The Bottom Line: A potent storm will begin early on Tuesday. Dangerous avalanche conditions may develop by the afternoon at mid and upper elevations. New and wind-drifted snow may not bond well to old snow surfaces. Monitor changing conditions throughout the day.
Snow and Avalanche Discussion
An incoming storm will bring heavy snowfall and a warming trend over the next 48 hours. Snow levels will start off near 2000ft. Expect increasing avalanche danger throughout the day Tuesday as snow accumulates. The majority of precipitation will come Tuesday night as snow levels rise to 4500ft.
Avalanche activity was minimal over the past 3 days in neighboring zones with generally few signs of unstable snow. Overall, the existing snowpack is strong. Areas near and above treeline have 80-100in of snow on the ground. Significantly less snow exists below treeline due to rain and high snow levels during the majority of storms so far this season.
Tue 22nd Jan 14:49 - Robert Hahn
Weather Synopsis for Tuesday night through Thursday
A potent warm front will bring heavy rain and snow to many areas as snow levels rise to just below Cascade Crest level in many areas. The one cold air holdout will be the East-North Cascades near Washington Pass. Through the evening hours, a battle will play out between the cool easterly flow and the warm air spreading in aloft. Snow levels will rise significantly during the day in some areas west of the Cascades such as Hurricane Ridge which is likely to experience a changeover to rain during the day, but most NWAC stations will remain snow because it will take time to wash out the low-level cold air due to that easterly flow. Many low and mid-elevation NWAC stations will experience significant snow totals before a changeover to rain overnight as the warm air ultimately wins the battle once the warm front moves through during the evening hours. The firehose will be aimed at the north-central Cascades Tuesday evening with ample precipitation spilling over to the east slopes as the low slides along the coast of Vancouver Island. At this time, the warm sector of the storm provides a "relative" dry window south of Snoqualmie Pass. The cold frontal passage will bring the heavier precipitation back to all areas after 10 PM.
On Wednesday, the surface low passes over central Washington State during the mid-day hours, creating a moderate convergence off the Olympic Mountains in its wake as northerly flow is ushered down behind the low. Northerly flow will also decrease and end shower activity in the North Cascades. Elsewhere, light to moderate rain and snow showers that will decrease throughout the day and evening hours across the Cascades, with the heaviest rain and snow shower activity Wednesday near Mt. Hood. Snow levels will decrease gradually during the day, but they will remain above Stevens Pass level.
Wednesday night and Thursday a mild warm northwesterly flow and generally fair weather will predominate with a ridge parked offshore.
Sun 20th Jan 09:00
January 20, 2019
The recent weather pattern of lower accumulation storms (by NW standards) and longer stretches of calm weather should continue as we move into late January. Since January 17th, incremental snow accumulations punctuated with rising freezing levels favored the south and eastern parts of the region. Storm instabilities have risen with storms and gradually subsided.
New Snow Problems
New Snow Problems
Storms over the past week have brought a range of layers from rain crusts, to heavy moist snow, to stiff drifts, to light dry powder. Some storm days, like the 18-19th, saw reactive, but very short-lived avalanches caused by heavy precipitation and wind. Even the longer-lasting avalanche problems, wind slabs, haven't persisted for more than a few days. Where the recent snow is stressing underlying weak layers, more dangerous avalanche conditions have prevailed.
Old Snow Problems
Old Snow Problems
Persistent weak layers (PWLs) have been a constant in the eastern zones of the Cascades this winter. As usual, they have been much less problematic at the Passes and west of the Cascade Crest. The latest PWL is a layer of surface hoar, buried around January 17th and found generally east of the Cascade Crest. Buried surface hoar is an active weak layer in the eastern zones and can be found to a limited extent on the eastern edge of the Stevens and Snoqualmie Pass zones. There few, if any, avalanches have been reported on the buried surface hoar. It may be most problematic in open, wind-sheltered terrain, especially well above the valley floor.
You are most likely to find other layers of old weak snow the further you move east from the Cascade crest. Here snowpacks are shallower, more variable, and generally weaker. In some locations, weak snow near the ground can still be found. These basal facets have hung around all season. Digging profiles and using snowpack tests is the best way to gain information about these old persistent weak layers. However, snowpack tests are just one piece of the puzzle. Your terrain decisions shouldn't hinge on any given test result. Because of the size of our forecast zones and the variability in the snowpack, it's important to make snow observations as you travel. We’ll keep watching these old layers, but let us know what you see while you are in the mountains.
New snow and wind will form fresh drifts primarily at mid and upper elevations. Conditions could change rapidly throughout the day Tuesday. Touchy wind slabs may develop in leeward terrain late in the day. Monitor snow accumulation and rate. Watch for blowing snow and cornice formation. If you see 6in or more of new snow, dial back your terrain selection to lower angled and supported slopes. Hollow sounding snow, textured snow surfaces, and smoothly rounded pillows are good indications wind slabs are present. Avoid wind-loaded convex slopes 35 degrees and steeper.
Aspects: All aspects.
Elevations: All elevations.