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BACKCOUNTRY AVALANCHE FORECAST

WEST SLOPES SOUTH - SOUTH OF I-90 TO COLUMBIA RIVER

ISSUED
Tuesday, February 18, 2020 - 6:00PM Tue, Feb 18, 2020 - 6:00PM
AUTHOR
Dallas Glass
THE BOTTOM LINE

Another warm sunny day in the West South zone could produce a few natural loose wet avalanches. Avalanche danger should be lowest in the morning and increase as the sun and warming temperatures have time to weaken the snow surface. Steer away from steep sunny slopes when you see new rollerballs or fresh fan-shaped debris

AVALANCHE DANGERi

Wednesday, February 19, 2020
Above Treeline Moderate (2)
Near Treeline Moderate (2)
Below Treeline Low (1)

OUTLOOKi

Thursday, February 20, 2020
Moderate (2)
Moderate (2)
Low (1)

OUTLOOKi

Thursday, February 20, 2020
2 2 1
Danger Scalei
  • Low (1)
  • Moderate (2)
  • Considerable (3)
  • High (4)
  • Extreme (5)

AVALANCHE PROBLEM #1i

  • Loose Wet

    Avalanche Problemi
  • Above Treeline
    Near Treeline
    Below Treeline
    Aspect/Elevationi
  • Unlikely
    Possible
    Likely
    Very Likely
    Almost Certain
    Likelihoodi
  • Small (D1)
    Large (D2)
    Very Large (D3)
    Historic (D4-5)
    Sizei
Photo for Loose Wet

Natural loose wet avalanche originating from a steep rocky slope in the Crystal backcountry. (2/18/2020)

Another round of sunny weather with warming temperatures could drive a few more natural loose wet avalanches Wednesday. The sun will need to overcome a solid overnight freeze, cold morning temperatures, and a chilly east breeze to impact the snow. As a result, you’ll be most likely to trigger avalanches in the afternoon and on sunny slopes protected from the cooling wind. Look for clues such as snow shedding from rocks and fresh rollerballs to key you into changing surface conditions. When you suspect the snow surface is becoming wet, seek out shaded terrain. Even though we expect these avalanches to be generally small, long sustained slopes could produce larger avalanches. 

Several days of sunshine created a more aspect dependent snowpack. A new surface curst formed on most slopes receiving sunshine, while shaded aspects remained soft and dry. If you travel to higher elevation shaded slopes, carefully evaluate steep lines for signs of wind drifted snow. These high northerly aspects are where you’re most likely to encounter an isolate ingering wind slab. Over the holiday weekend, several natural and human triggered slab avalanches ran in wind loaded pockets over the slick prominent crust from 2/13. (N-E aspects, 6000-6500’)

 

 

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

No Corresponding Mountain Weather Forecast Available