Avalanche Forecast
Issued: Dec 26th, 2017 12:57PM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Wind Slabs and Loose Dry.
NWAC,
Northwest Avalanche Center
Recent wind slabs are settling very slowly under cold temperatures and can be found on a variety of aspects near and above treeline. Shallow new snow may be masking wind slabs, approach exposed terrain with great caution.
***A strong storm cycle is expected Thursday through Friday. The avalanche danger will likely reach warning criteria. Pay close attention to forecasts and be prepared to alter plans as we move towards the weekend.***
Summary
Detailed Forecast
A ridge of high pressure should move east of the area Wednesday opening the door to increased Pacific frontal systems later this week.
The weather Wednesday should be cool with light winds and promote the continued slow settlement of recent storm layers or wind slabs in higher exposed terrain. Cool temperatures continue to slow this process and may help preserve wind slabs in higher terrain.
Watch for wind stiffened snow on a variety of aspects, including non-traditional aspects from recent northerly winds
There remains significant deep, soft snow out there from the past week. Unconsolidated snow, particularly around small trees, can present a non-avalanche, snow immersion hazard. Keep visual and verbal communication with your travel partners when traveling in treed terrain.
Also despite all this new snow, early season hazards still exist at some lower elevation locales and especially around creek beds that are not filled in.
Snowpack Discussion
Weak disturbances moved across the Cascades Friday and Christmas Eve depositing a total of about 4-14 inches of snow by Christmas morning.
In the Snoqualmie Pass area as of Tuesday, December 26th, a thin layer of freezing fog had produced a surface crust in low to mid elevation terrain, capping the recent new snow.
Periods of moderate to strong and shifting winds have occurred over the past several days to easily transport the cold loose new snow and cause both wind stiffened surfaces and build wind slabs in exposed terrain, especially near and below ridges near and above treeline.
The few recent avalanches reported since last Wednesday have involved wind slabs in higher terrain and on steep features such as terrain convexities.
The last major storm cycle began on December 15th, ending Wednesday, December 20th. Storm total snow during this period ranged from 1-4 feet throughout these zones and has been reported to be fairly well bonded to the old snow surface formed during the high pressure in early December.
In most wind sheltered areas near and below treeline, the snowpack consists of shallow very low density surface snow with slowly settling and gradually increasing density storm snow.
The December 15 layer varies from about 2-4 feet throughout the zones.
Observations
North
Tuesday, December 26, NWAC Observer Lee Lazzara traveled between 2000-6000 feet in the Swamp Ck drainage. The recent cold low density storm snow from this past week has been preserved and producing stable conditions with favorable density profile near and below treeline. Tests failed to produce results within the upper storm snow layers and the 12/15 layer was not a concern in this area. The wind slab problem in higher terrain was difficult to accurately assess as a result of shallow new snow and low visibility. Wind slabs appeared to be the main current problem in exposed terrain and specific terrain features, such as slope convexities. Wind slabs were featured on a variety of aspects due to shifting wind patterns over the past week.
A main take away was the preservation of low cohesion surface snow which may lead to a rapid increase in avalanche danger with the anticipated storm cycle later this week!
On Saturday, a skier on Mt Herman, reported no signs of instability via the NWAC Observations page.
Central
On Christmas Day, the Alpental Pro Patrol indicated the 24 hr storm snow of about 6 inches showed little to no wind affect. Ski cuts produced only shallow loose-dry slides confined to steep, isolated features and did not entrain significant snow or travel far in the paths.
The Alpental pro patrol on Sunday reported shallow wind slab forming due to east winds.
A skier on Humpback Mountain via the NWAC Observations page for Saturday reported getting caught by a small 2 foot wind slab on a W facing slope near the summit. No serious injuries were reported. Note that areas west of Snoqualmie typically have stronger winds during periods of offshore flow.
NWAC pro observer Jeremy Allyn was in the Chair Peak - Mt Roosevelt area on Saturday and reported windy conditions on the shoulder of Chair Peak. His party triggered 2 small wind slabs on the north slope of Chair Peak at about 4800 feet. Storm layers are healing slowly due to cold temperatures.
The Stevens pro patrol on Saturday found moderate CT result in storm layers, with hard or no results on the 12/15 layer down 85-90 cm in north slope pits at 4800 and 5800 feet. No wind slab was observed.
NWAC forecaster Robert Hahn was on Jove Peak on Friday and found deep trail breaking, small subtle wind slab on the ridge, storm slab no longer an issue, no signs of instability and good skiing.
A public observation from Stevens Pass Thursday reported triggering very small isolated wind slabs in wind loaded terrain. The party also noted a mostly right-side-up snowpack that was gaining strength.
South
A nice report via the NWAC Observations page from the Tatoosh on Saturday reported several triggered D1-1.5 loose dry avalanches on various steep slopes.
Professional observations from the Crystal Mountain backcountry Wednesday and Thursday are consistently demonstrating a well bonded and strengthening snowpack. Many newly formed cornices along the ridges failed easily under a travelers weight.
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
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: Dec 27th, 2017 12:57PM
