The Bottom Line: Light snow and wind will drive heightened avalanche danger at the highest slopes. Watch for fresh drifts, wind stiffened snow, and slippery old snow surfaces. Rain may fall at low elevations.
Snow and Avalanche Discussion
A variety of old snow surface exist from weak facets on shaded slopes to very firm, slick crusts on sunny slopes. New snow may not stick well to some of these interfaces. Check the bond at the new/old snow interface. Watch for slipping on firm crusts especially in terrain where a fall could have consequences or where stopping a fall may be difficult.
Check out the Regional Synopsis tab for more details.
Fri 1st Feb 14:20 - Dennis D'Amico
Weather Synopsis for Friday night through Sunday
A frontal system currently passing through western Washington this afternoon will weaken and split as it passes inland. Before it weakens, it is delivering a generous shot of moisture for the Mt. Baker area and to a lesser extent the Paradise/Crystal/Chinook Pass region. Snow levels have been around 4500 ft in the north and 6000 ft over the south Washington Cascades. Cool east flow caused a wintry mix earlier this morning at Snoqualmie Pass before a switch to rain. Only a minor cooling trend is expected tonight behind the front.
Showers should quickly diminish overnight for all areas except for Mt. Baker, where more moderate rain and snow showers will linger for the first half of the night. The Pacific Northwest should be in a lull for much of Saturday with weather systems to our south and north, but not much happening in our region. The cloud forecast is tricky, but a partial clearing trend is expected for the west slopes of the Cascades and Mt. Hood in the afternoon, with a better bet for sunshine along the east slopes of the Cascades.
The lull between weather systems should continue through Sunday morning. The exception may be some light rain and snow that brushes the Mt. Hood area as it rotates up from a cut-off low pressure well to the south.
As a strong upper level ridge amplifies well offshore near 150W, a deep upper low will move south and dig off of our waters on Sunday. As the low sets up offshore, it will begin to rotate light snow showers towards the Olympics and west slopes of the Cascades during the day.
Thu 31st Jan 09:00
January, 31, 2019
As we turn the corner to February we're coming out of a week-long high pressure ridge and into unsettled weather. The snowpack survived extremely warm temperatures and sunny skies over the week with minimal new wet avalanche activity reported. This break in the weather allowed for avalanche danger to steadily decline in all regions. With stormy weather, the danger is once again elevated.
We’ve heard a variety of stories from backcountry travelers over the past week. There have been reports of extremely firm slopes creating slide-for-life conditions. Others reported perfect spring like snow. Some encountered difficult breakable crust. And, for a lucky few, softer, drier, mid-winter snow has been found. A common thread in most zones is where precipitation falls as snow, it likely will be landing on slick surfaces. It's time to pay attention to the new old interface formed by our most recent storm.
While a high elevation rain event, around January 23rd, formed surface crust in many regions, it’s the constant melt-freeze cycles from the past week, that caused a divergence in the Northerly and Southerly snowpacks.
North: On shady slopes, things haven’t exactly been soft. The crust formed at the end of the last storm extends to high elevations (Mt. Hood 7000+ft, South Cascades 6500 ft, Passes and Central Cascades 6000 ft. and West-North 5500 ft.). Only areas in the East Cascades seemed to escape the wrath of this breakable crust. Without the help of the sun, shady slopes haven’t been softening even during this period of warm weather. Instead, the surface crust underwent some weakening. Observations found faceting on top of and below this crust. In some locations, this caused the crust to begin to degrade, becoming less supportive. Surface hoar has also been reported from the typical valley bottoms and sheltered terrain near water sources. At low elevations, rain may melt any weak snow on the surface. Slopes receiving significant dry snow should be suspect for a poor bond at eh interface buried on Thursday night.
South: On sunny aspects, the sun drove warming and melting of surface snow. Long, cool, winter nights allowed for the surface to freeze again. This repeating melt-freeze pattern created a thicker, firmer, and more supportable surface. On many days, weak surface snow, such as near surface facets or surface hoar, melted during the day limiting its development. On cooler days, very firm travel conditions were reported. Crusts may provide a poor bond for any snow falling on them.
It’s not uncommon for our east-side forecast zones to experience lingering persistent weak layers (PWL’s). This season, we’ve also seen several different PWL’s in our western zones. This break in the weather gave the snowpack time to gain strength in all zones.
West: While you may find some weak snow in the upper few inches of the snowpack, the mid and lower snowpack has been found to be quite strong. Firm rounded grains, stout crust, and strong frozen melt-forms make up the majority of the snowpack at this time.
East: The east-side snowpack continues to be highly variable. You may find deep strong snowpacks closer to the crest or you could encounter shallow weak snowpacks areas further east. While there are number of potentially weak interfaces, there are two more common layers we’ve got our eyes on.
- January 22nd surface hoar and small facets. You can find these just under the recent storm snow, about a foot down. .
- December 26th surface hoar. This layer can be found from 16” to 40” down and is still producing clean, planar shears with tests.
You are most likely to find these layers to be preserved on wind sheltered, shady, and open slopes above 5,500ft. You can find more defined weak layers where snowpack is less than 4 feet deep and variable especially east of Highway 97. Persistent weak layers have been “dormant” or unreactive during the week of high pressure. They may or may not become reactive with the next storm. As new snow accumulates, it will begin stressing these weak layers. We’ll keep tracking them to watch their progression..
The latest round of snow and wind will be just enough to raise the avalanche danger above treeline. Watch for shallow drifts on leeward sides of ridges. While any avalanches will be small, they may be easy to trigger on the old snow surface. Use small test slopes and quick tests, like a shovel tilt test, to check how the new snow is bonding to old surfaces.
Aspects: All aspects.