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Avalanche Forecast

Archived

Apr 24th, 2015–Apr 25th, 2015

Alpine
Natural avalanches unlikely, human triggered possible.
Treeline
Natural avalanches unlikely.
Below Treeline
Natural avalanches unlikely.
Alpine
Natural avalanches unlikely, human triggered possible.
Treeline
Natural avalanches unlikely.
Below Treeline
Natural avalanches unlikely.

Regions

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 Don't underestimate the effects of the increasing strong April sun which can make avalanches touchy on Saturday in areas of heavier storm snowfall.

Detailed Forecast

The low pressure system will shift east of the Cascade crest on Saturday. Showers mainly west of the crest should be mostly light and less extensive on Saturday. There should be little if any additional snowfall on Saturday.

Storm snow amounts are likely to vary a lot by Saturday morning from perhaps around 5 inches at Hurricane, to 5-10 inches west of the crest, but with about 1.5 feet or more at Mt Baker, around 1 foot at Mt Hood, and only light amounts at higher elevations east of the crest.

Avalanche problems Saturday will in turn vary a lot depending storm snow amounts. Don't underestimate the effects of the increasing strong April sun which could make avalanches touchy on Saturday. Areas with moderate amounts of new snow will give poorer skiing and areas with heavier snow are likely to be dangerous such as near and above treeline at Mt Baker. A bed surface may be provided by crusts from warm weather in mid-April.

Loose wet avalanches should become touchy and easily triggered on solar slopes in areas of heavier snowfall Saturday mid-morning through the afternoon. This is very likely on solar slopes but possible on all slopes in areas of heavier snowfall. Watch for wet surface snow that gets deeper than a few inches, pinwheels or initial small natural avalanches from rocks or cliffs that are signs to shift to lower angle slopes.

Firmer wind transported snow and wind slab will be most likely to linger on lee slopes. Storm slab should be shorter lived on Saturday. Both are also most likely in areas of heavier snowfall at higher elevations.

If you decide to venture into the back country on Saturday it should be best to find an area of lighter or no storm snowfall such as east of the crest.

Cornices won't be listed as an avalanche problem but avoid slopes below cornices and areas on ridge where it may be difficult to know if a cornice is present.

Remember that this forecast applies to elevations up to the Cascade crest and not to the high parts of the volcanoes where conditions at this time of year are often more dangerous.

The mid and lower snowpack is most areas should consist of stable rounded grains and crust from warm periods this winter. Many areas at low elevations especially in the Olympics and east of the crest do not have enough snow to cause an avalanche danger.

The work of the 3 NWAC forecasters is transitioning to other essential parts of the program before the NWAC closes for the summer.

 

Snowpack Discussion

After a cool, snowy early April we had about a week of warm dry weather in mid-April. This caused loose wet snow avalanches and consolidation. Layers from early April should be mostly stabilized for the time being and attention will be on new snow through the weekend.

A cold front moved across the Northwest on Thursday. The front is being followed by a low pressure system, south to southwest winds and a cool unstable air mass that is shifting across the Northwest on Friday. Storm snow amounts are likely to vary a lot by Saturday morning.

 

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.

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.