Avalanche Forecast

Issued: Apr 24th, 2015 11:00AM

The alpine rating is moderate, the treeline rating is low, and the below treeline rating is low. Known problems include Loose Wet, Wind Slabs and Storm Slabs.

Northwest Avalanche Center NWAC, Northwest Avalanche Center

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

Summary

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

An icon showing 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: East, South East, South, South West, West.

Elevations: Alpine.

Likelihood

Possible

Expected Size

1 - 1

Wind Slabs

An icon showing 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.

Elevations: Alpine.

Likelihood

Possible

Expected Size

1 - 1

Storm Slabs

An icon showing 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: Alpine.

Likelihood

Unlikely

Expected Size

1 - 1

Valid until: Apr 25th, 2015 11:00AM