Avalanche Forecast

Issued: Feb 17th, 2019 10:27AM

The alpine rating is below threshold, the treeline rating is below threshold, and the below treeline rating is below threshold.

Northwest Avalanche Center NWAC, Northwest Avalanche Center

The consequences of triggering a destructive persistent slab avalanche remain very high, but the likelihood of triggering such an avalanche is gradually decreasing. We continue to recommend navigating around complex terrain with exposure to large avalanche paths at any elevation. Stay safe by keeping your terrain selections simple.

Summary

Discussion

Snow and Avalanche Discussion

Nearly every zone in the Cascades contains a similar potentially dangerous snowpack structure. Each area has subtle differences, but they appear more alike than different. The most recent and compelling piece of information we have concerning the persistent slab is from Stevens Pass. On Saturday, two snowboarders triggered a persistent slab avalanche in Tunnel Creek outside the boundary of Stevens Pass ski area. Both were caught, carried and partially buried in the event and you can find more information here.

Human triggered persistent slab avalanche form Tunnel Creek: Photo: Stevens Pass Ski Patrol

Snowfall since February 8th is estimated to be 2-3 ft based on area Snotels and may be resting on an old weak snow/firm crust sandwich, creating ripe conditions for persistent slab avalanches. Any terrain, even low elevations, that had a shallow snowpack prior to this cycle should be treated as potential avalanche terrain (think northerly aspects with sagebrush). The likelihood of triggering a life-threatening avalanche continues to gradually decrease, but the high consequences should encourage you to choose low-angle, supported slopes or the safety of trees to avoid this problem.

If the sun warms the snow surfaces on Monday, expect loose snow avalanches and roller ball activity on steeper slopes.

Forecast Schedule and No Rating

At this time, we do not have enough specific snowpack information to issue an avalanche hazard rating for the East Slopes South zone. However, even when No Rating is applied, applicable avalanche conditions and backcountry travel advice will be provided throughout the season. When weather systems produce very dangerous avalanche conditions in adjacent zones, NWAC will issue an avalanche warning for this zone as well.

Snowpack Discussion

February 15, 2019

Since February 8th, the mountains (and low elevation cities) of the Pacific Northwest have experience cold and very storm weather. Significant snowfall has added up in all forecast zones. Records from Snoqualmie Pass DOT avalanche workers back to 1973 show that February 11-12th set a record for the most snow recorded in a 24hr period at that location. The table below shows storm totals starting February 8th through the morning of the 13th

5 day totals ending morning of Feb 13th

Water Equivilent (inches)

24hr storm totals

(inches)

Difference in Height of Snow (inches)

Hurricane Ridge

1.97

N/A

+ 30

Mt. Baker

1.94

44

 

Washington Pass

1.66

NA

+ 16

Stevens Pass

2.71

49

 

Snoqualmie Pass

3.91

80

 

Mission Ridge

1.86

38

 

Crystal

2.91

59

 

Paradise

4.55

N/A

 

White Pass

N/A

57 (4400ft)

+ 26 (5800ft)

Mt. Hood Meadows

4.70

43

 

Heavy precipitation brought many mountain regions to their tipping point. Avalanches ran readily with a peak of snowfall intensity. For Stevens Pass, Snoqualmie Pass, East Central, West South, Mt Hood, and possibly West Central zones we have good confirmation that this cycle happened from the night of February 11th through the 12th. In other zones, snow totals haven’t been significant enough for widespread avalanche cycles, or we lack data (like in the East South zone).

A natural persistent slab avalanche (D2.5) on a southeast aspect at 6,600ft. Grindstone Mtn in Icicle Canyon. Likely ran 2/12. Photo: Matt Primomo

The high rates of precipitation drove avalanches in the storm snow. Notably, a persistent weak layer of facets and surface hoar was buried in most zones on February 8th. Storms produced a widespread and prolonged cycle of avalanches on the February 8th interface, involving a variety of aspects and elevations. Local ski patrols, highway workers, and backcountry travelers reported extensive avalanching with widely propagating crowns and very sensitive conditions. With less stormy weather, observers have just begun to get a sense of the extent of the avalanche activity. Triggering persistent slab avalanches will be a concern for backcountry travelers in zones where the February 8th weak layer is active for at least the near, if not distant future. Stay tuned for more updates.

Large surface hoar near Snow Lake Divide on February 7, 2019 just before it was buried on the 8th. Photo: Jeremy Allyn

Valid until: Feb 18th, 2019 10:27AM