Avalanche Forecast

Issued: Feb 24th, 2017 10:00AM

The alpine rating is moderate, the treeline rating is moderate, and the below treeline rating is moderate. Known problems include Loose Wet and Wind Slabs.

Northwest Avalanche Center NWAC, Northwest Avalanche Center

Sunshine Saturday may lead to shallow wet snow conditions on steep solar aspects during the late morning to afternoon. Watch for snow rollerballs, snow falling from trees, and for wet surface snow more than the top few inches. Watch for cornice hazard as well.

Summary

Detailed Forecast

A brief period of high pressure should build over the area Saturday. This will allow for clearing and plenty of sunshine Saturday. Winds should remain light with very cool temperatures. 

Old wind slabs should continue to heal, becoming very isolated to specific terrain features, and very stubborn to human triggering Saturday. 

More sunshine is expected Saturday than previously this week. Direct sunshine and some daytime warming should gradually increase the potential for loose wet avalanches Saturday, mainly confined to steep solar aspects during the warmest part of the day. Watch for wet surface snow deeper than a few inches if you find yourself on sun exposed slopes during any extended sun breaks.

Loose dry avalanches have been triggered this week, but will not be indicated as an avalanche problem. You can do tests for loose dry avalanches by pushing snow onto small safe test slopes.

Avoid areas along ridges where there may be a cornice and slopes below cornices, since cornices can fail at any time.

Snowpack Discussion

Weather and Snowpack

The latest of several warm, wet SW streams of moisture this season arrived on Valentines Day 2/14 bringing another round of heavy rain, avalanches, crusts and consolidation through Thursday 2/16 along the Cascade west slope zones. Minor new snow amounts were deposited at the tail end of the storm.

A short period of fair weather on Friday, 2/17 caused another surface crust, especially on solar aspects.

A pair of storms last Sunday and Monday deposited about a foot of snow at most areas by Monday. This was combined with periods of moderate to strong W-SW winds, forming wind slabs at the time.

A slightly unstable weather pattern this week caused a mix of sun and light snow showers with cold temperatures and light winds from Tuesday through Friday. The weather this week helped to freshen the surface with a few, up to several inches of snow. This weather has all tended towards increased snowpack stability through the week, while maintaining some excellent touring conditions, especially on shaded terrain.

Recent Observations

North

NWAC observer Lee Lazzara was in the Mt. Herman-Artist Point area Monday 2/20 and found 15 cm (6") of denser new snow poorly bonded to less dense snow from earlier in storm cycle. In wind loaded areas, the slab was up to 60 cm (2 ft) deep.

Lee was back out in the Heliotrope ridge area on Thursday 2/23 on mostly N-E aspects and no results from ski tests. He found some reactivity in ECT tests due to wind slab from Sunday 20-60 cm below the surface. He also reported some small triggered, loose dry avalanches.

Central

On Tuesday 2/21, the Alpental patrol reported widespread 6" storm slabs, occasionally up to 10" in isolated locations during control work with explosives. The slabs were soft and generally didn't run far. Shallow loose avalanches were becoming touchy by midday with a slight rise in temperatures even with cloudy skies.

A report for the Snoqualmie area for Wednesday 2/22 via the NWAC Observations page indicates pockets of 20 cm likely storm or wind slab on a steep S-NE slope above 4500 ft.

NWAC observer Jeremy Allyn was in the Snoqualmie Pass area on Thursday 2/23 and found old wind slab weaknesses at 30-40 cm but little in the way of avalanche problems.

South

NWAC observer Ian Nicholson was at Paradise on Monday 2/20 and observed only minor wind transport of new snow. Ian also reported storm slab instabilities were a bit less touchy than in the Crystal area Monday. 

Backcountry reports from professionals in the Crystal area on Tuesday 2/21 indicated less sensitive avalanche conditions overall, but storm instabilities were still reactive to ski triggering about 20 cm (8") down on steep slopes.

NWAC forecasters Garth and Dennis were in the East Peak area Wednesday 2/22 finding improved mainly right side up conditions. Storm layers were still producing moderate test results, but not indicating propagation. Cornice drops onto previously loaded east facing slopes did not give significant results. Wind slabs seemed confined to isolated nearby ridges.

NWAC observer Ian Nicholson was in the East Peak area on Thursday 2/23, he also found recent snow was generally right side up with good conditions and good bonding to the 2/14 crust.  He noted small pockets of shallow potential 10-15 cm 1F wind slab.

Problems

Loose Wet

An icon showing 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

Possible

Expected Size

1 - 1

Wind Slabs

An icon showing 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.

Likelihood

Unlikely

Expected Size

1 - 1

Valid until: Feb 25th, 2017 10:00AM