Cascades - West Avalanche Forecast
Jan 10th, 2020 10:01AM
The alpine rating is Storm Slabs., the treeline rating is , and the below treeline rating is Known problems include
A potent winter storm will maintain very dangerous avalanche conditions at upper elevations. The new snow needs time to stabilize. Avoid travel on or underneath slopes 35 degrees and steeper.
Avalanche danger will likely peak overnight Friday as heavy snowfall continues. Very dangerous avalanche conditions could linger into the day Saturday, especially if we reach the high end of the forecast precipitation over the next 24 hours.
Friday brought strong and gusty westerly winds and 8-12in of cold low-density snow above 4000ft. Observers at Stevens Pass and in the Mt. Baker area reported fresh slabs forming throughout the day that became touchy to human-triggers in the afternoon, especially in wind loaded areas. On Thursday, a human-triggered cornice avalanche occurred in the Mt. Baker Backcountry on the east aspect of Mt. Herman at 5500ft (observation, photo). Two people were caught and carried in the avalanche, but uninjured.
January 9th, 2020 (The regional synopsis is updated every Thursday @ 6 pm)
As we said Happy New Year and rang in 2020, snow was turning to rain at many trailheads and lower elevation Passes, not exactly the fresh start winter recreationalists had in mind. The snowpack was already looking a little thin throughout the region, especially at lower elevations. Low snow in places like Snoqualmie Pass made backcountry travel difficult and hazardous. NWAC’s snow depth climatology report was showing snow depths 25-64% of normal to kick off the start of 2020.
Things can change quickly in the Pacific Northwest and they did as we entered an extended storm cycle between January 2nd to January 8th. Strong winds, fluctuating temperatures, and heavy precipitation offered few breaks in the weather over this period limiting observations and hampering travel. Despite periods of rain at lower elevations, most areas saw several feet of new snow with big jumps in total snow depths as a westerly storm track strongly favored the West Slopes of the Cascades and the Olympics for the highest precipitation totals.
Total Snow Depth (in) 1/2/20
Total Snow Depth (in) 1/8/20
Heather Meadows Mt Baker
Crystal Mt Green Valley
Paradise Mt Rainier
White Pass Upper
Mt Hood Meadows
We may have started with a shallow snowpack, but most locations increased their snowpack by 70% or more over this storm cycle!
During this extended and impressive storm cycle that included backcountry avalanche warnings, natural avalanches were reported in many areas Jan 6th-7th.
The Stevens Pass area was especially active over the period with over 100(!) avalanche observations made on the 6th and 7th. Professionals reported numerous avalanches in places that they hadn't previously observed avalanches and some paths avalanched multiple times in a 24 hour period. Observers reported a few very large (size D2.5-3) avalanches, originating at upper elevations with deeper crowns that likely formed from wind drifting. Topping off an active couple of days, warming temperatures lead to a widespread loose wet avalanche cycle.
The southern Washington Cascades, the Wentachee Mountains and Mt. Hood either saw less precipitation, warmer temperatures leading to more rain than snow, or some combination of the two and ended up with relatively less active avalanche conditions than areas further north.
A large natural avalanche on Rock Mountain near Berne along Hwy 2 east of Stevens Pass that released Jan 6th or 7th. Photo: Josh Hirshberg 1/7/20
Many small storm slabs released in the Crystal backcountry 1/6-1/7. Pinwheels in the photo suggest loose wet avalanche activity occurred when temperatures rose above freezing and snow turned to rain.
Another active and colder weather pattern is on it’s way. Enjoy yourself out there and be sure to check the forecast before heading out. Remember, NWAC is a community-supported avalanche center and when you submit an observation you make the forecast better!
It’s getting deeper! Photo: Jeremy Allyn
You can trigger large avalanches in new and wind loaded snow on slopes 35 degrees and steeper. Expect thicker and more reactive slabs at upper elevations, and in wind drifted areas. Avalanches could occur naturally at upper elevations, especially during periods of heavy precipitation and active wind loading. Watch for obvious signs of unstable snow like recent avalanches, shooting cracks, and collapses. Seek out low-angle, supported terrain away from overhead hazard. With all the new snow, anticipate loose dry avalanches as well. These sluffs could entrain quite a bit of snow, or even trigger slab avalanches as they run downslope.
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.
Valid until: Jan 11th, 2020 10:01AM