the recent storm snow overlies a melt-freeze crust buried Jan 16th. The bonding continues to be favorable to this crust from most recent observations.
Observations above treeline have been limited leading to a higher level of uncertainty in this terrain.
Buried surface hoar was observed on the 1/16 crust in the Cascade East - Central zone on Wednesday, January 17th. The layer was also found on the crust in the Twisp River drainage on Monday, January 22nd, but faceted grains below the crust were also concerning. The crust/PWL interface were the suspect culprit for a large avalanche on Sunday 1/21 in the Cascade East-North zone. This layer will need to be watched during the upcoming stormy period as the extent and distribution of these weak layers remain uncertain and likely confined to isolated areas.
Snowdepth still decreases substantially east of the Cascade crest. In many areas below treeline, there has not been enough snow to present an avalanche danger.
Note - The NWAC Washington Pass station is back online - nice work DOT! Precipitation and other sensor repairs to be planned...
Observations
North
On Thursday, NCH observed evidence of numerous recent wind slabs that had released in steep terrain above treeline in the Washington Pass area.
Wednesday, Jan 24th, NCMG observations on Delancy Ridge found nearly 3 ft of low cohesion storm snow in sheltered terrain below treeline with no avalanches noted. No reactivity was found at the 1/16 interface with the melt-freeze crust.
On Monday, snowpit tests by a snow professional on a SE aspect near treeline in the Twisp River Valley gave sudden collapses within faceted grains below the 1/16 crust. Test results on a NE aspect yielded consistent sudden planar results above the 1/16 crust with partially decomposed surface hoar on the interface.
NWAC received a second-hand report of a large skier-triggered avalanche on west-facing slope (likely near treeline) on Abernathy Peak in the upper Twisp River drainage. The avalanche propagated widely and was suspected to have failed on the 1/16 crust.
Central
A public observation from Wednesday in the Blewett Pass area identified basal facets at the bottom of the snowpack and surface hoar above the most recent crust. Snow cover was still regionally low in this area. Small skier triggered wind slabs were noted and rain briefly reached up to 5000' Wednesday morning.
Buried surface was found in the Icicle Creek drainage over the 1/16 crust over a week ago up to 6800 ft.
South
No recent 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: 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
Loose Dry
Release of dry unconsolidated snow. These avalanches typically occur within layers of soft snow near the surface of the snowpack. Loose Dry avalanches start at a point and entrain snow as they move downhill, forming a fan-shaped avalanche. Other names for loose-dry avalanches include point-release avalanches or sluffs. Loose Dry avalanches can trigger slab avalanches that break into deeper snow layers.
Loose Dry avalanches are usually relatively harmless to people. They can be hazardous if you are caught and carried into or over a terrain trap (e.g. gully, rocks, dense timber, cliff, crevasse) or down a long slope. Avoid traveling in or above terrain traps when Loose Dry avalanches are likely.
Loose Dry avalanche with the characteristic point initiation and fan shape.
Loose dry avalanches exist throughout the terrain, release at or below the trigger point, and can run in densely-treed areas. Avoid very steep slopes and terrain traps such as cliffs, gullies, or tree wells.
Aspects: All aspects.
Elevations: All elevations.
Likelihood
Expected Size
Valid until: Jan 26th, 2018 10:30PM
