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Mt Hood

Issued: 8:42 PM PST Friday, December 29, 2017
by Garth Ferber

Watch for firmer wind-transported snow near and above treeline on Saturday. 

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Avalanche Problems for Saturday

Wind Slabi

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Forecast for Saturday:

Rain Friday will give way to sharp cooling with heavy post-frontal rain and snow showers Friday night decreasing and ending Saturday morning. Crest-level winds will be strong out of the W-SW overnight, gradually weakening and turning to the W-NW on Saturday.

The sharp cooling trend expected Thursday night should mitigate Friday's loose wet and wet concerns.

New post-frontal snow is expected to be in the 2-4" range and should be of decreasing density. This new snow will be blown by the strong westerly crest-level flow and create additional wind slab concerns which add to some minor wind slab concerns in the above treeline band formed during the warmer portions of the Thursday-Friday storm cycle. Traditional NW through SE aspects have been favored for loading by the wind patterns during these storms, but due to especially strong winds, you might find wind slabs on non-traditional aspects in complex terrain. Refreezing crusts will provide bed-surfaces for wind slabs to slide on.

Avalanche Summary:

A very big change is underway compared to the past 5 days of quiet weather. The Northwest will experience a very active changeable weather pattern Thursday and Friday with moist systems crossing the Pacific Northwest.

Winds and precipitation ramped up on Mt Hood on Thursday evening bringing intense rain and snow. A brief respite was seen for part of the day Friday before a second frontal system swept across the northwest bringing further precipitation and intense winds. 

The NWAC weather graphs tell the story well, with cold air being over-run by significant warming and sustained moderate winds. Reports indicate that the snowpack has acted like a sponge absorbing the 4 or so inches of water that fell on the snowpack Thursday through Friday.

Prior to Thursday, it had been an "interesting" week for weather at Mt Hood with some mixed precipitation including yet more freezing rain Wednesday. A weather disturbance also moved across the southern WA Cascades and Mt Hood area Christmas Eve into Christmas Day, depositing 6-10 inches of snow before unfortunately ending with freezing rain and forming a surface crust. Prior to this storm, about 15-18 inches of low density snow fell at Mt Hood in the 24 hours ending Saturday morning 12/23.

Observations

On Friday, NWAC Pro Observer Laura Green was in Sand and Heather Canyons. In terrain from 4850-6600 feet elevation, she found moderate to hard tests with progressive collapse at crusts at crusts down 5 and 10 inches. She noted that the Hood snowpack absorbed the water "like a thirsty sponge." 

On Friday morning, Mt. Hood Meadows pro patrol reported the freezing rain crust to have been melted by the rainfall at 5200 feet, where where boot penetration was 18”. The upper snowpack was moist, not wet. Standing water was observed at 6500 feet with no avalanches. 

NWAC pro-observer Laura Green on Thursday reported that the freezing rain crust remained stout and predominant on the surface of the snowpack up to about 7300 ft.

On Wednesday morning 12/27, Mt. Hood Meadows Patrol reported a rain/freezing rain crust at 5200 ft thickening with elevation up to 6600 ft. Some wind transport of recent snow was occurring above treeline. 

On 12/26 the Mt Hood Meadows Patrol reported that wind slabs were more stubborn to trigger during control work. A 1 cm freezing rain crust had formed near the surface.

No Corresponding Mountain Weather Forecast Available