Avalanche Forecast
Issued: Feb 18th, 2018 1:00PM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Wind Slabs, Storm Slabs and Loose Dry.
NWAC,
Northwest Avalanche Center
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.
Summary
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
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.
Wind Slabs form in specific areas, and are confined to lee and cross-loaded terrain features. They can be avoided by sticking to sheltered or wind-scoured areas..
Wind Slab avalanche. Winds blew from left to right. The area above the ridge has been scoured, and the snow drifted into a wind slab on the slope below.
Wind slabs can take up to a week to stabilize. They are confined to lee and cross-loaded terrain features and can be avoided by sticking to sheltered or wind scoured areas.
Aspects: All aspects.
Elevations: Alpine, Treeline.
Likelihood
Expected Size
Storm Slabs
Release of a soft cohesive layer (a slab) of new snow that breaks within the storm snow or on the old snow surface. Storm-slab problems typically last between a few hours and few days. 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.
You can reduce your risk from Storm Slabs by waiting a day or two after a storm before venturing into steep terrain. Storm slabs are most dangerous on slopes with terrain traps, such as timber, gullies, over cliffs, or terrain features that make it difficult for a rider to escape off the side.
Storm slabs usually stabilize within a few days, and release at or below the trigger point. They exist throughout the terrain, and can be avoided by waiting for the storm snow to stabilize.
Aspects: All aspects.
Elevations: All elevations.
Likelihood
Expected Size
Loose Dry
Release of dry unconsolidated snow. These avalanches typically occur within layers of soft snow near the surface of the snowpack. Loose Dry 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. Loose Dry avalanches can trigger slab avalanches that break into deeper snow layers.
Loose Dry avalanches are usually relatively harmless to people. They can be hazardous if you are caught and carried into or over a terrain trap (e.g. gully, rocks, dense timber, cliff, crevasse) or down a long slope. Avoid traveling in or above terrain traps when Loose Dry avalanches are likely.
Loose Dry avalanche with the characteristic point initiation and fan shape.
Loose dry avalanches exist throughout the terrain, release at or below the trigger point, and can run in densely-treed areas. Avoid very steep slopes and terrain traps such as cliffs, gullies, or tree wells.
Aspects: All aspects.
Elevations: All elevations.
Likelihood
Expected Size
Valid until: Feb 19th, 2018 1:00PM
