Dashboard Regions Weather Stations Radar Alerts Glossary
Contact About
Log In

Register for an account and never miss a forecast again!

Register

Avalanche Forecast

Archived

Jan 1st, 2019–Jan 2nd, 2019

Alpine
Natural avalanches unlikely, human triggered possible.
Treeline
Natural avalanches unlikely, human triggered possible.
Below Treeline
Natural avalanches unlikely.
Alpine
Natural avalanches possible, human triggered probable.
Treeline
Natural avalanches possible, human triggered probable.
Below Treeline
Natural avalanches possible, human triggered probable.

Regions

.

It remains possible to trigger avalanches in wind-loaded areas near and above treeline. A weak layer of buried surface hoar still lingers above 4500ft. Any avalanche triggered on this layer could break widely across terrain features.

Discussion

Snowpack and Avalanche Discussion

A widespread avalanche cycle occurred on December 29th in the West North zone. Several natural and human-triggered avalanches were reported in Heather Meadows and Schreibers Meadow, as well as in neighboring forecast zones. These avalanches failed in unusual ways on a layer of buried surface hoar. Reported avalanches had very wide propagation, shallow crowns, and some even failed mid-slope.

Fluctuating snow levels, rain up to 5000ft, and strong winds during the storm ending on December 29th created variable surface conditions. Below treeline, the snowpack is capped with a stout rain crust. Above 4500ft, snow totals increase rapidly with up to 2ft of settled dry snow in sheltered areas. Strong and gusty winds have sculpted snow surfaces and created deep drifts.

Photo: Natural avalanches observed on December 30th with impressive propagation. These occurred between 4800-5400ft on north and east aspects of Table Mountain. The flows from these avalanches entrained wet snow and converged creating deep and overlapping and debris piles.

 

Snowpack Discussion

Happy New Year!

Thanks to all of you who volunteer, send observations, and support NWAC in various ways - we appreciate it.   

December of 2018 was fun (from a forecasting perspective) with three pronounced avalanche cycles, a couple different persistent weak layers, some rain events, and a flurry of human-triggered avalanches to ring in the New Year. Most importantly, it seems that we made it through the last days of 2018 without anyone getting seriously hurt by an avalanche.

The deep (Dec 9) layer responsible for many of the avalanches early in the month no longer seems to be a problem in the western zones. That said, it is still possible to trigger an avalanche on its counterpart (or basal facets) in the eastern areas.

A widespread layer of surface hoar formed around Christmas. Late December storms preserved this layer in areas above the rain line and we have numerous (more than a dozen) reports of people triggering avalanches on it in the last three days. At least 4 people were caught and carried during this period, but so far we have no reports of serious injury. Most of these avalanches were soft slabs, D1-D2+, but there were several harder wind slabs in the mix.

It appears that the layer is most reactive and/or prevalent in the Crystal Mountain backcountry and in the mountains around Leavenworth and west of Mazama.

Surface Hoar can be an especially tricky and persistent weak layer. Read more about it here.

Problems

Wind Slabs

Wind Slab avalanches are the 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.

Persistent Slabs

Persistent Slab avalanches are the release of a cohesive layer of 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 Slab.