Avalanche Forecast

Issued: Mar 18th, 2016 11:15AM

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

Northwest Avalanche Center NWAC, Northwest Avalanche Center

Check the forecast for a mixed bag of spring avalanche problems and evaluate snow and terrain carefully on Saturday.

Summary

Detailed Forecast

Increasing mid and high clouds should be seen on Saturday. The best chance of some light rain or snow showers should be over the Olympics. Any rain and snow amounts will be light and should not cause a change in snow conditions. East winds should decrease over the Cascade crest and in the Cascade passes on Saturday.

The avalanche forecast won't change much from Friday.

Loose wet avalanches may still be possible on steeper solar aspects on Saturday. Loose wet avalanches may start small but could have the ability to entrain recent moist snow in mainly on steep solar terrain. High clouds Friday night may limit a refreeze of surface snow by Saturday morning.

Storm winds 3/13-15 likely built wind slab on lee aspects in the upper elevations of the above treeline band. This wind slab will mostly likely be found on NW-SE facing slopes. But east winds the past couple days will also have built wind slab on northwest to southwest aspects in places along the Cascade crest and in the Cascade passes. Hence the the wind slab avalanche problem will be indicated on all aspects in the near and above treeline.

Wind slab may also be locally possible in the below treeline on west slopes in areas that had east winds the past couple days. Firm wind transported snow and snowpack cracking can be good signs of wind slab layers.

Recently formed cornices have grown large and can become sensitive when it is warm and sunny. Avoid areas on ridges or summits where there may be a cornice and avoid slopes below cornices in the spring.

Although it won't be listed as an avalanche problem avoid areas below steep slopes or rock faces where unusual glide avalanches can release unexpectedly. See the recent observations above for a good example in the Mt Baker backcountry.

Snowpack Discussion

Weather and Snowpack

Storms moved across the Northwest at a nearly daily frequency the past few weeks with fluctuating but generally moderate snow levels. Two deep surface low-pressure systems moved across the Olympic Peninsula last week with the first passing last Thursday 3/10, and the next on Sunday, 3/13. These systems caused very stormy weather.  

Cool and showery weather Monday and Tuesday with  prolonged westerly winds deposited additional snow at lower snow levels. Storm totals from Thursday 3/10 through Tuesday morning 3/15 were 2 - 3.5 feet!

Daytime warming and settlement have allowed recent storm-related weak layers to gradually settle and mostly stabilize. The stabilizing has been verified by pro-observers and snow safety personnel over the past few days.

Recent storms also built large cornices along ridges in many areas with some recent releases.

The main story Thursday and Friday 3/17-18 has been fair weather and east winds.

The mid and lower snowpack along the west slopes should generally be a stable mix of crusts and layers of moist and rounded snow crystals.

Recent Observations

The Mt. Baker pro-patrol reported yet another glide avalanche to ground (5-6 feet deep!) on a steep rock face around 3500 feet in the below treeline band off of Shuskan Arm on Tuesday 3/15. Powerful glide avalanches remain possible in isolated terrain features such as on steep slopes with smooth bed surfaces and especially where the slabs are unsupported from below. These avalanches are not predictable in the sense that they are not tied to short term warming or rain events. Specifically in the Mt. Baker backcountry, we want to highlight the low likelihood-high consequence of this sporadic but ongoing avalanche hazard during this wet winter.  

NWAC pro-observer, Lee Lazzara  was in the Mt Baker backcountry Tuesday 3/15 near Table Mt and Mt Herman. The main takeaway was that wind slabs were still sensitive to ski release on steep open slopes, with a skier triggered size D1 wind slab on a steep NE facing slope. The mid-March solar radiation was quickly settling storm slab instabilities and causing 5-10 cm on wet surface snow on solar aspects, producing several small loose-wet slides.  

Pro-observer Dallas Glass was in the Kendall Peak area of Snoqualmie Pass Wednesday 3/16. He found that while density differences persisted in the layering of recent storm snow, propagation was unlikely and that overall instabilities had settled out within the 60 cm of storm snow over a 3/12 rain crust. As the sun poked out mid-day Wednesday, generally small loose wet avalanches were observed in steep terrain of Kendall Peak on S-SW aspects.

Lee was back in the Mt Baker backcountry on Thursday 3/17 and reported good stability in the below and near treeline with little softening of surface crusts on solar slopes and good skiing on non-solar slopes. Plumes of snow from the high summits of Shuksan and Baker indicated lots of wind transport in the above tree line and high elevations.

Dallas was out in the Paradise area on Thursday 3/17 and found east winds transporting snow to SW to NW aspects. Several small to large loose wet avalanches were seen on solar slopes. About 80-90 cm of cold dry powder was found on non-solar sheltered slopes.

A skier on Turns All Year reported east winds and a small triggered wind slab below treeline in the Tatoosh on Thursday 3/17.

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: All elevations.

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: All aspects.

Elevations: Alpine, Treeline.

Likelihood

Possible

Expected Size

1 - 1

Cornices

An icon showing Cornices

Cornice Fall is the release of an overhanging mass of snow that forms as the wind moves snow over a sharp terrain feature, such as a ridge, and deposits snow on the downwind (leeward) side. Cornices range in size from small wind lips of soft snow to large overhangs of hard snow that are 30 feet (10 meters) or taller. They can break off the terrain suddenly and pull back onto the ridge top and catch people by surprise even on the flat ground above the slope. Even small cornices can have enough mass to be destructive and deadly. Cornice Fall can entrain loose surface snow or trigger slab avalanches.

 

Cornices can never be trusted and avoiding them is necessary for safe backcountry travel. Stay well back from ridgeline areas with cornices. They often overhang the ridge edge can be triggered remotely. Avoid areas underneath cornices. Even small Cornice Fall can trigger a larger avalanche and large Cornice Fall can easily crush a human. Periods of significant temperature warm-up are times to be particularly aware.

 

A corniced ridgeline. A large cornice has formed at the top of the ridge. A smaller cornice has formed to the left of the trees from crossloading.

Cornices are easy to identify and are confined to lee and cross-loaded ridges, sub-ridges, and sharp convexities. They are easiest to trigger during periods of rapid growth (new snow and wind), rapid warming, and during rain-on-snow events. Cornices often catch people by surprise when they break farther back onto flatter areas than expected.

Aspects: North, North East, East, South East, North West.

Elevations: Alpine, Treeline.

Likelihood

Possible

Expected Size

1 - 1

Valid until: Mar 19th, 2016 11:15AM