Avalanche Forecast

A complex storm system will bring increasing rain and snow, rising snow levels and W-SW winds on Sunday.  See the mountain weather forecast tab for more details, but the moderate westerly winds should gradually erode the cold air pool on the east side progressively from south to north during the day Sunday. Significant enough precipitation for slab formation should spill over to the higher-elevation east side locations, particularly in the central zone, where moderate precipitation rates are expected both Sunday and Sunday night, with slightly decreased amounts further north and light to moderate amounts in the south.

Strong winds will quickly build unstable wind slab on lee slopes at higher elevations Sunday. Avoid freshly wind loaded avalanche terrain. New snow will build upon the light amounts of new snow received Friday night. New storm and wind slab may fail at a few different interfaces; within unstable new storm layers caused by the rising temperatures and increasing precipitation rates, at the recent snow/new snow interface or down to the previous snow surface comprised of various crusts or weak persistent grain types. These storm slabs will be particularly touchy on the "various crust or weak persistent grain types" interface on the east slopes of the Cascades where this high variability is likely to have been preserved. Also, remember that the underlying interfaces change with elevation, due to the inversion that persisted for so long.

Loose wet avalanches are not expected to be a significant problem in these zones where the lower elevations expecting a snow to rain transition have shallow snowpack and little new snow to work with.

Remember that beneath any new snow received during this storm, a myriad of early season travel terrain hazards exist, especially lower elevations. Expect terrain hazards with poorly covered rocks, vegetation and creeks, particularly on south-facing terrain, at lower elevations, and on exposed ridges where wind events have stripped much of the seasons snowcover.

Following 12 days of high pressure, the first front crossed the Cascades Friday morning. It wasn't much of a front as the high pressure weakened it significantly, with only a brief period of light rain and snow, New snow accumulations of 1 to 4 inches were common along the east slopes at higher elevations.

The shallow new snow has fallen on a tremendous variety of surfaces (often over very short spatial distances). These older surfaces include melt-freeze crusts, surface hoar, and near surface facets which developed during the prolonged high pressure period of warm and sunny weather, which also returned our snowpack to early season conditions in many areas. The underlying snowpack of 1.5-4 feet along the east slopes is stable with settled old snow from prior to the warm period sitting over the strong Thanksgiving rain crust, buried about 1-2 feet. East-side snowdepths are highest in the Washington Pass area and lowest in the southeast zone.

Reports indicate that in many areas along the east slopes up to about 6000 feet there was extensive recent surface hoar and near surface faceted growth, due to the strong and consistent temperature inversion. However, the top of the inversion has not been well documented in many east side locations. The distribution of these persistent grain types will be an important distinction as greater loading occurs during the incoming storm cycle.

No human-triggered avalanches have been reported in over a week. Some natural small loose wet slides had been occurring during the high pressure spell, mainly on steep sun exposed slopes, but have become a thing of the past as colder, stormier weather moved in on Friday. 

Observations

North

NCMG was near Washington Pass on north-facing terrain on Wednesday and observed continued development of surface hoar (over 2 cm) below 6000 ft and faceting of the upper snowpack in thin rocky areas in the above treeline zone as well. They report no signs of instability in spite of increasing terrain coverage showing signs of wind effect.

Central

No recent observations, but see the Stevens Pass zone observations as reports there may be relevant.

South

No recent observations.