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Avalanche Forecast

Archived

Dec 24th, 2016–Dec 25th, 2016

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

Regions

Mt Hood.

Wind slab should be possible mainly above treeline on Sunday due to recent west-northwest winds. Watch for firmer wind transported snow mainly on northwest to southeast slopes.These layers could be more reactive where they build over a hard smooth crust. The surface crust formed last week is strong and hard enough to present an out of control fall danger. 

Detailed Forecast

Christmas should be cold and mostly sunny with generally light winds across the Cascade range including Mt. Hood. 

Wind slab should be possible mainly above treeline on Sunday due to recent west-northwest winds. Watch for firmer wind transported snow mainly on northwest to southeast slopes. These layers could be more reactive where they build over a hard smooth crust.

The surface crust formed last week is strong and hard enough to present an out of control fall danger. Avoid steep slopes where the slide for life hazard is present.

Snowpack Discussion

Weather and Snowpack

Strong westerly flow directed two Pacific frontal systems into the PNW Sunday night and again Monday night (12/18-12/19) with generally 3 inches of water accumulating at NWAC stations at Mt Hood through early Tuesday morning.

Unfortunately, much of the heavy precipitation fell in liquid form with rain likely reaching up to at least 7000 feet late Monday night and Tuesday morning. 

 A sharp cooling trend followed mid-day Tuesday with about 2 inches of snow in post-frontal showers.  A strengthening rain crust was noted near and below treeline by late in the day Tuesday with the arrival of colder air.  

A fair day was seen on Wednesday.

Another front crossed the Northwest on Thursday followed by an upper trough on Friday. This has only caused about 5 inches of snow at NWAC stations at Mt Hood. 

NW winds near and above treeline were moderate at Mt. Hood Friday night through mid-day Saturday. 

Recent Observations

Reports from the Mt Hood Meadows pro-patrol Wednesday reported a significantly different snowpack following rain, avalanches and cooling. A stout surface crust was found on all elevations up to at least 7200 feet. On exposed terrain, the crust was very supportable while in treed terrain the crust ranged from breakable to supportable.

A report via the NWAC Observations - Recent Observations tab for the Cooper Spur area on Wednesday indicated a thick surface crust that likely presented an out of control fall danger in the above treeline. Local reactive wind slab to 1 foot deep was reported in the near treeline.

The Mt Hood Meadows pro-patrol on Friday afternoon only reported shallow unconsolidated new snow on the thick crust up thought they only went up to 6600 feet. Windy conditions were reported above 6600 feet.

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.