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Avalanche Forecast

Archived

Feb 4th, 2016–Feb 5th, 2016

Alpine
Natural avalanches possible, human triggered probable.
Treeline
Natural avalanches possible, human triggered probable.
Below Treeline
Natural avalanches possible, human triggered probable.
Alpine
Natural avalanches possible, human triggered probable.
Treeline
Natural avalanches unlikely, human triggered possible.
Below Treeline
Natural avalanches unlikely, human triggered possible.

Regions

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A mixed bag of ugly snow conditions and a considerable avalanche danger in all the elevation bands is expected Friday. Careful snowpack evaluation will be needed along the east slopes on Friday.

Detailed Forecast

The next weather system will move strongly to the Olympics and north Cascades Friday late morning and spread to the rest of the Cascades by Friday evening with fairly high snow levels.

A fairly broad brush will be applied to the avalanche forecast for Friday. Expect a mixed bag of ugly snow conditions and a considerable avalanche danger in all the elevation band along the east slopes.

Wet snow and rain or warm temperatures should further build or activate wind and storm slab mainly in the above tree line band.

Recent snow will be available for activation by rain and warm temperatures elsewhere for loose wet avalanches. Watch for surface wet snow deeper than a few inches and pinwheels or small natural loose wet avalanches that usually precede large loose wet avalanches.

But new snow and cooler temperatures should be seen by Saturday.

 

Snowpack Discussion

Weather and Snowpack

Two fair weather periods in January allowed surface hoar and near surface faceting to occur. These persistent weak layers were buried intact on Jan 3rd and 11th throughout the Cascades.

A warm front about January 21st caused up to about a foot of snow in the northeast zone. Freezing rain or rain created a crust in the central east and southeast zones.

Another wet and warm front was seen Jan 27-28th. This brought up to another 1.5 ft of snow in the northeast zone and likely formed another crust in the central and southeast zones. 

A strong occluded front with strong winds crossed the Northwest last Friday and Saturday. NWAC stations along the east slopes had about 4-14 inches of new snow by Saturday morning.

Cool, benign weather followed Sunday through Tuesday. A sun crust formed on many solar slopes and surface hoar was seen on many non-solar slopes.

A cold front crossed the Northwest on Wednesday. A warm front is causing more snow mainly in the south Cascades on today. NWAC stations along the east slopes for the 2 days ending Friday morning should have about 2-7 inches of increasing damp or wet upside down snow.

Recent Observations

The January 3rd and 11th layers were reactive with widespread triggering occurring during the late January storm cycles. The latest observations indicate that these layers have gradually become non-reactive and the persistent slab problem has been removed from the east slope zones. You can still check for these layers in snow pits in your specific area of travel.

NWAC pro-observer Jeff Ward was in the Chumstick Mountain area Monday. The January 3rd and 11th layers were found at about 75-85 cm and 70-80 cm and gave hard or no results in tests. Jeff noted isolated, stubborn to release small wind slab near ridges and new widespread surface hoar.

Jeff was in the Stevens Pass area Tuesday. He generally found a stable upper snowpack but observed new surface hoar growth most prominent in Highland Bowl near ridgecrest.

Tom Curtis observed healthy surface hoar growth in the Blewett Pass area on all aspects Monday and in the Icicle Creek drainage near and above 4500 feet Tuesday. Increased cloud cover in the Washington Pass zone may have limited surface hoar development in this area.

The surface hoar from early this week will need to be watched in case it makes new snow Wednesday and Thursday more reactive.

Problems

Wind Slabs

Wind Slab avalanches are the 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.

Storm Slabs

Storm Slab avalanches are the release of a cohesive layer (a slab) of new snow that breaks within new snow or on the old snow surface. Storm-slabs typically last between a few hours and few days (following snowfall). Storm-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.

Loose Wet

Loose Wet avalanches are the 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. 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.