Avalanche Forecast

Increasing strong southwest flow aloft will carry a major deepening low pressure system and cold front across the Northwest on Monday night and Tuesday. Expect moderate to strong shifting alpine winds Monday night and Tuesday, about 5-15 inches of new snow by Tuesday early morning, and more new snow Tuesday.

 New storm slab is likely to form Monday night and Tuesday in areas with rapidly accumulating new snow.

New wind slab will form on lee slopes mainly near and above treeline Monday night and Tuesday morning. This is most likely on northwest to southeast aspects but is possible on other aspects. Watch for firmer wind transported snow.

New storm and wind slab may fail within new storm layers, at the interface from Sunday, or down to crusts or weak persistent grain types from mid December. Slabs may be particularly touchy where crusts or weak persistent grain types have been preserved. 

 A peak in the avalanche danger should be seen Tuesday. You may encounter a greatly increasing avalanche danger with elevation on Tuesday. Avoid travel in terrain connected to higher start zones.

Remember that beneath any new snow received during this storm, a myriad of early season terrain hazards exist, especially at 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 long days of high pressure and a temperature inversion, the first frontal system in quite awhile crossed the Cascades Friday. It wasn't much of a front with only a brief period of light rain and snow. New snow accumulations of 1 to 5 inches were common along the east slopes at higher elevations through Saturday morning.

The shallow new snow fell on a variety of surfaces including melt-freeze crusts, surface hoar, and near surface facets up to about 5-6000 feet which was about the level of the inversion during the high pressure. The distribution of these persistent grain types will be an important distinction as greater loading occurs during the incoming storm cycle.

Warm frontal moisture that arrived on Sunday brought periods of light rain and snow to the east slopes of the Cascades, mainly near the Cascade crest with a sharp decline in precipitation further to the east. Above freezing temperatures were noted at Dirty Face summit and the Lyman Lake Snotel, suggesting light rain pushed up to near 6000 feet in many areas. The warmer temperatures and light rain may have helped wet or potentially destroy recently buried persistent weak layers at lower elevations but more direct information is needed to confirm.

An approaching stronger system with moist west flow then brought about 8-15 inches of new snow to the northeast zone on Monday morning, with much less in the central east and south east zones.

The underlying snowpack of 1-4 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.

Observations

North

On Sunday, NCMG found surface hoar buried intact up to 6000' in the Washington Pass area by 10-15 cm of recent snowfall. No signs of recent avalanche activity were noted. 

Central

The Mission pro patrol reported only a trace of new snow on a consolidated, shallow base on Monday.

There are no other recent observations, but see the Stevens Pass zone observations as reports there may be relevant.

South

No recent observations.