Avalanche Forecast

Issued: Apr 26th, 2017 11:00AM

The alpine rating is below threshold, the treeline rating is below threshold, and the below treeline rating is below threshold.

Northwest Avalanche Center NWAC, Northwest Avalanche Center

New snow can be very touchy and conditions can change very rapidly after spring storms due to increasing spring sun or solar effects and warmer daytime temperatures. Dangerous avalanche conditions are expected in avalanche terrain near and above treeline on Thursday. Careful snowpack evaluation and cautious routefinding will be essential along the Cascade west slopes especially where there has been heavier snowfall.

Summary

Detailed Forecast

Cool northwest flow, orographic snow showers and convergence zone effects are expected on Thursday.

New snow can be very touchy after spring storms due to increasing spring sun or solar effects and warmer daytime temperatures.

Loose wet avalanches near and above treeline are likely due to the new snow and sun or solar effects. Increasing spring sun or solar effects can rapidly activate loose wet snow and result in a dramatic increase in avalanche danger.

New or recent wind slab is likely near and above treeline on lee aspects. NWAC stations along the Cascade crest are having moderate to very strong west winds on Wednesday. So wind slab is most likely on NW to SE aspects but watch for it on other aspects in areas of varied terrain.

New or recent storm slab is likely in areas near and above treeline that had rapidly accumulating new snow.

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.

Snowpack Discussion

The active late season pattern continues in the Pacific Northwest!

Last week from Monday 4/17 to Thursday 4/20 the NWAC weather station at Mt. Baker picked up about 3 inches of water equivalent (WE) while other NWAC stations along the Cascade west slopespicked up 1 - 2 inches of WE. Much of this WE fell as snow above 5000 feet in the north, 5500 feet in the central and 6000 feet in the south Washington Cascades.

A short break was seen last Friday.

The current storm cycle began on Saturday. For the 4 days ending Wednesday morning NWAC stations along the Cascade west slopes have had .8-1.9 WE. Much of this WE has also fallen as snow above about 5000 feet in the north, 5500 feet in the central and 6000 feet in the south Washington Cascades.

Note that this will be a quite a bit of new snow for this time of year!

Recent observations

NWAC observer Lee Lazzara was out on Friday 4/21 in the Baker backcountry on Ptarmigan Ridge. Lee observed numerous loose wet slides on all aspects near and below treeline. A natural 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 falls were noted. Note the massive cornices in the photo! 

Photo by Lee Lazzara, 4-21-17.

The Chinook Pass DOT crew on Monday reported very touchy size 1-2 loose wet avalanches on all slopes steeper than about 30 degrees by ski tests that were largest below 5500 feet.

Further triggered touchy loose wet avalanches were seen on Tuesday by the Chinook DOT crew on steep solar slopes that start small and were confined to gullies but became large by the time they reached the road elevation.

Valid until: Apr 27th, 2017 11:00AM