Avalanche Forecast

The main concern on Thursday and going forward should be for small areas of slab layers on northerly or lee slopes. This should be mainly northwest to southeast slopes near or above tree line, although note facet/crust combos were found on west aspects in the Olympics. Such layers may be hard to trigger but require more caution and analysis before jumping onto a lee or northerly slope.

Freezing levels should warm considerably on Thursday but persistent and moderate east winds up to crest level should locally keep conditions cooler and prevent snow surfaces on solar aspects from softening much.  Away from the Cascade crest there will be a better chance of small wet loose avalanches.  Avoid steep solar aspects or gullies in the sun if you see surface snow there deeper than a few inches or natural wet loose avalanches.

 

 

The most recent storm cycle to impact the Northwest lasted about a week and ended around January 14th. The storm produced about 2-4 feet of snow at most NWAC sites near and west of the crest with somewhat lower amounts near Hurricane Ridge. The tail end of the storm cycle 12-14 January was accompanied by a warming trend that brought rain to mid and low elevations and upside down heavy dense snow and slab layers to high elevations and caused a natural avalanche cycle.  Crowns of slab avalanches size D2 to D3 are still visible on many slopes throughout the Olympics and Cascades on lee slopes near and above tree-line.  

Nearly a week of mostly clear weather and warm temperatures occurred through January 21st.  Varying amounts of low clouds and temperature inversions kept lower elevations cooler. This warm weather allowed for additional small wet loose snow avalanches on solar aspects mainly near and above treeline. A weak front passed through Tuesday night and brought an increase in cloud cover, some very light rain or flurries, and mixed the lower atmosphere to allow upper mountain temperatures to cool and lower mountain temperatures to warm.

The warm days and cooler nights lately have been producing generally stable surface crusts on solar aspects at all elevations until mid-day softening occurs, and on most other aspects up to mid-mountain where a rain crust exists. A generally favorable mid and lower snowpack of crust layers and melt form crystals exists throughout the west slopes. The avalanche danger is low at lower elevations where there is little snow and and ample terrain and vegetation anchors. There is little snow cover on solar aspects even into the near tree-line zone near Hurricane Ridge. (see photo below). 

Recent layers of concern: On northerly or shaded aspects near and above treeline not affected by the warm temperatures or weak January sun small areas of surviving slab layers may still be found. Strong radiational cooling at night has increased temperature gradients and faceting in the upper snowpack.  NWAC observers at Hurricane and Nason Ridge (near Stevens Pass) the past couple days are reporting moderate or hard compression and extended column results with sudden collapse or sudden planar fracture characteristics. These results are found in the upper snow pack, as far down as 65 cm near Nason Ridge, releasing due to crusts and thin faceted snow layers. Due to the moderate and hard results, weak layers, would likely require larger triggers, perhaps a cornice or a snow mobile but are becoming more likely to propagate across the slope over time. The upper snowpack is becoming more spatially variable during this ongoing dry spell. If future loads buried these weak layers they would be classified as persistent slabs. Widespread reports of surface hoar growth this week will have to withstand additional sunshine, warm temperatures and easterly winds to become an issue when we finally receive more precipitation. 

ECTP29 SC Q1 at 67 cm due to crust and facet layers at Nason Ridge, NWAC observer Tom Curtis, 20 January.

Faceted crystals from the upper snow pack, Klahhane Ridge near Hurricane Ridge, NWAC observer Katy Reid, 20 January.

Quite a difference on north and south slopes in the vicinity of Hurricane Ridge,NWAC observer Katy Reid, 20 January.

Report of the cornice/avalanche fatality on Lewis Peak available at https://www.nwac.us/accidents/accident-reports/ 

Local non-avalanche hazard Snoqualmie Pass area:  On 13 January a heavy local freezing rain event occurred in the Snoqualmie Pass area above about 4500 feet, covering the surface with about a 2-3 inch ice crust.  This crust should break down over time but a fall on a slope on this surface could result in a long and dangerous ride!