Avalanche Forecast

Archived

Apr 4th, 2017–Apr 5th, 2017

Alpine

Natural avalanches unlikely, human triggered possible.

Treeline

Natural avalanches unlikely, human triggered possible.

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.

While less new avalanche danger is expected at Mt Hood than in the Washington Cascades you will need to use extra caution on specific terrain features and evaluate snow and terrain carefully on Wednesday. The forecast at Mt Hood is quite uncertain and be prepared to scale back your plans if snow and rain is heavier than expected on Wednesday.

Detailed Forecast

Heads up because winter isn't done yet and the quiet weather and snowpack seen the past couple days is going to see significant snow and rain depending on location on Wednesday!

A front will stall over the Northwest on Wednesday with a series of waves moving south to north along the front. Precipitation should fall as snow in the above treeline and perhaps down into the near treeline zones. Rain and snow is expected to be less at Mt Hood than in the Washington Cascades.

New shallow storm slab may build in the above and perhaps down into the near treeline at Mt Hood where there is rapidly accumulating new snowfall. New storm slab should be very touchy especially if there is a daytime warning trend.

Loose wet avalanches should be possible in the near and below treeline at Mt Hood. Watch for wet surface snow deeper than a few inches, pinwheels and initial natural releases that indicate an increasing loose wet avalanche danger.

Recent cornices are large and will get new loads on Wednesday. Natural cornice releases and resulting slab avalanches are dangerous and unpredictable. Give cornices a wide berth if traveling along ridge-lines and avoid slopes below large cornices. See a blog post regarding cornices here.

New wind slabs won't be added to the list of avalanche problems at Mt Hood on Wednesday but may also build in the above and perhaps down into the near treeline. New wind slab should also be very touchy especially if there is a daytime warning trend.

While less new avalanche danger is expected at Mt Hood than in the Washington Cascades on Wednesday the forecast here is quite uncertain. Be prepared to scale back your plans if snow and rain is heavier than expected.

Snowpack Discussion

Weather and Snowpack

March was wet and wild for weather and avalanches in the Cascades.

The last major system in March was seen Tuesday and Wednesday when a strong low pressure system brought rising snow levels and locally heavy precipitation along the west slopes of the Cascades including Mt. Hood. After receiving a few inches of snow Tuesday night, Mt. Hood saw periods of moderate to heavy rain to above 7000 feet on Wednesday.

Cooling and showery weather Thursday allowed wet snow to begin refreezing with 2-5 inches of new snow at NWAC stations at Mt Hood. The winds diminished Thursday and combined with daytime warming, this allowed for wind slab and storm slab to begin stabilizing.

A weak front crossed the Northwest on Saturday morning, causing light rain mainly along the Cascade west slopes. This was followed by an upper trough on that caused some light amounts of snow along the Cascade west slopes at much cooler temperatures on Sunday.

Fair but cool weather has been seen in the Olympics and Cascades Monday and Tuesday. Reports generally indicate strong surface or near surface crust layers and shallow recent snow.

Recent Observations

The storm limited observations on Wednesday, but rain quickly penetrated the upper snowpack in the Mt. Hood Meadows base area by late morning.

NWAC pro-observer Laura Green was in the White River area in the 4500-6000 ft range on Friday and reported 4-13 cm of recent snow on the P crust from mid week. Pit tests gave hard results that did not indicate propagation on a layer at 30 cm. She noted natural wind slab releases in the above treeline and loose wet avalanches below from Thursday.

The Meadows pro-patrol on Saturday and Sunday reported triggered loose wet avalanches becoming possible on solar slopes. Otherwise the recent crust is the predominate snow surface there with small shallow areas of wind transported snow been soaked by rain and frozen in place.

Problems

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 Wet

Loose Wet avalanches are the 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. 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.

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 drifts 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.