Avalanche Forecast
Regions: Olympics.
You can still trigger on a highly destructive slab avalanche that releases down to a snow/old snow interface. While the likelihood of triggering such an avalanche is gradually decreasing, the consequences are high. Stay safe navigating around terrain capable of producing a large avalanche, such as steep, open, or unsupported slopes.
Discussion
Snow and Avalanche Discussion
A small slab avalanche was triggered by patrol in-bounds at the Hurricane Ridge ski hill on Sunday on a convex slope. While our primary concern is the persistent slab problem, you might still trigger a smaller avalanche on subtle interfaces within the upper snowpack.
NWAC professionals traveled in the Hurricane Ridge area Friday. Crowns were still visible from the widespread avalanche cycle from earlier in the week. Most avalanches were in the large range (D1.5-2.5). It was impressive how many avalanches released at low elevations below treeline. Check out our Instagram feed for some photos from Friday.
Matt S. and Robert H. found weak snow (a mix of facets and buried surface hoar) above a firm crust on both south and north aspects below treeline with about 1.5-3 feet of settled storm snow on top. This means you need to consider the potential for large and deadly persistent slab avalanches across the entire compass. One skier triggered avalanche occurred in a chute on a NE aspect, but avalanche details are sparse.
4.5†of new snow fell Saturday afternoon through Sunday morning and now sits on top of the firmer slab.
There are two other hazards to consider Monday.
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When the sun comes out, you may see rollerballs and small loose avalanches from steep, rocky slopes. Don’t let these easy to predict avalanches catch you off guard.
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The snow out there is deep, especially around tree wells. Be careful when traveling near small trees. Keep your partners in sight, and maintain voice contact.
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 Equivalent (inches)
24hr storm totals
(inches)
Difference in Height of Snow (inches)
Hurricane Ridge
1.97
N/A
+ 30
Mt. Baker
1.94
44
Â
Washington Pass
1.66
NA
+ 16
Stevens Pass
2.71
49
Â
Snoqualmie Pass
3.91
80
Â
Mission Ridge
1.86
38
Â
Crystal
2.91
59
Â
Paradise
4.55
N/A
Â
White Pass
N/A
57 (4400ft)
+ 26 (5800ft)
Mt. Hood Meadows
4.70
43
Â
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
Avalanche Problems
Persistent Slabs
A thick and firm slab (3 ft) of storm snow from the past week sits on a 3-5 mm slightly rounding facet layer over 5 cm supportable crust. The most recent reports of loud whumphing were from Hurricane Hill on Saturday. The lack of reported whumphs suggests that the slab/weak layer combination is beginning to gain strength. You are less likely to experience the obvious indicators of instability such as whumpfs, sudden collapses in the snowpack, or shooting cracks in the snow on Monday. However, there are broad similarities on a regional scale with the current slab and weak layer characteristics. A large persistent slab avalanche caught two snowboarders in the Stevens Pass area on Saturday and that should give you pause before venturing onto larger slopes throughout the region.
Persistent slabs can break widely across terrain features, come down on top of you, and can be triggered from a long distance away. These avalanches are difficult to predict and can break wide and in surprising ways by wrapping around terrain features. Ask yourself, if a very large avalanche released from the slope above, would I be in the runout zone? If the answer is yes or if you can’t answer the question, then choose much simpler terrain that’s much lower angled and not connected to anything capable of producing a very large avalanche.
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: Possible
Expected Size: 1 - 2