Avalanche Forecast Cascades - West

Date Issued: Valid Until:

Robert Hahn,

Avalanche Forecast

Fri Jan. 18th · 6:00PM

Alpine

Danger Ratings High

Treeline

Danger Ratings Considerable

Below Treeline

Danger Ratings Moderate
Loose Wet Loose Wet
Storm Slabs Storm Slabs

Alpine

Danger Ratings Considerable

Treeline

Danger Ratings Considerable

Below Treeline

Danger Ratings Moderate
The Bottom Line: A strong storm will impact the West-Central zone Friday night and Saturday adding more snow/ rain to the area and keeping the avalanche danger elevated. Be patient Saturday. Stay off of open slopes greater than 35 degrees where you can trigger avalanches within the new snow.

Snow and Avalanche Discussion

Another round of heavy precipitation Friday night into Saturday will only serve to add more load to the snowpack and keep the avalanche hazard elevated. The avalanche danger will likely peak overnight Friday/Saturday as the heaviest and warmest precipitation impacts the area. However, heavy snow showers and moderate winds may cause very dangerous avalanche conditions to linger into the daylight hours. As the weather cools, and the snow showers decrease, avalanche danger will slowly decline, but if you find yourself in areas of sustained heavier snow showers, expect that the avalanche danger may locally maintain or increase.

We received reports of natural, human-triggered, and explosive triggered avalanches from the Mt Baker area Friday and given similar recent weather impacts, we suspect that the Mountain Loop may behave similarly. At least one of these avalanches released after several people traveled on the same slope. Avalanches occurred on W-N-NE aspects above 4800 ft, where mostly cold dry snow accumulated. A subtle weakness found between Thursday and Friday’s storm snow seemed to be the culprit in most of these events. We expect similar weaknesses within the storm snow again Saturday in the Mountain Loop.

Weather Forecast

Sat 19th Jan 14:37 - Dennis D'Amico

Weather Synopsis for Saturday night through Monday

Leftover showers should wind down this evening as a weak low pressure system develops along the old frontal boundary to our south. This system will develop and lift north Sunday morning, spreading light rain and snow into Mt. Hood and perhaps the south Washington Cascades. Clouds and light precipitation should decrease near and north of Snoqualmie Pass in the afternoon as the system pulls further east.

Shortwave ridging anchored offshore will flop into the region late Sunday night and Monday making for a fair weather day. Relatively cooler air will filter into the region Monday and no significant winds are expected over the short term.  

Regional Synopsis

Wed 16th Jan 09:00

January 16, 2019

Since Friday January 11, we’ve enjoyed a spell of generally nice weather in the mountains. This allowed for some great views, enjoyable outings, and lots of snow observations. A more active weather pattern beginning Thursday January 17th will bring this nice weather to a close.

During this time period, the snow surfaces around the area have changed dramatically. This forms the foundation for a few current trends we are seeing in the mountains.

New Snow Problems

Reports from around the area indicate a wide variety of snow surface conditions prior to new snow on January 17th. We’ve heard about breakable crust, very icy surfaces, sugar facets, surface hoar, and rime. What snow surface you encounter can depend on aspect, elevations, and general location.

As a series of winter storms impact the area, how will the new snow bond to the old snow surface. This can be tricky to predict. As the snow starts to pile up make lots of shallow snow observations. Shovel tilt test, hand shears, and small slope test can all help you track how the new snow is bonding to the old snow at different aspects and elevations. Stop and take a look at where the snow is failing. Do you see large grains of snow? Do they look like feathers? Do the act like sugar? How far below the current snow surface are they?

Old Snow Problems

There are still lingering persistent weak layers in the eastern zones of the cascades. You are most likely to find weak older snow in areas further east from the Cascade crest where snowpacks are shallower, more variable, and generally weaker. In some locations weak snow near the ground can still be found. These basal facets have hung around all season. The only way to gain information about these old persistent weak layers is to get out your shovel and dig. Because of the size of our forecast zones and the variability in the snowpack, it's important to make snow observations as you travel. We’ll keep watching these old layers, but let us know what you see while you are in the mountains.

Problems

Loose Wet

Loose Wet

Below the rain-snow line, you will find wet snow surfaces. This lower elevation snowpack is well situated to handle more water. That said, when we have wet snow on the surface, you may trigger loose avalanches on steeper slopes. These small avalanches can be a problem if they catch you off guard, or carry you into a terrain trap, such as gullies, creeks, and trees. As temperatures cool and the snow surface dries, these avalanches will become less and less likely.

Aspects: All aspects.

Elevations: Below Treeline.

Likelihood

Possible

Expected Size

1 - 1
Storm Slabs

Storm Slabs

Fluctuating freezing levels and periods of heavy precipitation will create weak layers within the new storm snow. Storm slabs are more likely to fail naturally during periods of high precipitation rates and when the wind is actively blowing snow. As snow showers and winds decrease, storm slabs will slowly stabilize. This is a good time to be patient and allow these storm slabs to gain strength. Stay off of open slopes greater than 35 degrees where you can trigger avalanches within the new storm snow. If you travel to higher elevations, do not linger in locations where avalanches can run and stop. You can look for unstable storm snow as you travel. Small slope test, hand shears, and hand pits can help you identify weaknesses within the new snow.


Aspects: All aspects.

Elevations: All elevations.

Likelihood

Very Likely

Expected Size

2 - 2