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Avalanche Forecast

Archived

Apr 28th, 2017–Apr 29th, 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 unlikely, human triggered possible.
Treeline
Natural avalanches unlikely, human triggered possible.
Below Treeline
Natural avalanches unlikely, human triggered possible.

Regions

Mt Hood.

Watch for wet and weak surface snow from daytime warming and filtered sunshine. This can rapidly activate the loose wet snow and result in a rapid increase in avalanche danger. Continue to give cornices a wide berth if traveling along ridge-lines and avoid slopes below cornices. A good way to manage spring time risk is to start early and be out of avalanche terrain by afternoon.

Detailed Forecast

Increasing clouds Saturday with light rain and snow developing in the afternoon. Only very light amounts of precipitation if any are expected during the daylight hours Saturday.

Loose wet avalanches should be possible on solar slopes. Filtered sun and rapid daytime warming can quickly melt nighttime surface crusts and rapidly activate loose-wet snow and result in an increase in avalanche danger.  Watch for surface wet snow deeper than a few inches, pinwheels and initial small loose wet avalanches that indicate an increasing loose wet avalanche danger.

New or recent wind slabs are mainly expected in steep lee terrain above treeline, though expected to be less reactive by Saturday. NWAC stations on Mt Hood have had moderate to very strong westerly winds on Wednesday and Thursday. So wind slabs are most likely on NW to SE aspects, but watch for firmer wind transported snow that is a sign of wind slab on other aspects in areas of varied terrain.

Cornices are still large, so give them a wide safety margin. Natural cornice releases and resulting slab avalanches are dangerous and unpredictable. Give potential cornices a wide berth if traveling along ridge-lines and avoid slopes below cornices. See a blog post regarding cornices here.

Avoid unsupported slopes with overhanging blocks of snow and smooth rock underneath. Glide avalanches are difficult to predict and can release at any time, not just during the heat of the day.

Note that this forecast applies up to the Cascade crest level and does not apply to higher elevations on the volcanoes.

 

Snowpack Discussion

A cool, snowy pattern has been seen in late April mainly for the Cascade west slopes and especially for the Mt Hood area.

Last week from Monday 4/17 to Thursday 4/20 the NWAC weather stations at Mt Hood picked up 1 - 2 inches of WE. Much of this WE fell as snow above about 6000 feet.

A fair day was seen last Friday with most NWAC stations in the Olympics and Cascades reaching the 40's and 50's.

The current storm cycle began on Saturday. In the 6 days ending Friday morning, 4/28 the NWAC stations at Mt Hood had 4-6 inches WE, with an average snowline from 5500-6000 feet! 

Much of this storm snow has settled to about 4-8 inches below treeline, but amounts to 1-2 feet above treeline! 

Recent observations

NWAC observer Laura Green did a short tour at the closed Meadows ski area on Monday and turned in an observation via the NWAC Observations page. In windy conditions she visited E-SE slopes in the below tree line. She found wind loading to E slopes and about 12 inches of rapidly accumulated snow that was resulting in touchy storm slab and a high danger.

Observations from the Mt Hood Meadows patrol Friday indicated there was a period of sub freezing rain recently, forming a clear ice layer up to about 7300 feet. As a result, wind slabs were isolated, but some hard slabs of 1-3 feet were released with explosives above treeline on wind loaded terrain. No slabs were reactive to human trigger Friday. 

Problems

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.

Wind Slabs

Wind Slab avalanches are the 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.

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.