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Avalanche Forecast

Archived

Mar 5th, 2014–Mar 6th, 2014

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

Regions

Stevens Pass.

Very dangerous avalanche conditions should persist Thursday, especially near and above treeline where the strongest winds are expected and where weaker bonds to the early March crust should remain. 

Detailed Forecast

Another strong warm front should cause more moderate to heavy rain or snow at higher snow levels Wednesday night with a bit of a break Thursday morning.  Another front should renew moderate rain or snow at lower snow levels during the day Thursday along with strong winds. 

Five day water equivalent nearing 4-7 inches and snowfall of about 3-6 feet should be the expected storm totals along the west slope Cascade zones ending Thursday afternoon.

Due to rain continuing to reach higher elevations Wednesday night before gradually cooling Thursday, continued  wet loose avalanches remain a concern at mid and lower elevations.  This should be at least up to the near treeline zones. Steep slopes that recently received heavy snowfall should be primed for wet loose avalanches. Natural snowballing and natural releases are usually precursors to this type of avalanche.

Wind slab will be a likely concern mainly on lee slopes above treeline but gradually lowering to near treeline as cooling occurs Thursday. This should be mainly northwest to southeast aspects. Watch for signs of wind transported snow, such as trees void of snow and sculpted surface snow patterns and deposited pillows of denser wind slab.  Recall a weak bond may remain to the early March crust and may lead to some areas of very sensitive avalanches. 

Storm slab will also be listed as a concern above treeline on a wider variety of aspects. Wetter denser new snow accumulating over lower density snow in the above treeline zone will contribute to this concern.

 

Snowpack Discussion

Recent Weather

An extended mild and dry period occurred the latter half of January generally forming a crust in all areas.

Very cold weather followed in early February which produced light low density snowfall and facets just above the late January crust.

A two week storm cycle ended a week ago causing 7-12 feet of snowfall and many avalanche cycles near and west of the crest.

Warm dry weather was seen late last week from Wednesday Through Saturday causing wet snow avalanches and melt/freeze crusts throughout the region. Surface cold air also redeveloped east of the crest last weekend. 

Temperatures dropped significantly by Sunday prior to the onset of a new storm cycle.  This caused poor bonding of the initial snowfall to the newly formed crust layer in most areas. 

A series of fronts have moved across the area since Sunday at the rate of at least one per day. Freezing levels have been generally on the rise through these events causing rain or wet snow conditions to about 5500 ft in the north, 6000 ft central and 7000 ft or higher in the south.

Washington Cascades near and west of the crest

The latest storm cycle began on Sunday. An avalanche cycle was seen on Sunday night and Monday morning. DOT crews at Stevens and Snoqualmie reported widespread natural and triggered 1-2 foot avalanches Sunday night and Monday morning which closed Snoqualmie Pass. Poor bonds to the melt-freeze crust from late last week have been reported with the crust likely providing bed surfaces.

Snow levels have been rising and temperatures warming Tuesday afternoon through Wednesday with heavy rain or wet snow producing several avalanche cycles with the latest reports from the Alpental ski patrol where another round of natural avalanches were seen Wednesday morning with mostly loose wet snow avalanches.  However some larger natural releases were noted above cliff bands where unsupported slabs released above rock faces slopes.  Some of these releases deposited significant snow on the slopes below. 

There have been no recent reports above treeline as to the extent of dry wind slab and storm slab development.  However, with the significant precipitation received and very strong winds, direct observations are not needed to determine that very dangerous avalanche conditions are likely near and above treeline.  

The late January crust and facet layers remain a concern and are now very deeply buried and have been getting tested by the recent very heavy snowfall. Large avalanches have been seen at Stevens and Snoqualmie the past week. The Stevens Pass ski patrol produced this video of explosive control producing a 10-15 foot avalanche, you might want to turn down your speaker volume first. A large natural hard slab avalanche with a 10 foot crown released on a steep north slope on Chair Peak near Alpental Saturday 22 February and caught 3 skiers. Here is a link to more information about deep persistent slabs which are unusual in our area and not predictable.

Problems

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.

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.

Deep Persistent Slabs

Deep Persistent Slab avalanches are the release of a thick cohesive layer of hard snow (a slab), when the bond breaks between the slab and an underlying persistent weak layer deep in the snowpack. The most common persistent weak layers involved in deep, persistent slabs are depth hoar or facets surrounding a deeply buried crust. Deep Persistent Slabs are typically hard to trigger, are very destructive and dangerous due to the large mass of snow involved, and can persist for months once developed. They are often triggered from areas where the snow is shallow and weak, and are particularly difficult to forecast for and manage.