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Avalanche Forecast

Archived

Mar 12th, 2017–Mar 13th, 2017

Alpine
Natural avalanches possible, human triggered probable.
Treeline
Natural avalanches possible, human triggered probable.
Below Treeline
Natural avalanches unlikely, human triggered possible.
Alpine
Natural and human triggered avalanches likely.
Treeline
Natural avalanches possible, human triggered probable.
Below Treeline
Natural avalanches possible, human triggered probable.

Regions

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Mother Nature is throwing a lot of avalanche problems at us right now so review the forecast to remind yourself of what to watch for if you are planning a trip into the backcountry on Sunday. Re-evaluate or ratchet back your plans if you encounter significantly more new snow than described at higher elevations.

Detailed Forecast

A short break between frontal systems should be seen Sunday. Light showers mainly over the Cascade west slopes should taper off in the morning. Renewed light rain or snow should be seen mainly over the Olympics and northwest to central west Cascades as moisture from the warm front of the next system reaches the Northwest. Re-evaulate your plans if the weather is not turning out as expected.

Mother Nature is throwing a lot of avalanche problems at us right now so review the forecast to remind yourself of what to watch for if you are planning a trip into the backcountry.

New or previous wind slab should be suspected mainly on NW-SE aspects above treeline due to recent SW-W winds. But watch for firmer wind transported snow on all aspects especially in areas of complex terrain.

New sensitive storm slab is most likely above treeline in areas that receive more than a few inches of rapidly accumulating snowfall.

The persistent slab problem was returned to the Northeast zone due to recent avalanches and some reactive tests on the Valentine's Day or 2/17 crust. Avalanches stepping down to these depths would be large and very dangerous.

Loose wet avalanches are most likely on solar slopes in the near and below treeline especially if you are in an area with sunshine or little cloud cover. Watch for surface wet snow deeper than a few inches, pinwheels and initial small releases that will get bigger the longer the sun heats the snowpack.

Dangerous avalanche conditions are expected Sunday. We only got a taste of the spring stabilizing process the past couple days. Avalanches may step down or entrain deeper layers and be large and dangerous especially if they reach the Valentine's Day crust layer.

Snowpack Discussion

Weather and Snowpack

The first week or so of March was very cool and snowy. NWAC and NRCS stations indicate about 2 feet of snow in the northeast Cascades with less elsewhere along the Cascade east slopes. This gave the snowpack a test and caused an avalanche cycle of several types of avalanches in the Cacades.

A strong frontal system brought increasing precipitation and winds along with a warming trend to the Cascades on Thursday. On Thursday night rain may have pushed up to about 4000 feet in the northeast Cascades and about 5000 feet in the southeast Cascades. By Friday morning NWAC stations along the Cascade east slopes had about .35-.9 in of WE  with only 0-5 inches of snow mainly in the northeast Cascades.

Another front crossed the Cascades on Saturday causing more SW-W winds, snow at higher elevations, and rain at low elevations. It looks like there is up to a few inches of new snow east of the crest on Sunday morning.

Recent Observations

North

On Wednesday, the NCMG on Delancey Ridge observed crowns from recent natural storm slabs about 20-30 cm deep on S-SW aspects near and above 5500 feet. In a test pit on a SE aspect at about 5400 feet, the 2/17 melt freeze crust gave hard but sudden planar results in compression tests failing on facets above the crust.

The NCMG were out again on Thursday near Washington Pass and report a 2/17 crust and the Valentine's Day layer at about 80-130 cm down. Deep tap tests of the Valentine's Day crust at 95 cm on gave results 1 out of 4 tests; PSTs gave results that ranged from no result to PST End so there is variation in the reactivity of the Valentine's Day layer.

On Saturday a report for Delancy Ridge via the NWAC Observations page indicates many natural large loose wet avalanches on solar slopes.

The NC Heli Guides on Saturday report significant recent warming and settlement. Recent natural size 1.5-2 wind and storm slab were seen in the Silver Star area. Pits in the upper Willow drainage had generally right side up layers with the Valentine's Day crust seen at 100-128 cm.

Central

The Mission Ridge patrol reported surprisingly minor results during avalanche control Thursday morning despite the new snow and moderate west winds. The new snow was generally bonding well with only a few areas of soft wind slab releasing below ridge-lines. The releases were shallow and did not step down to any deeper layers.  

South

No recent observations. 

Problems

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.

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.