Saturday, May 20, 2017

Avalanche Special Advisory

Posted on the NWAC page:

Avalanche Special Advisory
Issued: Thu, May 18, 2017 at 4:07 PM PST
Expires: Mon, May 22, 2017 at 6:00 PM PST

Snowpack Analysis:

Say goodbye to winter and hello to warmer weather and a spring avalanche cycle.

Recent Weather

A strong front, and then an upper low pressure system and cold unstable air mass crossed the Northwest on Monday night and Tuesday. The strongest winds and thunderstorms, and heaviest snow were generally in the south Cascades especially Mt Hood.

Snow for the 2 days ending Wednesday morning was about 4-6 inches at Hurricane, 6-24 inches along the west slopes of the Washington Cascades with the most on the volcanoes and generally above the pass levels, 20-30 inches at Mt Hood, and 5-10 inches along the Cascade east slopes at higher elevations. This is a lot of snow for this time of year!

Recent Avalanche Observations

As of Thursday there isn't much for observations, here is what we've got.
Communications to the base weather station are out at Mt Baker due to lightning on May 4th. But the ski area crew there reported about 15 in of storm snow there on Tuesday with limited visibility but with natural loose wet avalanches seen in the vicinity of the base of the area.

Via the NWAC Observations page, backcountry skiers at Washington Pass on Tuesday reported 5-8 inches of storm snow that was not well bonded to the previous frozen sun affected snow. On south slopes they triggered several storm slab avalanches that gave debris piles 4 feet deep x 150 feet wide.
The WSDOT crew at Chinook Pass on Wednesday reported that skis and explosives triggered 6-12 inches loose wet and storm slab avalanches on specific solar slopes that were large relative to the paths.

NWAC observer Laura Green took a lower elevation trip up to 5800 feet on Mt Hood on Tuesday. Stormy weather with strong winds, graupel and thunder made for an exciting tour. She found about 12 inches of storm snow without signs of instability on lower angle NE to SE slopes in trees on her below treeline tour.

Detailed Forecast for Friday:

Weather Forecast

The weather has finally shifted from the recent winter-like pattern to an overall sunnier warmer weather pattern. An upper ridge will gradually build over the US coastal waters through the weekend and early next week.

Mostly sunny weather with gradually warmer temperatures should be seen Friday. Freezing levels should rise to the 7-9000 foot range from the north to the south Cascades Friday.

A weak system should move over the building ridge and over the Northwest on Friday night and Saturday morning. This should cause a push of clouds into the western Washington lowlands and along the west slopes of the north to central Cascades. Some light showers are indicated in convergence mainly between Puget Sound and the central Cascades Friday night and Saturday morning. But sunnier weather is likely by Saturday afternoon. Freezing levels should rise to the 8,000-11,000 foot range from the north to the south Cascades Saturday.

Sunny weather and stronger warming should be seen on Sunday and early next week. Freezing levels should rise to the 11,000-12,000 foot range from the north to the south Cascades Sunday and even higher early next week.

Avalanche Forecast and Travel Advice

There will be competing factors of stabilizing and warming this weekend and early next week. But due to the recent snow and lack of warm weather so far this spring we expect a cycle of loose wet snow avalanches.

Loose wet snow avalanches may occur sooner on more direct solar slopes and later on less direct, non-solar slopes this weekend and early next week. These avalanches should be most likely during the warmer, sunny daytime hours. Large or very large loose wet snow avalanches are likely on the volcanoes where there has been the most recent snow. But remember that even small, loose wet snow avalanches are powerful and dangerous.

Watch for the lack of an overnight refreeze of surface snow, wet snow deeper than boot top and initial pinwheels and initial small loose wet snow avalanches that indicate an increasing loose wet avalanche danger. It is always a good plan to be away from avalanche terrain by the warmest midday and afternoon hours. Careful snowpack evaluation and cautious routefinding will be essential this weekend this weekend and early next week.

Many people are likely to be out this weekend due to the good snow cover and the delay of good weather this spring. Don't head out expecting normal spring conditions and passively follow others and forget to evaluate conditions for yourself. Avoid situations where an avalanche can be triggered onto you by others from above. This has been the cause of fatal accidents in the Northwest in past seasons.

Other Types of Avalanche Problems:

Nearby glide cracks and extensive loose wet avalanches can indicate that wet slab avalanches are possible.

Wind slab may still be possible on previous lee slopes at higher elevations.

Cornices should be starting to melt back but may remain unstable during prolonged warmer weather. Avoid potential cornices which can break well back from the edge along ridges and avoid traveling in areas underneath cornices on ridges above. See our blog post about cornices here.

Don't linger and move one a time if you decide to travel under rock slabs holding snow where sudden glide avalanches can release.

NWAC forecasts and statements do not apply to where conditions are likely to be more dangerous above the crest level on the volcanoes.