Avalanche Forecast

A weak surface low pressure system will move east along the Washington - Oregon border on Monday morning. An upper shortwave trough will follow and move to the east over the Northwest on Monday afternoon. Further new snow is expected above about 5000 feet on Monday, mainly on the volcanoes from Mt Rainer to Mt Hood. Heavy snow above about 5000 feet is expected is expected at Mt Hood.

At this time of year given significant new snow and spring sun effects, you have to expect serious spring avalanche problems in areas with significant new snow if you venture into the backcountry.

At Mt Hood very dangerous avalanche conditions are expected on Monday. Travel in avalanche terrain is not recommended at Mt Hood on Monday.

On the volcanoes of the south Washington Cascades from Mt Rainier south dangerous avalanche conditions are expected on Monday. You will need to make careful snowpack evaluations and use cautious routefinding in these areas.

In other areas continue to evaluate snow and terrain carefully and identify features of concern on Monday.

New storm slab will be seen in areas with new rapidly accumulating new snow. This is most likely on the south volcanoes and especially at Mt Hood.

Above treeline areas of new wind slabs may develop on lee aspects in areas with significant new snow. This is also most likely on the south volcanoes and especially at Mt Hood.

Loose wet avalanches are most likely in areas with significant new snow and sun or solar effects.

Cornices are still large, so give them a wide safety margin. Natural cornice releases and resulting slab avalanches are dangerous and unpredictable. Give cornices a wide berth if traveling along ridge-lines and avoid slopes below large cornices. See a blog post regarding cornices here.

Avoid unsupported slopes with overhanging blocks of snow and smooth rock underneath. Glide avalanches are difficult to predict and can release at any time, not just during the heat of the day.

The active weather pattern continues into late April in the Pacific Northwest.

Last week from Monday 4/17 to Thursday 4/20 Mt. Baker picked up about 3 inches of water equivalent (WE) while other areas along the west slopes picked up 1 - 1.5 inches of WE. Much or all of this fell as snow above 5000 feet in the north and 5500-6000 feet in the central and south Washington Cascades. Natural loose wet avalanches likely occurred throughout this period as the snow-line oscillated or on solar aspects at lower elevations during sunbreaks. 

Friday was a warm day with most NWAC stations in the Olympics and Cascades reaching into the 40s and 50s. Breezy E-SE winds were seen in the Passes and in the southwest Cascades including Mt. Rainier and Crystal.

A renewed active weather pattern Saturday brought periods of light rain and snow showers through the day, with a slow cooling trend. These were mainly rain showers below about 5500 feet.   

It continued to snow on Sunday and Sunday night mainly on the volcanoes including at Mt Hood. Most areas near and above treeline on the volcanoes will have about 3-6 inches of new snow on Monday morning.

Recent observations

The Chinook Pass DOT crew on Thursday reported loose wet avalanches becoming increasingly sensitive during sunbreaks below 5000 feet with ski cuts and explosives, with larger slides gouging down to older wet snow. A 12-16 inch wind slab was triggered below the ridge crest with explosives.

NWAC observer Lee Lazzara was out on Friday in the Baker backcountry on Ptarmigan Ridge. Lee observed numerous loose wet slides on all aspects near and below treeline. Anatural cornice fall along Ptarmigan Ridge ripped out a D2.5 wind slab on a north aspect (see photo below, thanks Lee!), and other natural cornice fall was noted as well. Note the massive cornices in the photo! 

Photo by Lee Lazzara, 4-21-17.

Laura Green checked in from the Mt. Hood Meadows pro-patrol Friday. During morning avalanche control work with explosives, stubborn wind slabs were not widespread, but propagating well on lee NE - ESE aspects between 6600 and 7800 feet. Crowns were up to 2 feet deep. Recent and natural wind slab avalanches had occurred in White River Canyon on similar aspects and elevations.