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Avalanche Forecast

Archived

Feb 18th, 2017–Feb 19th, 2017

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

Regions

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Up to a few inches of new snow should be seen on Sunday. Evaluate snow and terrain carefully for possible wind slab on Sunday.

Detailed Forecast

Moderately increasing south to southwest alpine winds and up to a few inches of new snow may be seen on Sunday along the Cascade east slopes. Temperatures shouldn't change much again on Sunday.

This should bring a minor avalanche danger increase on Sunday.

Watch for firmer wind transported snow on potential lee slopes. Alpine winds at the tail end of the last storm and on Sunday will have been mostly south to southwest so north to southeast aspects will be indicated.

Snowpack Discussion

Weather and Snowpack

High pressure Saturday to Tuesday 2/11-2/14 brought fair weather and warming temperatures. Temperatures reached the 30's and 40's in many areas along the Cascade east slopes. This caused snowpack settlement, some small loose wet avalanche activity and left a melt-freeze crust in many areas.

Yet another atmospheric river arrived Tuesday night.

In the Northeast zone the NWAC station at Washington Pass received about 1.25 inches of water equivalent by Thursday morning, resulting in about 10-14 inches of moist snow over a melt-freeze crust from the fair weather. Reports also indicated a thin surface freezing-rain crust to near treeline.

In the Central East and Southeast zones light to moderate snow with warmer temperatures and some high snow levels occurred with localized freezing rain along the lower east slopes. Then above freezing temperatures occurred during precipitation with light rain likely up to about 5-6000 feet in the Central East and up to about 7000 feet in the Southeast.

Recent Observations

North

NWAC observer Jeff Ward was out Thursday, 2/16 and reported the recent about 10 inches of  moist storm snow was being transported and building wind slabs through the day. There was evidence of a few natural wind slab releases and a natural storm slab was noted releasing sometime Thursday. A thin freezing rain crust formed mid-storm cycle and was yet to form a good bond as of Thursday.

More NCMG reports on Friday in the Hairpin Valley near Washington Pass indicated shears at the interface of the storm snow and the melt-freeze crust, down about 14 inches. But ski test gave no results and no avalanches were seen. Snowmobiles were high marking also with no triggered slides, also indicating good stabilizing of recent storm snow.

The NCMG were out again in the Hairpin Valley on Saturday and found slab structures lacking in the recent storm snow with ski tests even on steep convex rolls not giving results.

Central

On Thursday 2/16, warm temperatures had reached the lower east slopes and numerous wet snow avalanches had released in Tumwater Canyon.

Reports from the Mission Ridge pro-patrol Friday indicated that the previous wet snowpack had locked up and formed a strong surface crust in most terrain with little to no new snow above.

South

No recent observations. 

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.