Avalanche Forecast

Issued: Mar 20th, 2017 12:24PM

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

Northwest Avalanche Center NWAC, Northwest Avalanche Center

Above treeline, the avalanche danger should slightly increase Tuesday due to shallow fresh wind slabs near ridges. At lower elevations, shallow wet snow conditions should maintain the possibility of loose-wet avalanches. Watch for overhead hazard, such as recently formed large cornices.

Summary

Detailed Forecast

A weak disturbance will spread increasing light rain and snow at gradually rising freezing levels overnight Monday. Only light amounts of precipitation are expected overnight. This should begin to wet and weaken shallow surface snow layers.

Another band of moisture Tuesday should bring periods of generally light rain and snow at moderate freezing levels.  Southerly winds should be moderate along the ridges Tuesday.

The new snow and wind will likely form small to large fresh wind slabs. Human triggered avalanches may include some of the lower density snow from Saturday. Additionally, we have seen large cornice-fall avalanches that could trigger potentially very large wind-slabs formed during the last storm cycle or as new snow is added.

At lower elevations, light rain should maintain shallow wet snow conditions and make loose-wet snow avalanches possible.

It is always a good plan to travel well back from ridges, suspected of cornice formation, or on steep slopes below cornices.

Snowpack Discussion

Weather and Snowpack

The first week or so of March was very cool and snowy. NWAC stations along the west slopes of the Cascades piled up about 3 to 8 ft of snow with the most at Mt Baker.

The 2nd week of March was equally active with non-stop Pacific frontal systems pummeling the PNW. Unfortunately, these systems delivered far more rain than snow. At least two regional avalanche cycles occurred during the stretch. Significant snowpack consolidation occurred over this period due to rainfall and warmer temperatures. 

After a short respite from the active weather pattern on Thursday, another strong low pressure system brought several inches of rain to the west slopes of the Cascades outside the Cascade Passes Friday night. 2.5 to 4 inches of water was seen at Paradise, Crystal and Mt. Baker, respectively over the 24 hours ending at 5 PM PDT Saturday, mainly in the form of rain. Stevens and Snoqualmie Pass, picked up 6-8 inches of new snow Friday night before changing to rain later Saturday morning.

Rapid cooling with the frontal passage produced about 8 inches of snow at Mt. Baker and in a convergence band which set up in the North Central Cascades with decreasing amounts seen elsewhere along the west slopes of the Cascades late Saturday morning through Saturday evening (Stevens - <5", Snoqualmie 4", Crystal <3"). Sunday was cool with light winds with a general clearing trend seen throughout the Cascades. 

High clouds have pushed into our region Monday ahead of the next system, with some solar warming crusts from Sunday present on most aspects in the near tree-line and on solar aspects at higher elevations.

Recent Observations

North

NWAC observer Lee Lazzara was in the Ptarmigan Ridge area of the Mt Baker backcountry Sunday.  There was widespread evidence of the most recent wet snow avalanche cycle from Friday night with debris at the bottom of many avalanche paths. Crowns from wet slabs were especially large below treeline. Lee found the most recent rain crust very supportable above treeline, but still in the process of re-freezing below 4500 feet. Small loose wet slides released naturally and ran well on steeper solar slopes during clearing in the afternoon.  About 20 cm (8") of recent storm snow was evenly distributed regardless of elevation. Isolated pockets of shallow wind slab were found above treeline, but no obvious loading pattern was discernable. 

A massive and destructive size 3.5+ natural avalanche was witnessed on the leeward upper alpine zone of Ruby yesterday. The slide was triggered by a large, natural cornice fall and ran over ~1400' vertically, and a far greater distance laterally. The crown was just below ridge-crest and varied in height from 7-25 ft. 

Central

Alpental pro-patrol reported easy and widespread ski triggered 6-8" storm slabs on the upper half of Alpental Saturday morning with rain reaching the summit. Loose wet avalanches were easily triggered later in the day on steeper slopes. Stevens Pass pro-patrol had similar results Saturday morning with widespread sensitive, loose wet slides, up to size 2, natural and human triggered during the warming and switch to rain. Stevens Pass reported no new avalanche activity as of Monday morning.

South

Crystal patrol reported a natural cycle Friday night, presumably loose wet, but little to no results during avalanche control work Saturday morning.  On Sunday, Crystal pro-patrol reported a stout rain crust on the upper mountain with about 5-8 cm (2-3 in.) of well bonded snow on top of the crust. 

Problems

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, West, North West.

Elevations: Alpine.

Likelihood

Possible

Expected Size

1 - 2

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.

Elevations: Treeline, Below Treeline.

Likelihood

Possible

Expected Size

1 - 1

Valid until: Mar 21st, 2017 12:24PM