Avalanche Forecast

Archived

Feb 17th, 2018–Feb 18th, 2018

Alpine

Natural and human triggered avalanches likely.

Treeline

Natural and human triggered avalanches likely.

Below Treeline

Natural avalanches possible, human triggered probable.

Alpine

Natural avalanches possible, human triggered probable.

Treeline

Natural avalanches possible, human triggered probable.

Below Treeline

Natural avalanches unlikely, human triggered possible.

Regions

Mt Hood.

Travel in avalanche terrain is not recommended Sunday. Moderate to heavy snowfall paired with strong winds Saturday night into Sunday morning will create very dangerous avalanche conditions especially near and above treeline Sunday. Avoid slopes steeper than 30 degrees or terrain where avalanches could release from above.

Detailed Forecast

Travel in avalanche terrain is not recommended Sunday. Moderate to heavy snowfall paired with strong winds Saturday night into Sunday morning will create very dangerous avalanche conditions especially near and above treeline. Avoid slopes connected to large start zones higher in the terrain where wind slab avalanches are capable of producing very large and destructive slides.

A strong cooling trend should help mitigate the storm slab likelihood, but at the same time preserve storm slab instabilities formed overnight. In non-wind affected terrain, avoid open slopes greater than 35 degrees and allow new storm slab instabilities time to stabilize.

New snow will likely bond poorly to a slick crust that formed Saturday morning near and below treeline. This would be a likely bed surface for any avalanche that steps down to deeper layers.

Snowpack Discussion

A strong storm system brought heavy precipitation in the form of snow, ice and rain to Mt. Hood Saturday along with moderate winds increasing to strong and gusty winds in the afternoon (WSW gusts 40-70 mph were common at the lower NWAC wind sites!). Scoured surfaces surfaces are expected on windward slopes as well as wind effected snow well into the below treeline band. Despite heavy precipitation, only about 4 inches of new snow was recorded at the Meadows and Timberline base plots through 5 pm Saturday. The rain-line is estimated to have reached near 5500 feet Saturday mid-day before cooling ensued.

Prior to this system, 12-18 inches of storm snow fell late last week and sits on a strong firm crust layer formed earlier in the month. There are no significant layers of concern in the mid and lower snowpack.

Observations

On Saturday morning, a new rain or freezing rain crust was noted up to 6600 ft. Rime ice had covered most surfaces. Strong winds prevented observations later in the day.

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.

Storm Slabs

Storm Slab avalanches are the release of a cohesive layer (a slab) of new snow that breaks within new snow or on the old snow surface. Storm-slabs typically last between a few hours and few days (following snowfall). Storm-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.