Avalanche Forecast
Regions: Snoqualmie Pass.
A continuing mix of spring and winter avalanche conditions should be seen on Sunday.
Detailed Forecast
A short wave ridge, weak surface high pressure and drier air mass in indicated to move to the Northwest on Sunday. Cloudy conditions in the morning should give way to partly sunny weather in the afternoon near and west of the crest although high clouds are indicated. Temperatures and freezing levels should rise a fair amount by Sunday afternoon.
Natural or triggered wet loose avalanches seem likely and more extensive than other concerns on Sunday. The late April sun can be counted on to act on new snow deeper that a couple inches on any steep solar slope. This will be mainly in the ATL and NTL zones but possible in the BTL zone. Watch for surface wet snow deeper than a few inches, pinwheels or roller balls which usually precede larger wet loose avalanches at all elevations.
Small new areas of wind slab from Saturday will be likely on lee slopes. This will mainly be on north to southeast slopes in the ATL zone. Watch for signs of firmer wind transported snow or cracking snow.
Small areas of new shallow storm slab from Saturday will also be possible on sheltered slopes in the ATL zone. But this should only be possible in areas that get a least several inches of snow.
Snowpack Discussion
A warm front caused snow and warming on Wednesday. This was followed by a cold front with you guessed it snow and cooling on Thursday. By Friday morning 48 hour snowfall at NWAC sites in this area above about 5000 feet was about 7-13 inches.
The Alpental ski patrol reports that the snow and warming Wednesday caused a natural avalanche cycle in their area.
NWAC observer Jeff Hambelton was in the Mt Baker area on Thursday and reported easily ski triggered small to medium storm slab and wet loose avalanches on the south slopes of Mt Herman. Storm and wind slab were also responsive to ski cuts by the Mt Baker ski patrol Friday morning.
Storm slab and wet loose releases on Mt Herman 17 April by Jeff Hambelton.
Otherwise less activity was reported during a relative break in the weather on by Friday. NWAC observer Dallas Glass noted wind had redistributed snow in the vicinity of Paradise but did not observe instability.
A general profile of surface layers on non-solar slopes in this area Saturday morning above about 5000 feet should be the 3-10 inches of snow from Thursday generally bonded to a crust from about Wednesday night over wetter snow from Wednesday.
Much less snow and consequently much less avalanche activity has been seen below about 4-5000 feet in all areas the past couple days.
Another front is crossing the Cascades this afternoon. A short wave should rapidly follow the front across the Northwest this afternoon and evening. A convergence zone is indicated by the WRF model in the Glacier Peak to Washington Pass area this evening. Then showers should decrease tonight and end Sunday morning. This should generally cause southwest winds and 3-9 inches of new snow above in the ATL zone with cooler temperatures and lower snow levels by Sunday morning.
Avalanche Problems
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: All elevations.
Likelihood: Likely
Expected Size: 1 - 1
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: Likely
Expected Size: 1 - 1
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: Likely
Expected Size: 1 - 1