Avalanche Forecast
Issued: Feb 13th, 2020 10:00AM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Wind Slabs.
NWAC,
Northwest Avalanche Center
New snow and strong to extreme winds are creating increasing widespread hazard near and above treeline, with slabs building into exposed below treeline terrain. Adjust your mindset and limit your terrain selection to slopes less than 35 degrees in response to the major increase avalanche hazard. Look for active snow transport, recent cornices near ridgelines, and cracking snow surfaces where large slabs may be increasingly sensitive to trigger on lee slopes.
Summary
Discussion
Mt. Hood experienced a major wind event on Tuesday that scoured near and above treeline terrain, leaving hard surfaces and minimal avalanche danger, with a sun crust on aspects exposed to the sun up to near treeline. Below treeline, several inches of dry snow sits atop a 1” knife hard crust. The wind left minimal avalanche danger, but all that will change quickly Thursday night with another wind event accompanied by fresh snow.
Below treeline, the snowpack depth is far below normal and you are likely to encounter early-season-like conditions. Expect 0-16" of snow depth below 4000 ft elevation and 2-3 ft or less of snow depth on solar aspects at 4000-5000 ft elevations.
Temperatures were mild until a cold front passed on Thursday causing temperatures to drop below freezing at Mt. Hood Meadows in the afternoon. Winds were moderate to strong all day at 6500 ft and should ramp up further Thursday night the onset of moderate snowfall.
Snowpack Discussion
February 13, 2020 (The regional synopsis is updated every Thursday @ 6 pm)
Heart of Winter
The action has been non-stop so far in 2020 with several widespread natural avalanche cycles and a few recent close calls. The active weather pattern has kept us all on our toes, especially January’s barrage of storms bringing seemingly endless precipitation and dramatic snowpack growth. Ongoing snow, wind, and rain continued into February, and a not-so-ordinary atmospheric river event recently left its mark on the region. The second week of February brought the first stretch of high pressure in weeks, allowing the snowpack to gain strength and the avalanche danger to ease between storms. Now, in the heart of winter, we have a deep and healthy snowpack with snow depths throughout the Cascades and Olympics near 100% of normal. Looking ahead, each day brings new changes to the upper snowpack, and a dynamic pattern with direct action events (storm-driven avalanche danger) will likely be par for the course.
Atmospheric River Aftermath
Model simulation for February 5-6th, 2020 showing an Atmospheric River (AR) with a less than common northwest-southeast orientation as it impacts the region. This orientation allowed for strong westerly winds and more favorable upslope flow than a more typical AR approaching from the southwest. Image courtesy of the Center for Western Weather and Water Extremes, UC San Diego. (Link)
An atmospheric river impacted the region on February 5th-8th, causing a string of notable events. This storm favored the Central Cascades and Stevens Pass in particular, which experienced continuous heavy snow and rain for 86 hours, amounting to almost 70in of snow with about 7.5in of water equivalent. Not surprisingly, atmospheric rivers often go hand in hand with avalanche warnings, which were issued for 3 consecutive days at Stevens Pass from February 5th-7th, along with high danger in all other zones. Heavy rain fell at low elevations and even caused a significant mudslide on SR 410 between Enumclaw and Crystal Mountain, closing the road for 4 days and knocking out communications to 9 mountain weather stations for a week. As the AR exited the Northwest, and natural avalanche activity tapered off, conditions still remained touchy to human traffic on February 8th and 9th. Several triggered avalanches were reported that weekend, most notable of which was a close call near Mt. Baker Ski Area:
On February 8th, a skier was fully buried in an avalanche adjacent to Mt. Baker Ski Area. The avalanche was triggered by a traveler from a different party. Mt. Baker Ski Patrol was on the scene immediately, located the victim quickly, dug them out, and cleared the airway. The individual survived and reported no injuries. The avalanche was about 1ft deep and eventually broke up to 500ft wide. NNW aspect 5500ft. Photo: Mt. Baker Ski Patrol
Clear skies on Sunday, February 9th gave observers a chance to document the widespread avalanche cycle in the Stevens Pass zone that occurred February 5th-8th, including this view of crowns from large natural avalanches in the Berne Camp Chutes with Glacier Peak in the background. Photo: Matt Primomo
High Pressure before President’s Day Weekend
The week of February 10th brought the longest stretch of dry weather so far in 2020. A notable northwest wind event redistributed snow throughout the region and drove an isolated wind slab problem in most zones. Generally, it was the quietest few days avalanche-wise in weeks. However, a significant human-triggered avalanche occurred near White Pass on February 12th. Fortunately, no one was caught or injured. The incident provided a good reminder that even during periods of lower avalanche danger when avalanches are unlikely, outlier events can and do happen. The winter snowpack will always pose some level of uncertainty, and big triggers like cornice fall can produce surprising results.
The crown of a human-triggered avalanche on a northeast aspect at 6700ft in the Hogsback area near White Pass. Two travelers unintentionally triggered a cornice, which dropped onto the slope below and triggered a very large avalanche. 2/12/20 Photo: White Pass Ski Patrol
Problems
Wind Slabs
What falls out of the sky Thursday night and Friday will be the snow available for transport. Strong to extreme winds sustain during this entire period and will create increasingly dangerous conditions with elevation and as the day wears on. Look for these increasingly large slabs to build on lee slopes. Above treeline or on polar aspects, they rest on old wind-buffed surfaces while on solar aspects they may lie on a knife hard crust, making the slabs particularly reactive and widely propagating. With reactive slabs building around you, seek terrain less than 35 degrees and avoid exposure to wind-loaded or corniced slopes (which have become large to very large in many areas of the Cascades). Below treeline, winds may cross-load more open features, creating slabs you can trigger. If the storm is juicier than we expect, you might find a storm slab you can trigger if 8” of new snow sits on top of the old snow interface, potentially creating more widespread concerns.
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: All elevations.
Likelihood
Expected Size
Valid until: Feb 14th, 2020 10:00AM
