Avalanche Forecast
Issued: Jan 25th, 2017 11:47AM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Wind Slabs and Loose Wet.
NWAC,
Northwest Avalanche Center
Thursday's avalanche danger will consist of a mix of lingering wind slab near and above treeline and loose wet avalanches on solar slopes if extended sunbreaks materialize during the afternoon.
Summary
Detailed Forecast
Generally cloudy weather with light winds should be seen on Thursday although there may be some sunbreaks in the afternoon. A slow warming trend begins Thursday.
These conditions will continue to help recent wind and storm slabs settle and further stabilize. Storm slabs will not be listed as an avalanche problem, but particularly in the Stevens and Snoqualmie Pass areas, storm slabs may still be poorly bonded to the underlying freezing rain crust in isolated areas.
Lingering wind slabs are most likely on SW-NW-NE aspects in the Passes and the Crystal area due to strong S-E transport winds over the weekend. In the Mt Baker area, look for wind slabs primarily on NW-E-SE aspects. Watch for recent wind effects near and below ridgelines and avoid steep slopes with obvious signs of recent wind loading. Wind slabs will likely extend into the upper portion of the below treeline band in the Crystal Mountain, Snoqualmie and Stevens Pass areas.
Watch for loose wet snow conditions on steep solar slopes if the sun pops out for an extended period of time on Thursday afternoon.
Snowpack Discussion
Weather and Snowpack
An arctic air mass settled over the Northwest with fair, cold weather from the New Year through the second week of January. An atmospheric river moved over the Northwest 1/17-1/18 with highly variable weather seen throughout the Cascades during this event. Heavy rain was seen up to about 5500 feet in the north Cascades and up to about 6500 feet in the south with snow at higher elevations. Three day precipitation totals through Thursday 1/19 were roughly 8 inches at Mt Baker, 5 inches at Crystal and 1-2 inches at the Passes. Sleet and freezing rain Tuesday 1/17 at the Passes changed to snow at higher elevations at Stevens and Snoqualmie on Wednesday 1/18. At Stevens and Snoqualmie Pass the freezing rain crust is about 1 inch (2-3 cm) thick.
From last Thursday 1/19 through Sunday 1/22, generally 1-2 feet of snow has accumulated along the west slopes of the Cascades with the highest totals seen at Mt. Baker. Strong S-SE winds Saturday night through Sunday morning built unstable wind slabs on lee slopes primarily in the central and south Washington Cascades. Cloudy and cool weather with little to no new snow accumulation occurred Tuesday and Wednesday.
Recent Observations
North
NWAC observer Lee Lazzara was in the Mt. Baker backcountry on Wednesday 1/25 and observed a generally stable snowpack for the terrain he traveled through. Winds slabs were present but generally unreactive near and above treeline. Wind slabs were found on more traditional NW-E-SE aspects. The most recent storm snow was well bonded to the 1/17 rain crust in the Mt. Baker area. Several glide cracks from the rain event are still visible and at least one natural glide avalanche occurred last weekend.
Central
On Sunday 1/22, NWAC pro-observers Dallas Glass and Ian Nicholson were in the Stevens and Snoqualmie Pass areas. Strong south to east winds were actively transporting recent storm snow and building wind slabs on all but windward slopes. Many windward slopes, NE-S facing, were scoured to the 1/17 freezing rain crust. Storm slabs were gaining some strength but remained touchy in some areas below treeline. In both areas, recent storm snow was highly variable due to wind effects, but in sheltered areas averaged 12-16 inches.
From the NWAC Observations page easy shears and some signs of propagation were seen Sunday at the 1/17 interface near Lanham Lake at Stevens Pass. A skier reported triggering a large storm slab in a chute above Snow Lake on Sunday.
NWAC pro-observer Jeff Ward was on north slopes on Rock Mountain near Stevens Pass on Monday and found 50-65 cm of storm snow on the 1/17 crust with good stability and no recent avalanche activity.
A skier via the NWAC Observations page for the Source Lake-Bryant area on Monday reported 15-60 cm of storm snow on the 1/17 crust. Previous wind transport was evident at higher elevations. A skier triggered a large storm slab on a N-NE slope at about 4000 feet which ran on the 1/17 crust. Natural and ski triggered loose wet avalanches were also seen on solar slopes.
NWAC-ers Dennis D'Amico and Matt Schonwald were on Roaring Ridge in the Snoqualmie Pass area Tuesday 1/24. On N through E aspects at 4400 ft they found about 30 cm of right side up storm snow well bonded to the uneven and thick 1/17 freezing rain crust.
South
The Crystal pro-patrol reported extremely sensitive new wind slabs on NW aspects Sunday morning. About 4-6 inches of new storm snow were transported by very strong S-SE winds overnight and early Sunday morning. This built very sensitive wind slabs on lee slopes by early Sunday. Wind slabs, mostly in the 6-8" range, released sympathetically and were running long distances. Shooting cracks were extending over 100 feet from skis.
By Monday and Tuesday the Crystal pro-patrol reported that previous reactive wind deposited snow had bonded and greatly stabilized with no new avalanches.
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 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: Jan 26th, 2017 11:47AM
