The Bottom Line: If you are heading to the alpine, less traveled terrain, or areas with a shallow snowpack, use caution on slopes 35 degrees and steeper. Stay off of steep slopes if you see shooting cracks in the snow or experience collapses.
Snow and Avalanche Discussion
The current snow surface is a mix of supportable and breakable sun and wind crusts, crunchy styrofoam, and recycled powder. Avalanches aside, one of the main hazards out there tomorrow will be firm, icy snow. A widespread natural avalanche cycle occurred throughout the region during the storm on January 22nd and 23rd. Several large to very large avalanches were observed in the Salmon la Sac, Icicle Creek Canyon, and on Dirtyface near Lake Wenatchee. Many of these slides are suspected to have run within storm snow or on the January 3rd melt freeze crust. Check out the Regional Synopsis tab for more details on the storm and avalanche activity. Thanks for your observations, keep them coming!
Tue 29th Jan 14:44
- Robert Hahn
Weather Synopsis for Tuesday night through Thursday
The ridge of high pressure continues to dominate the weather with the ridge axis just offshore. Expect generally mostly clear skies in the mountains, light alpine winds, and some easterly flow. Tuesday night and Wednesday, the pool of cold air east of the Cascade Crest will diminish as the trough that brought it is now far to the east. The pressure gradient across the Cascades will also weaken and easterly flow through the mountain gaps will lose strength. Meanwhile, cutoff low well offshore heads ESE towards northern California with quiescent weather remaining over our region and the ridge starts breaking down from the north with moisture spilling into British Columbia. The Pacific Northwest is in between. More fog is anticipated Tuesday night and Wednesday with very little wind to mix the atmosphere.
The same pattern continues Wednesday night and Thursday with a low pressure developing west of Vancouver Island. Moisture continues spilling into British Columbia with clouds increasing in northwest Washington state. On Thursday there may be a chance of light drizzle in the mountains. Easterly flow will increase slightly with the approaching trough.
Sat 26th Jan 09:00
The late January pause...
Now that we’re well into winter and in the midst of a prolonged period of quieter weather, this is a good time to assess the current snowpack. Most areas are hovering around 75% of average snowpack depth for the season to date. Many of this season’s storms have brought at least some rain to mid elevations at the passes and west of the Cascade Crest. Multiple periods of cool, clear weather formed persistent weak layers.
A prolonged period of warm quiet weather followed a storm on January 22-23rd that impacted most of the region. This weather system changed snow at the surface of the snowpack, drove an avalanche cycle, and tested or changed the latest persistent weak layers. The storm ramped up late on the 22nd in most areas. Intense precipitation (mostly snow) fell by dark with strong wind and warming temperatures. In the early morning hours of the 23rd snow switched to rain at the passes and western zones. Cold air to the east maintained low-density snowfall at most locations well east of the crest. In some areas, especially east of the crest, a new weak layer may have been buried at the January 22nd interface.
January 23rd avalanche cycle
The bulk of recently reported avalanche activity occurred on January 23rd with high precipitation rates and wind loading in the early morning hours or with liquid water in the snow from rain or warming and sun during the day.
At the passes and in the western zones, reported avalanches were generally running either at the new/old snow interface or within the new snow. Observers reported a mix of soft slab, wet loose avalanches from near and below treeline with a few reports of shallow slabs above treeline. As of January 24th observations at upper elevations have been limited, due to lack of visibility and access.
In the eastern zones at areas like Washington Pass, Icicle Canyon, and the Wenatchee Mountains, some reported slab avalanches were confirmed or suspected to have involved persistent weak layers. Most notable is the January 17th surface hoar/near surface facets. There were a few reports of widely propagating avalanches, up to destructive size 3. Wind loading was a factor in many avalanches that released at upper elevations. Observers reported a widespread point releases on steep sunny slopes in the new snow where skies cleared on the 23rd.
Persistent Weak Layers (PWLs)
The latest round of snow, rain, and warming that tipped the balance of the snowpack was a good test of existing persistent weak layers. While the ever-changing snowpack keeps us busy tracking changes, some trends are apparent.
West Slopes and Passes: With a few exceptions, persistent weak layers have gained enough strength through rounding or melt-freeze to no longer be a concern for triggering avalanches. The main uncertainty lies with the January 17th surface hoar at upper elevations near Stevens Pass. This PWL is no longer a concern below treeline or in other zones. It appears that surface hoar generally wasn’t a player in avalanche activity near and west of the Cascade Crest. At low elevations, it has either been wetted by rain or is capped with crusts and moist snow. There’s reason to prioritize other issues, like challenging travel conditions, below treeline.
Eastern Cascades: Triggering avalanches on persistent weak layers is possible east of the Cascade Crest. How much of a concern and exactly which PWL depends on where you are, north to south and east to west, in the range. The January 17th surface hoar/near surface facets is the main concern in the upper snowpack. Areas of shallower snowpack that lie further east of the Cascade Crest will have weaker snow and more pronounced weak layers in the mid to lower snowpack. In the Wenatchee Mounains you may find another layer of surface hoar (January 3rd) in the middle of the snowpack as well as weak facets near the ground. In places, like Washington Pass, the January 22nd surface hoar may become an issue with future storms.
A number of potential weak interfaces are worth checking for; The January 22nd surface hoar, small facets above a thick melt freeze crust from January 3rd, and the December 26th surface hoar are the most prevalent. In the Wenatchee Mountains and east of Highway 97, the snowpack is often less than 4 feet deep and highly variable in distribution. These layers are likely to be more reactive in this area. A pair of thin freezing rain crusts associated with facets above, between, and below are the main concern. You are most likely to find these layers to be preserved on wind sheltered, shady, and open slopes above 5,500ft.
Look for shooting cracks, and listen closely for whumphs. Human-triggered slides are becoming unlikely, but careful evaluation of these layers with snowpack tests are still warranted. Use the terrain to your advantage, avoiding areas with thick slabs adjacent to shallow, rocky slopes. Steep, unsupported slopes on shaded aspects are also worth steering clear of. In steep, rocky, and south facing slopes you may be able to initiate an isolated loose wet avalanche.