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Avalanche Forecast

Archived

Dec 23rd, 2016–Dec 24th, 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 in the ATL on Saturday due to recent west winds. Avoid steep hard slopes where there will be fall consequences if you are not confident you can manage this problem by walking or using ski or boot crampons.

Detailed Forecast

An upper trough will exit the Northwest on Saturday and high pressure will begin to build offshore. Light snow showers mainly along the west slopes Saturday morning should give way to partial clearing Saturday afternoon with light west to northwest winds and cool temperatures.

Wind slab should be possible mainly in the ATL on Saturday due to recent west 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 following the storms early this week is strong and hard enough to present an out of control fall danger. Avoid steep hard slopes where there will be fall consequences if you are not confident you can manage this problem by walking or using ski or boot crampons.

Snowpack Discussion

Weather and Snowpack

Strong westerly flow directed two Pacific frontal systems into the PNW Sunday night and again Monday night 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 about 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. West winds seen Thursday and Friday should become light by Friday night with a cooling trend Friday.

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 ATL. Local reactive wind slab to 1 foot deep was reported in the NTL.

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.