Avalanche Forecast
Regions: Mt Hood.
Mother Nature will throw a lot of serious avalanche problems at us again on Monday. The exact outcome of the snow, rain and warmer temperatures is hard to predict but we have to expect multiple types of potentially large or very large avalanches. Very dangerous avalanche conditions are expected in the above treeline areas at Mt Hood on Monday and travel in avalanche terrain is not recommended in these areas. Careful snowpack evaluation and cautious routefinding will be required in other areas.
Detailed Forecast
A front should sag slowly south over the Northwest on Monday. Waves of moisture moving along the front should cause a very wet day in the Olympics and Cascades. Periods of moderate to heavy rain or snow should be heaviest along the Cascade west slopes with rising snow levels. The heavier rain may not reach Mt Hood until Monday afternoon.
Mother Nature will throw a lot of serious avalanche problems at us again on Monday. The exact outcome of the snow, rain and warmer temperatures is hard to predict but we have to expect multiple types of potentially large or very large avalanches.
Loose wet avalanches are very likely during intense wet snow or rain.
Wet slab avalanches are most likely in areas of intense rainfall where rain lubricates layers in the snowpack. Glide cracks can indicate areas where wet slab avalanches are likely.
Avoid areas on ridges where there are potential cornices and slopes below cornices. Cornices will be weakened and prone to failure on Monday due to loading by wet snow and rain and warm temperatures.Large cornices have been reported from many areas and have been involved in recent accidents and close calls.
Very dangerous avalanche conditions are expected in the above treeline areas at Mt Hood on Monday and travel in avalanche terrain is not recommended in these areas. Careful snowpack evaluation and cautious routefinding will be required in other areas.
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.
A strong frontal system brought increasing precipitation and winds along with a warming trend to the Cascades on Thursday. On Thursday night rain pushed up to about 7000 feet at Mt Hood. By Friday morning NWAC stations at Mt Hood had about 1.6 in of WE but alas only rain to show for it. This gave the snowpack a huge test and caused an avalanche cycle at Mt Hood.
Another front crossed the Cascades on Saturday causing more SW-W winds, snow at higher elevations, and rain at low elevations. NWAC stations at Mt Hood had about 2-6 in of new snow on Sunday morning with the most on the upper mountain.
Recent Observations
The Meadows patrol on Thursday reported that rainfall was saturating the upper snowpack allowing several feet of boot penetration by mid-day.
NWAC observer Laura Green was out on Friday and reported new large cornice triggered slab avalanches in White River Canyon on SE aspects in the 6500-8500 ft range, and in Heather Canyon on a NE aspect in the 6100-6800 ft range. Very large debris was seen from new avalanches in Newton Canyon. Laura reports that in the 5200-6600 ft range the upper snowpack of 4F wet and some lower density drier layers.
Avalanche Problems
Loose Wet
Release of wet unconsolidated snow or slush. These avalanches typically occur within layers of wet snow near the surface of the snowpack, but they may quickly gouge into lower snowpack layers. Like Loose Dry avalanches, they start at a point and entrain snow as they move downhill, forming a fan-shaped avalanche. They generally move slowly, but can contain enough mass to cause significant damage to trees, cars or buildings. Other names for loose-wet avalanches include point-release avalanches or sluffs. Loose Wet avalanches can trigger slab avalanches that break into deeper snow layers.
Travel when the snow surface is colder and stronger. Plan your trips to avoid crossing on or under very steep slopes in the afternoon. Move to colder, shadier slopes once the snow surface turns slushly. Avoid steep, sunlit slopes above terrain traps, cliffs areas and long sustained steep pitches.
Several loose wet avalanches, and lots of pinwheels and roller balls.
Loose wet avalanches occur where water is running through the snowpack, and release at or below the trigger point. Avoid terrain traps such as cliffs, gullies, or tree wells. Exit avalanche terrain when you see pinwheels, roller balls, a slushy surface, or during rain-on-snow events.
Aspects: All aspects.
Elevations: All elevations.
Likelihood: Very Likely
Expected Size: 1 - 1
Wet Slabs
Release of a cohesive layer of snow (a slab) that is generally moist or wet when the flow of liquid water weakens the bond between the slab and the surface below (snow or ground). They often occur during prolonged warming events and/or rain-on-snow events. Wet Slab avalanches can be very destructive.
Avoid terrain where and when you suspect Wet Slab avalanche activity. Give yourself a wide safety buffer to handle the uncertainty
A Wet Slab avalanche. In this avalanche, the meltwater pooled above a dusty layer of snow. Note all the smaller wet loose avalanches to either side.
Wet slabs occur when there is liquid water in the snowpack, and can release during the first few days of a warming period. Travel early in the day and avoiding avalanche paths when you see pinwheels, roller balls, loose wet avalanches, and during rain-on-snow events.
Aspects: All aspects.
Elevations: All elevations.
Likelihood: Likely
Expected Size: 1 - 2
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, North West.
Elevations: Alpine, Treeline.
Likelihood: Likely
Expected Size: 1 - 2