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Avalanche Forecast

Archived

Jan 7th, 2017–Jan 8th, 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 possible, human triggered probable.
Treeline
Natural avalanches unlikely, human triggered possible.
Below Treeline
Natural avalanches unlikely, human triggered possible.

Regions

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Avoid backcountry travel on slopes with previous firm wind slab where you could trigger the slab from an area where the wind slab is shallower. Watch for new wind slab that is expected to form during the day along the east slopes on Sunday especially in the south part.

Detailed Forecast

A moderate warm front will move south to north over the Northwest on Sunday. S-SE winds should increase on Sunday and shift to S-SW in the south Cascades Sunday afternoon. Several or more inches of new snow seems possible along the east slopes especially in the south part by the end of the daylight hours Sunday with a warming trend so this will be where the main danger increase is expected. Less new snow is expected by the end of the daylight hours in the north part.

Watch for new wind slab mainly on northwest to southeast slopes in the near and above treeline on Sunday. Previous wind slab will linger on other slope aspects in all the terrain bands and will have been slow to heal due to the cold temperatures.Watch for firmer wind transported snow.

Continue to identify the 12/17 buried PWL in snowpits and avoid areas where the overlying snowpack is shallower where affecting this layer would be more likely.

Snowpack Discussion

Weather and Snowpack

A cold dry Arctic air mass was over the Northwest last week. The main weather event last week was a day or so of nuclear east winds on Wednesday that went on a powder wrecking rampage, scouring windward slopes, and re-distributing it to lee slopes or other areas.

It sounds like the winds may have eliminated much of the faceted surface snow and surface hoar that may have formed out there in the near and above treeline during the cold weather. But these weak surface crystals may still be found in sheltered areas below treeline and will need to be avoided where buried by snowfall especially above terrain traps.

Recent Observations

There haven't been many recent observations from the east slopes.

On New Years Day reports from multiple snow pits by the the Mission Ridge pro-patrol indicated that basal facets remained intact but showing signs of rounding and bonding.

Th Mission Ridge ski patrol had their hands full on Monday morning with about 34 inches of new snow and strong northeast winds. It was not possible for them to visit lee S-W slopes where new, deep wind slab was expected.

The NCMG were in the Cuthroat drainage near  Washington Pass area on Wednesday and found cold temperatures and evidence of stubborn wind slab up to size 1.5 in the near and above treeline.

The Mission Ridge pro-patrol on Saturday reported that the deep low density heavy snow from early last week had greatly consolidated and stabilized.

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.

Persistent Slabs

Persistent Slab avalanches are the release of a cohesive layer of snow (a slab) in the middle to upper snowpack, when the bond to an underlying persistent weak layer breaks. Persistent layers include: surface hoar, depth hoar, near-surface facets, or faceted snow. Persistent weak layers can continue to produce avalanches for days, weeks or even months, making them especially dangerous and tricky. As additional snow and wind events build a thicker slab on top of the persistent weak layer, this avalanche problem may develop into a Deep Persistent Slab.