The Bottom Line: Expect the warmest temperatures of the winter, so far. Warming, sun, and old weak snow layers at will maintain dangerous conditions. Choose your routes with these in mind. Step back if you find signs of instability such as shooting cracks, hear whumphs, or find slabby snow over weaker snow.
Snow and Avalanche Discussion
As we round out January, the sun is becoming stronger already. This may have caused a small wet loose avalanches on sunny slopes holding cold snow.
A storm on the 23rd caused a widespread natural avalanche cycle throughout the Cascades. Many large paths ran in the area, entraining wet snow as the debris ran through the water saturated lower elevation snowpack. Most of these appear to have ran within the storm snow, but some may have taken out persistent weak layers. It appears that it rained up to around 5,000ft, but uncertainty remains regarding how far down the water percolated into the snowpack.
Sat 26th Jan 14:48
- Robert Hahn
Weather Synopsis for Saturday night through Monday
A high-amplitude late-January ridge centered over our region on Sunday continues to dominate the weather pattern through early next week. The ridge is bringing some of the warmest temperatures our higher elevation stations have seen in several months, while lower elevation stations have managed to warm up as well in spite of the overall inversion (a pattern where colder air sinks and pools in valleys). Overnight, the inversion should gain some strength as clear skies and light winds continue, with patchy very low-level fog developing once again. The fog should reduce in extent during the daytime.
A shortwave weather system riding over the ridge Saturday night and moving down the Rockies into Montana on Sunday, resulting in the ridge axis moving slightly offshore where it will remain into early next week. This is likely to drop freezing levels by 2000 ft relative to Saturday. The upper-level flow should shift northerly while a colder high pressure from interior British Columbia drops southward along with it late Sunday, bringing cold air to the North Cascades and areas east of the Cascade Crest. It will also drive an offshore pressure gradient. Winds will pick up Sunday night and Monday.
Fri 25th 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. Some of these are still present, depending on where you are in the mountains.
Zooming in to current conditions, a storm on January 22-23rd impacted most forecast zones. 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 the 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 in most areas, 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, notably the January 17th surface hoar/near surface facets. Some widely propagating and/or large avalanches were reported, 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 already apparent.
West Slopes and Passes: Where the January 17th interface, (surface hoar and facets) had been a concern below treeline, it seems that it generally wasn’t a player in avalanche activity. At low elevations, the PWL has either been wetted by liquid water or is now capped with crusts and moist snow. While we’re still digging to look for the weak layer, there’s good reason to prioritize other issues below treeline. Uncertainty still exists about the January 17th interface at upper elevations.
Eastern Cascades: Persistent weak layers are still of concern. How much of a concern and exactly which PWL depends on where you are, north to south and east to west, in the range. Overall, January 17th surface hoar/near surface facets would be the main PWL to look for 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 some places, the January 22nd surface hoar may become an issue with future storms.
Large avalanches will be easier to trigger after the recent storm added to the slab over the weak layer.
To the east of Hwy 97, the main concern lies in triggering the whole snowpack to the ground, and a funky layer of crusts, facets, and surface hoar in the mid-pack down a foot or two. As recently as the 24th, an observer noted an "earth shattering WHUMPH!" at 5,000ft near Blewett Pass. Here, the snowpack is shallow and largely variable. Avoid areas with thick slabs adjacent to thin, rocky slopes.
Further west, the main layer of concern is a layer of surface hoar that was buried on the 17th. You can find this layer down a foot or two west of Hwy 97, and will be most readily preserved on sheltered, shaded open slopes from 4,000ft to 6,000ft. Look for shooting cracks, and listen closely for whumphs. Snowpack tests can also help identify these weak layers. Lack of evidence in one profile or test often isn't enough information to make sound decisions from. Avoid getting up underneath steep slopes, travel conservatively with these layers in mind. Put a large buffer of terrain between where you travel and any steep slopes. Slides may wrap around terrain features and surprise you and your group.
2 - 3
Where snow is redistributed, dense slabs have formed. More freshly drifted slabs are the easiest to trigger. When you find more than 6” of snow has drifted in or accumulated, it’s time to evaluate it. You can check how the new snow is bonding by using small, inconsequential test slopes and quick hand pits. Is there strong over weak? Are the slabs cohesive enough to crack? If so, it may be best to steer clear of firm, slabby snow on steep slopes.