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Avalanche Forecast

Archived

Feb 16th, 2017–Feb 17th, 2017

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

Regions

Stevens Pass.

Cooling should reduce the potential for wet slab and wet loose avalanches through the day on Friday. Use caution on steeper and unsupported terrain, especially in areas lacking a strong near-surface crust. Watch for shallow fresh wind slabs near ridges.

Detailed Forecast

Continued cooling with light to moderate snow showers expected overnight Thursday and early Friday. Friday should remain mostly cloudy with a chance of a few light showers.

This weather will help to decrease the avalanche danger by allowing the remaining liquid water in the upper snowpack to slowly drain and slowly refreeze to form a near surface crust.  However, the recent heavy rains have likely left significant liquid water in the upper snowpack, making wet snow avalanches still possible Friday.  

Wet slab avalanches are hard to predict and potentially powerful due to the high water content within the slab. The wet slab potential is decreasing through Friday, but extra caution is advised on steeper and unsupported terrain, especially in areas lacking a strong near-surface crust

Watch for areas where new snow received by Friday may be greater than expected, allowing  shallow new wind slabs to have developed near ridgelines.

Snowpack Discussion

Weather and Snowpack

A rain event beginning last Wednesday 2/8, left a strong rain crust with varying amounts, from about 5-15 inches, of snow accumulating in most areas, by Saturday 2/11. Strong west winds, exposed a firm and slick crust on windward aspects in many areas.  

High pressure last Saturday to Tuesday brought increasing sunshine and warm temperatures. Temperatures reached the 40's and 50's in most areas of the Cascades by Monday and Tuesday. This caused about 6-12 inches of snowpack settlement.  

Another atmospheric river impacted the PNW beginning Tuesday night. Heavy precipitation at high snow levels fell in all the west slopes of the Cascades zones. Two day rain amounts ranged mostly from 1.5 - 4.5 inches of water, with the Mt Baker area receiving nearly 7 inches of rain by Thursday morning.

Localized freezing rain occurred during this event along the lower slopes of the Cascade Passes, where a variable strength, freezing rain crust formed at mid and lower elevations. 

A slow cooling trend began overnight Wednesday with light to moderate snow showers depositing 2-6 inches of new snow by Thursday evening at mid and upper elevations.

The wet snowpack continues to drain and is beginning to slowly refreeze as of Thursday evening. 

Recent Observations

North

Thursday 2/16, the Mt Baker pro patrol reported a very large naturally triggered slab at the east end of Shuksan Arm that released sometime during the rain event. The initial estimates of the crown height are 20 feet!  

Central

Alpental pro-patrol did not perform avalanche control Wednesday, but reported a 1/2" freezing rain crust up to about mid-mountain with rain falling on the upper mountain. 

South

No observations on Wednesday.

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.

Wet Slabs

Wet Slab avalanches are the release of a cohesive layer of snow (a slab) that is generally moist or wet when the flow of liquid water weakens the bond between the slab and the surface below (snow or ground). They often occur during prolonged warming events and/or rain-on-snow events. Wet Slabs can be very unpredictable and destructive.

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.