Avalanche Forecast
Regions: Mt Hood.
Very strong winds and snow showers should combine to form new wind slabs on lee slopes, especially NW-E facing. Watch for earlier storm slabs that may be masked by shallow colder surface snow. Dangerous avalanche conditions are expected near and above treeline Thursday.
Detailed Forecast
Yet another atmospheric river is slamming the Mt Hood area Wednesday, bringing stormy wet weather with rising snow levels through Wednesday night.
The heavy precipitation at rising freezing levels should produce one or more natural avalanche cycles during the warmest periods, mainly Wednesday night.Â
By Thursday morning, cooling and light to moderate showers with continued strong winds are expected.Â
The cooling should help wet snow to begin consolidating and cause a lowering danger during the day with shallow new snow accumulations expected.Â
However, near and above treeline, very strong winds should build new areas of wind slab on a variety of aspects, but mainly on NW-E facing slopes below ridges.Â
Be especially aware that new shallow lower density snow may mask earlier storm slab formations, especially on slopes that did not avalanche late Wednesday or overnight Wednesday.Â
Snowpack Discussion
Weather and Snowpack
Last week was wet and relatively mild with a series of active fronts transiting the region resulting in fluctuating freezing levels and periods of very strong winds (100+ mph gusts were not uncommon at the Mt Hood Meadows Cascade Express station).
The latter half of last week and the weekend featured more rain than snow at NWAC sites on Mt. Hood. For perspective, the averaged freezing level measured in March thus far has been 6600 feet at Salem; more akin to spring than late winter.Â
A front Sunday and a cooler upper trough Monday brought some snow with 2 day storm totals of about 8-9 inches at Mt Hood ending Tuesday morning.
A very strong storm arrived Wednesday morning bringing heavy snowfall/rain at rising temperatures and very strong S-SW ridgetop winds.
The mid and lower snow pack at Mt Hood should be a stable mix of crusts and layers of moist and rounded snow crystals.
Recent Observations
On Saturday the Mt. Hood Meadows pro-patrol reported wet snow up to about 6600 feet with a thin crust above and a well consolidated snowpack in their area. Following the cool down Saturday night, pockets of shallow new wind slab were seen above treeline by Sunday morning.Â
On Monday there was about 5 inches of new snow and the Mt Hood Meadows pro-patrol reported a variety of conditions including isolated hard 12-18 inch wind slab on ENE slopes above treeline, wide spread sensitive 4-8 inch storm slab near treeline and small ski triggered loose wet avalanches below treeline on solar slopes.
The Mt Hood Meadows pro-patrol found that new and recent storm snow layers were less sensitive on Tuesday and that cooling had stabilized previous loose wet snow. Direct back country observations were not made on Tuesday but areas of 4-6 inch wind slab were still expected there.
Quickly, Wednesday morning, new extensive wind slabs were forming on lee slopes near and above treeline. Explosive control Wednesday produced widespread slab releases on wind loaded N-NE facing terrain with crown depths of 2 feet common.Â
Storm slabs Wednesday morning changed to loose-wet avalanches by midday as temperatures rose and snow changed to rain below treeline. Â
Avalanche 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: North, North East, East, South East, North West.
Elevations: Alpine, Treeline.
Likelihood: Likely
Expected Size: 1 - 1
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: Alpine, Treeline.
Likelihood: Possible
Expected Size: 1 - 1