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Archived

Avalanche Forecast

Mar 20th, 2016–Mar 21st, 2016
Alpine
2: Moderate
The avalanche danger rating in the alpine will be moderate
Treeline
2: Moderate
The avalanche danger rating at treeline will be moderate
Below Treeline
2: Moderate
The avalanche danger rating below treeline will be moderate
Alpine
2: Moderate
The avalanche danger rating in the alpine will be moderate
Treeline
2: Moderate
The avalanche danger rating at treeline will be moderate
Below Treeline
2: Moderate
The avalanche danger rating below treeline will be moderate

Regions: Olympics.

A mixture of typical spring avalanche problems exists with no one problem dominating. Evaluate the local snow and terrain carefully on Monday.

Detailed Forecast

Cooler temperatures, cloudy and showery weather is expected Monday.  This weather should maintain areas of wet and weak snow below about 4-5000 feet in the Below Treeline zone. Moderate to strong winds at the higher elevations should build new shallow wind slabs on some leeward exposed ridges, mainly NW-E facing.  Firm wind transported snow and snowpack cracking can be good signs of wind slab layers.

Recently formed cornices have grown large and can become sensitive during mild weather, especially following nights with above freezing temperatures.  Avoid areas on ridges or summits where there may be a cornice and avoid slopes below cornices in the spring. Cooler temperatures Monday may help temporarily strengthen cornices Monday.

Snowpack Discussion

Weather and Snowpack

A combination of sun and mild temperatures for several days gave way to mild temperatures and now light rain Sunday. The recent weather over the past five days has allowed for the 2-3 feet of storm snow that accumulated from about March 9-15 to settle and stabilize. Wind, sun, warmth, freezing and now rain have drastically changed snow conditions over the past several days. Surface snow conditions have become highly variable, ranging from some well settled old snow on steep shaded slopes and a mix of surface crusts, wind buffed surfaces or shallow wet snow on other aspects. Earlier formed avalanche problems of wind and storm slab have very likely settled and stabilized over the past several days of mild weather. 

The most recent storms did build large cornices along ridges with several releases noted over the past few days.

The mid and lower snowpack in the Olympics should generally be a stable mix of crusts and layers of moist and rounded snow crystals.

Recent Observations

A Hurricane Ridge Park ranger found variable surface crusts Saturday morning, as overnight temperatures remained well over freezing. Similar conditions were found Sunday with the snowpack settling an additional 20 cm (8 inches) at Hurricane Ridge since Saturday morning, an additional sign of a strengthening snowpack.  

Hurricane Weather Station

Internet communications to the station have been disrupted at the NWAC office. The ISP inadvertently disconnected our service and we are anticipating the resumption of service, sometime Monday, March 21. We apologize for the outage and thank you for your patience.

Avalanche Problems

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: Below Treeline.

Likelihood: Possible

Expected Size: 1 - 1

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, North West.

Elevations: Alpine, Treeline.

Likelihood: Possible

Expected Size: 1 - 1

Cornices

Cornice Fall is the release of an overhanging mass of snow that forms as the wind moves snow over a sharp terrain feature, such as a ridge, and deposits snow on the downwind (leeward) side. Cornices range in size from small wind lips of soft snow to large overhangs of hard snow that are 30 feet (10 meters) or taller. They can break off the terrain suddenly and pull back onto the ridge top and catch people by surprise even on the flat ground above the slope. Even small cornices can have enough mass to be destructive and deadly. Cornice Fall can entrain loose surface snow or trigger slab avalanches.

 

Cornices can never be trusted and avoiding them is necessary for safe backcountry travel. Stay well back from ridgeline areas with cornices. They often overhang the ridge edge can be triggered remotely. Avoid areas underneath cornices. Even small Cornice Fall can trigger a larger avalanche and large Cornice Fall can easily crush a human. Periods of significant temperature warm-up are times to be particularly aware.

 

A corniced ridgeline. A large cornice has formed at the top of the ridge. A smaller cornice has formed to the left of the trees from crossloading.

Cornices are easy to identify and are confined to lee and cross-loaded ridges, sub-ridges, and sharp convexities. They are easiest to trigger during periods of rapid growth (new snow and wind), rapid warming, and during rain-on-snow events. Cornices often catch people by surprise when they break farther back onto flatter areas than expected.

Aspects: North, North East, East, South East, North West.

Elevations: Alpine, Treeline.

Likelihood: Unlikely

Expected Size: 1 - 1