Avalanche Forecast
Issued: Feb 14th, 2019 11:21AM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Persistent Slabs and Wind Slabs.
NWAC,
Northwest Avalanche Center
We're on the tail end of a major avalanche cycle resulting in many large and wide-breaking avalanches. Less stormy weather on Friday will bring a decrease in the danger rating, but the serious risk of triggering dangerous avalanches remains. If you venture in the backcountry be certain of your ability to avoid slopes steeper than 30 degrees and put plenty of space between yourself and large avalanche paths.
Summary
Discussion
Observers have reported a widespread and prolonged cycle of avalanches involving a variety of aspects and elevations. Common characteristics of these avalanches are very widely propagating crowns about 3 feet deep. Local ski patrols, highway workers, and backcountry travelers all reported extensive avalanching on the night of the 11th, into the 12th, with triggered slides and a few naturals continuing into the 13th. While some avalanches ran in storm layers in the upper snowpack, many appear to be running an old weak faceted layer over 3 feet below the surface. Visibility and dangerous conditions have limited our ability to gather more detailed information.
Mission Ridge has received over 40 inches of snow since February 8th. Low elevation terrain, including slopes close to towns, are now locations of concern for avalanches. On Tuesday, forecaster Matt Primomo reported natural avalanches and found a very weak and avalanche prone snowpack on the Rooster Comb, on the outskirts of South Wenatchee. Expect similar conditions near the towns of Peshastin and Leavenworth. In addition to avalanches, all the deep snow is building concern for Snow Immersion Suffocation, or tree well hazards higher in the mountains. Travel with partners and keep them in sight.
A natural avalanche in the Icicle Canyon. Grindstone Mtn, SE, 6,600ft. Photo: Matt Primomo
Snowpack Discussion
Since February 8th, the mountains (and low elevation cities) of the Pacific Northwest have experienced cold and very stormy 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
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).
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.
Problems
Persistent Slabs
If you venture into the backcountry, you must be confident in your ability to put enough distance between you and slopes that could produce life-threatening avalanches. Stay far out from under big avalanche paths and make sure not to get too close to start zones. 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 break in ways much wider and in more surprising ways than avalanches that we typically see in the Northwest.
The snowpack has seen a rapid and major change. Give the snow some time to adjust. Continue to stick to slopes under 30 degrees and smaller terrain features. Observers have consistently reported; 1) Remotely triggered avalanches. 2) Widespread collapsing of the snow underfoot or a "whumphing" sound. 3) A layer of weak, sugary facets and surface hoar resting on a crust 4-5 feet below the snow surface. Watch for these signs indicating that you can trigger a Persistent Slab 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
Expected Size
Wind Slabs
On Thursday, easterly wind drifted snow at upper elevations, formed fresh slabs, and further stressed an already tenuous snowpack. Avoid any slope over 30 degrees where you see recently drifted snow. Today, you can't simply avoid or manage the storm instabilities. Any avalanche triggered in the recent snow could result in a larger and more deadly persistent slab avalanche. In sheltered areas you could still trigger large storm slab avalanches in the snow that fell in the past week. The upper layers may slide easily.
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: Alpine, Treeline.
Likelihood
Expected Size
Valid until: Feb 15th, 2019 11:21AM
