Avalanche Forecast
Issued: Apr 8th, 2018 11:44AM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Wind Slabs and Loose Wet.
NWAC,
Northwest Avalanche Center
The avalanche danger will rise Monday as sunshine and warm temperatures quickly make recent snow unstable. You can encounter a variety of avalanche dangers, including Wind Slabs, Loose Wet avalanches, Cornices and Glide avalanches. Use visual clues such as fresh cornices, wind drifted pillows and cracks in the snow all indicating that you could trigger a Wind Slab avalanche. Give fresh Cornices a wide margin of safety and limit exposure on steep sun exposed slopes or where the surface snow is becoming wet.
Summary
Detailed Forecast
Rapid clearing, sunshine and warming is expected Monday. This should cause an increase in the avalanche danger Monday, following the weekend's strong winter-like storm cycle.
Fresh wind slabs will be easy to trigger on steep lee slopes below ridges at higher elevations and features where wind drifted snow exists.
Recent storm snow and old snow will become wet, making Loose Wet avalanches possible, especially on steep sun exposed slopes during the late morning and afternoon. Watch for signs of wet snow conditions such as sluffing, roller-balling and any natural Loose Wet avalanches.
Cornices have grown large in many areas and will become fragile with the sun and warming. Give cornices a large margin of safety and avoid travel on slopes below, as cornice failures may trigger large avalanches on slopes below.
Snowpack Discussion
A strong storm over the weekend caused rain Saturday to high elevations followed by cooling and new storm snow by Sunday afternoon. A wide range of new snow amounts will exist along the east slopes of the WA Cascades with the greatest amounts nearer the crest and at higher elevations. The new snow was transported by periods strong winds that built fresh Wind Slabs on a variety of aspects, especially below ridges.
The recent storm built fresh cornices along exposed ridges.
Up to 6 inches of snow was received last week. Strong W-NW winds redistributed recent snow onto lee slopes above treeline forming shallow wind slabs.
Older layers of weak snow can be found in the snowpack. We have limited information about the distribution and reactivity of these layers leading to a higher level of uncertainty. If you have any information or observations on layers within the snowpack please consider submitting them to NWAC via our public observations page. The exact weak layer and depth depends on your location. Common weak layers are:
- 3/25: Buried surface hoar layer found on shaded aspects near and above treeline.
- 3/22: Buried surface hoar layer found on shaded aspects near and above treeline.
- 2/13 Facets above a firm and thick melt-freeze crust can be found in many locations near and above treeline. This layer is thought to be dormant at this time but may re-awaken with a substantial weather input.
Rainfall and continued above freezing temperatures Friday night and Saturday morning may allow Wet Slabs to fail at these interfaces. Peristent Slabs may be reintroduced when we receive more snowpack information following this storm cycle.
Observations
North
On 3/23, professionals near Washington Pass reported the 3/8 buried surface hoar was reported as 1.5 feet (45cm) below the snow surface.
Central
On Friday 4/6, professional observer Matt Primomo traveled up the Icicle to above Colchuck Lake. Matt found a moist and well settled snowpack below 5800' with no persistent weak layers. The 2/8 crust was 1 m down. At 6700' on a NNE aspect, Matt found preserved stellars 14 in (35 cm) down and showing a likelihood to propagate in snowpack tests. Dry snow was found at this elevation below the top 8 " (20 cm). Higher in the terrain, recent wind slab avalanches likely released on this layer. Matt also observed natural loose wet avalanches and one wet slab avalanche above treeline on a SE aspect.
On Saturday 3/31, an observer reported a large slab avalanche that likely failed on a persistent weak layer 4-5 feet below the surface in the Enchantment Mountians
On Friday 3/30, Matt Primomo was on Dirtyface Mountain where he found two buried surface hoar layers (3/22) on shaded aspects at variable depths within the two feet of the snowpack. Test results indicated the potential for propagation on both weak layers. Snow pits indicate a weakening 2/8 crust.
Problems
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: All aspects.
Elevations: All elevations.
Likelihood
Expected Size
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
Expected Size
Valid until: Apr 9th, 2018 11:44AM
