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Avalanche Forecast

Archived

Feb 18th, 2018–Feb 19th, 2018

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

Regions

Mt Hood.

Cold temperatures will slow recent storm and wind slabs from stabilizing. N-NW winds above treeline will transport recent snowfall onto lee slopes Monday, forming fresh wind slabs. Due to shifting winds, avoid steep wind loaded terrain especially near and above treeline on all aspects. Small loose dry avalanches are likely on very steep slopes and could become problematic around terrain traps.

Detailed Forecast

Cold temperatures will slow recent wind slabs from stabilizing. N-NW winds above treeline will transport recent snowfall onto lee slopes Monday, forming fresh wind slabs. Due to shifting winds, avoid steep wind loaded terrain especially near and above treeline on all aspects. Wind sculpted features, cracks in the snow, pillows, cornices, and variable height of recent snow are all indicators that you could trigger a new or old Wind Slab avalanche.

Even with showers winding down Sunday night, storm slabs may still be sensitive to human triggering on some steeper slopes Monday. Once again, cold temperatures will slow recent storm instabilities from healing quickly. With over 2 feet of new snow, storm slab avalanches comprised of lower density snow would still pack a punch. 

Small loose dry avalanches are likely on very steep slopes. They will entrain recent low density snowfall and could become problematic around terrain traps. 

Snowpack Discussion

Winds finally decreased Sunday afternoon but snow continued to pile up at Mt. Hood with steady snow showers continuing throughout the day. Over 2 feet of snow has been received in the last 24 hours along with much colder temperatures moving in. 

The strong storm system that preceded the colder weather 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!). The rain-line reached near 6000 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 Sunday, Mt. Hood Meadow reported generally soft storm or wind slab avalanches breaking within the new storm snow. Despite significant winds occurring mid-day near treeline, the usual firmer wind slabs present on Mt. Hood were not observed. No observations were available above treeline. 

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.

Loose Dry

Loose Dry avalanches are the release of dry unconsolidated snow and typically occur within layers of soft snow near the surface of the snowpack. These avalanches start at a point and entrain snow as they move downhill, forming a fan-shaped avalanche. Other names for loose-dry avalanches include point-release avalanches or sluffs.