Avalanche Forecast

Issued: Feb 16th, 2019 10:03AM

The alpine rating is considerable, the treeline rating is considerable, and the below treeline rating is considerable. Known problems include Persistent Slabs.

Northwest Avalanche Center NWAC, Northwest Avalanche Center

A poor snowpack structure can be found in the area, even below treeline. If you venture into the mountains, avoid slopes steeper than 30 degrees, stick to well supported slopes, and put plenty of space between yourself and steep slopes. 

Summary

Discussion

Snow and Avalanche Discussion:

Recent avalanches have had very widely propagating crowns 2 to 3 feet deep. They've been running on a layer of faceted snow over a stout crust, or underneath this crust where it is thin and decomposed. Limited information has come in from near and above treeline. Mild weather with some sun has created a melt freeze crust on solar aspects over the past few days. The warm weather is helping to heal instabilities at the very low elevations and particularly on southerly aspects. The snowpack is staying cold in the shade. This is where best snow conditions can be found, but it is also where the structure is most concerning.  

Observers on shaded aspects of Tumwater Mountain on Saturday reported a number of small collapses, and one very large one. Observers on Blewett Pass are finding very weak facets underneath a decomposed crust on northerly aspects above 4,000ft. On Friday, reports of numerous tree shaking collapses came in from north aspects in Squilchuck State Park. Observers in the Salmon la Sac area on Friday found 3ft of snow over the facet/crust layer of concern at 4,800ft, but few signs of instability. A concerning snowpack structure can be found in Icicle Creek on the north half of the compass, where recent storm snow rests on a few inches of facets over a freezing rain crust. Here, roughly 100 recent slab avalanches were observed on the 13th, some were over 1,000ft wide. Low elevation, shaded slopes near towns and cities are still locations of concern for avalanches. A very weak basal structure with large faceted grains near the ground can be expected on shaded aspects in the Wenatchee foothills. Roofs may avalanche on Sunday if they haven’t already. Don't linger under roofs.

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

Problems

Persistent Slabs

An icon showing Persistent Slabs

It is not worth trying to outsmart this problem. A problematic snowpack structure can be found throughout the zone. 2 to 3 feet of recent snow rests on facets over a crust, a weak layer that makes me cringe.  You won't find me on any steep slopes this weekend. If you decide to go into avalanche terrain this weekend, be sure of your ability to avoid slopes steep enough to avalanche. Measure your slope angles. This is not the time to go into big, complex terrain, and don't go near steep slopes that may be connected to where you are. These persistent slabs have been breaking incredibly widely across terrain features. The reactivity is decreasing with the calm weather, though that doesn't mean they are healed up yet. It appears the worst structure can be found in the East Central zone than anywhere else in the state. 

Professional guides and avalanche workers are continuing to avoid steep slopes, and sticking to well supported features. Even small, steep slopes below treeline that I rarely think much about may be exactly the type of place to avoid this weekend. Why? Observations of:

1. Lots of recent avalanches with crowns well over 500ft wide.

2. Continued widespread collapsing, which makes a "whumphing" sound as air escapes from the collapsed weak layer, sometimes shaking trees a long distance away. 

3. A layer of weak, sugary facets on, or beneath a crust 2 to 3 feet below the snow surface. 

Release of a cohesive layer of soft to hard snow (a slab) in the middle to upper snowpack, when the bond to an underlying persistent weak layer breaks. Persistent layers include: surface hoar, depth hoar, near-surface facets, or faceted snow. Persistent weak layers can continue to produce avalanches for days, weeks or even months, making them especially dangerous and tricky. As additional snow and wind events build a thicker slab on top of the persistent weak layer, this avalanche problem may develop into a Deep Persistent Slabs.

 

The best ways to manage the risk from Persistent Slabs is to make conservative terrain choices. They can be triggered by light loads and weeks after the last storm. The slabs often propagate in surprising and unpredictable ways. This makes this problem difficult to predict and manage and requires a wide safety buffer to handle the uncertainty.

 

This Persistent Slab was triggered remotely, failed on a layer of faceted snow in the middle of the snowpack, and crossed several terrain features.

Persistent slabs can be triggered by light loads and weeks after the last storm. You can trigger them remotely and they often propagate across and beyond terrain features that would otherwise confine wind and storm slabs. Give yourself a wide safety buffer to handle the uncertainty.

Aspects: All aspects.

Elevations: All elevations.

Likelihood

Likely

Expected Size

1 - 2

Valid until: Feb 17th, 2019 10:03AM