Avalanche Forecast
Regions: Mt Hood.
There should be enough new snow and wind to elevate storm related hazards on Friday and create dangerous conditions primarily near and above treeline. Watch for small loose wet avalanches below treeline. Cornices have recently proven dangerous and unpredictable and capable of triggering very large avalanches.
Detailed Forecast
A frontal system will slowly move across the Cascades Thursday night with modest amounts of new snow expected above 5500 feet on Mt. Hood area. Skies should remain cloudy on Friday with light to moderate showers and additional snow accumulations near and above treeline throughout the day. Snow levels should remain fairly consisent in the 5000-5500 ft range on Friday. Â
There should be just enough new snow and wind to elevate storm related hazards on Friday and create locally dangerous conditions primarily near and above treeline.
Recent winds have been mostly S-SW so firmer wind slab should be found mainly on NW-SE slopes. But there have been winds from other directions lately such as local east winds on Monday so watch for firmer wind transported snow on all aspects, especially in areas of complex terrain.
New storm snow is expected to bond well to the existing snow surface Thursday night but storm slab may still become locally sensitive in areas that receive rapid accumulation Thursday night or Friday. Â
Loose wet avalanches won't make the cut to be listed as an avalanche problem but are still possible in the below treeline band. Watch for surface wet snow deeper than a few inches, pinwheels and increasing natural releases. Avoid steeper slopes with terrain traps where small loose wet avalanches could have unintended consequences.Â
Make sure to avoid areas on ridges where there may be a cornice and slopes below cornices! Natural cornice releases and resulting slab avalanches are unpredictable. See a blog post regarding cornices here.
Snowpack Discussion
Weather and Snowpack
The first week or so of March was very cool and snowy. NWAC stations at Mt Hood piled up about 6-7 ft of snow.
The 2nd week of March was equally active with non-stop Pacific frontal systems pummeling the PNW. Unfortunately, these systems delivered far more rain than snow. At least two regional avalanche cycles occurred during the stretch. Significant snowpack consolidation occurred over this period due to rainfall and warmer temperatures.Â
Another strong low pressure system brought about an inch of predominately rain to the NWAC Mt. Hood stations Friday 3/17. Rapid cooling Saturday morning was followed by snow showers with light new snow accumulation.Â
Sunday was cool with light winds and mostly sunny skies at Mt. Hood. Increasing clouds Monday with moderate daytime warming allowed additional slow snowpack settlement.Â
Weaker fronts crossed the Northwest on Tuesday and Wednesday. 3-4 inches of new snow accumulated Wednesday night at the Mt. Hood Meadows and Timberline weather stations. Mostly sunny skies were seen during the day on Thursday.Â
Recent Observations
Observations from Mt Hood Meadows on Tuesday indicated light rain had maintained shallow moist to wet surface snow, but no natural or triggered loose wet avalanches were reported. Above treeline the strong winds and light snowfall began to build shallow wind slab.
On Thursday, Mt. Hood Meadows reported widespread but stubborn shallow hard wind slab, 1-2 ft deep, on NE slopes between 6000 and 7300 ft. Small loose wet slides remained possible on steeper solar slopes.
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, West, 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: All elevations.
Likelihood: Possible
Expected Size: 1 - 1
Cornices
Cornice Fall is the release of an overhanging mass of snow that forms as the wind moves snow over a sharp terrain feature, such as a ridge, and deposits snow on the downwind (leeward) side. Cornices range in size from small wind lips of soft snow to large overhangs of hard snow that are 30 feet (10 meters) or taller. They can break off the terrain suddenly and pull back onto the ridge top and catch people by surprise even on the flat ground above the slope. Even small cornices can have enough mass to be destructive and deadly. Cornice Fall can entrain loose surface snow or trigger slab avalanches.
Cornices can never be trusted and avoiding them is necessary for safe backcountry travel. Stay well back from ridgeline areas with cornices. They often overhang the ridge edge can be triggered remotely. Avoid areas underneath cornices. Even small Cornice Fall can trigger a larger avalanche and large Cornice Fall can easily crush a human. Periods of significant temperature warm-up are times to be particularly aware.
A corniced ridgeline. A large cornice has formed at the top of the ridge. A smaller cornice has formed to the left of the trees from crossloading.
Cornices are easy to identify and are confined to lee and cross-loaded ridges, sub-ridges, and sharp convexities. They are easiest to trigger during periods of rapid growth (new snow and wind), rapid warming, and during rain-on-snow events. Cornices often catch people by surprise when they break farther back onto flatter areas than expected.
Aspects: North, North East, East, South East, North West.
Elevations: Alpine.
Likelihood: Unlikely
Expected Size: 1 - 2