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Avalanche Forecast

Archived

Feb 16th, 2017–Feb 17th, 2017

Alpine
Natural avalanches possible, human triggered probable.
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

Mt Hood.

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 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, especially on steeper and unsupported terrain, and in areas lacking a strong near-surface crust. 

Watch for fresh wind slabs that formed overnight and early Friday, mainly easterly facing terrain near ridges near and above treeline.

Snowpack Discussion

Weather and Snowpack

A rain event beginning last Wednesday 2/8, left a strong rain crust with about 6-8 inches of snow accumulating 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 by Monday and Tuesday. This caused about 8 inches of snowpack settlement.  

Another atmospheric river impacted the PNW beginning Tuesday night. Heavy precipitation at high snow levels fell at Mt Hood. Two day rain amounts ranged from over 2 to nearly 3 inches of water by Thursday morning.

A slow cooling trend began overnight Wednesday with light to moderate snow showers depositing 4-6 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

A very large and impressive 6-8 ft natural slab avalanche released sometime last week, likely between  2/8 - 2/10, from about the 8000 ft level in Newton Canyon. 

Photo by Paul Klein, Mt. Hood Meadows

NWAC observers Matt Schonwald and Laura Green toured out of Timberline on Tuesday. Snow surfaces consisted of a melt freeze crust, some shallow wind slab, and scoured glaze ice. The only significant shear observed was around 15-20 cm down, where the fresh wind slab interfaced the 2/9 rain crust. However, extended column tests did not indicate propagation was likely and no avalanche activity was observed. 

A quick report late Thursday afternoon from Mt Hood Meadows indicated the new 4-6 inches of snow received late Thrusday was being transported and forming new wind slab layers on a variety of lee slopes from NE-SE facing below ridges. The bonding to the underlying wet snow appears to be good initially.

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.

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.

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.