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Avalanche Forecast

Archived

Dec 20th, 2016–Dec 21st, 2016

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

Regions

Mt Hood.

Large and dangerous wind slabs should be less likely to trigger Wednesday but wind loaded slopes should be avoided for at least one more day especially above treeline. Small loose wet avalanches are possible on steep solar slopes below treeline with daytime warming.   

Detailed Forecast

Storm slabs should have settled out by Wednesday, shifting the focus to lingering wind slab mainly found above treeline and the potential for small loose wet avalanches on steep solar slopes below treeline. 

Large and dangerous wind slabs should be less likely to trigger Wednesday but wind loaded slopes should be avoided for at least one more day especially above treeline. Small loose wet avalanches are possible on steep solar slopes below treeline with daytime warming.   

Snowpack Discussion

Weather and Snowpack

Clear and cold weather from Wednesday, December 14th to Friday, December 17th allowed widespread surface hoar and near surface faceted snow to develop in the Cascades and Mt. Hood. Thin sun crusts formed on steeper solar slopes during sunny periods.

However, persistent weak layers likely have not survived in the Mt. Hood zone due to the copious water amounts received over this latest storm cycle. 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 Mt. Hood NWAC stations 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.  

Westerly winds were especially strong with the 2nd system late Monday night and into Tuesday with gusty winds mixing down into usually more wind sheltered terrain. 

Recent Observations

NWAC pro-observer Laura Green was out in the Newton and Clark drainages on Friday a found a mix of surface conditions of recent snow and 3-4 mm surface hoar with a thin sun crust on solar slopes. The upper snow pack on N-NE slopes was right side up with no direct signs of instability.

The Mt Hood Meadows pro-patrol on Sunday reports fairly widespread surface hoar and near surface faceted snow up to about 7300 feet. Calm winds have allowed it to develop or grow the past couple days.

Avalanche control observations were limited Tuesday morning and the below treeline snowpack appeared to be handling the heavy rain remarkably well with little natural or human avalanche activity observed. However, by late Tuesday afternoon and after some partial clearing, debris from a very large wind slab avalanche was observed above treeline on a ENE aspect at 7500 feet in the Mt. Hood Meadows ski area. This R4.5 - D3 avalanche likely occurred Monday night and cleared out two start zones running roughly 1500 feet vertical. From a distance, the crown was estimated to be several feet thick! 

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.

Loose Wet

Loose Wet avalanches are the release of wet unconsolidated snow or slush. These avalanches typically occur within layers of wet snow near the surface of the snowpack, but they may quickly gouge into lower snowpack layers. Like Loose Dry Avalanches, they start at a point and entrain snow as they move downhill, forming a fan-shaped avalanche. Other names for loose-wet avalanches include point-release avalanches or sluffs. Loose Wet avalanches can trigger slab avalanches that break into deeper snow layers.