Mountain Weather Forecast

Weather Synopsis

With this Christmas and Boxing Day day storm Santa delivers a serious punch. This storm is likely to go down as one of the most significant of the year in terms of snow accumulations and contends as a significant mountain wind event.

The system has plenty of moisture to work with as revealed by a bload plume of moisture extending 750 miles to the WSW. As of Christmas afternoon, moderate to heavy snow is spreading throughout the region ahead of a warm front advancing into the area.

The warm front will bring modest warming Wednesday night along with the heaviest sustained precipitation. Expect 1-3" per hour sustained snowfall rates for the west slopes of the Cascades above the 2500-3500 rain/snow line. East flow enables plenty of spillover E of the Cascade Crest where 0.5-2" per hour snowfall rates are expected. Moderate to strong winds will generally peak in the overnight hours as the storm's first wave moves over the region, with the strongest winds likely around Mt. Hood.

On Boxing Day, models continue to indicate the passage of a secondary low across the Straight of Juan Da Fuca into the N Cascades from the morning into the afternoon hours. This will bring very intense W winds wrapping around the south side of the low as a bent-back occlusion ("poisonous tail"). Models are indicating sustained winds of 30-50 mph for areas favored by W winds, so you might expect to find some areas gusting above Hurricane force. The winds will be accompanied by decreasing moderate to heavy snow tapering to showers by the afternoon. Embedded convergence should focus on the southern mountain loop area in the wake of the low center.

Expect snow levels to rise to around 4000 ft west of the Cascade Crest by 4 AM Thursday, but Snoqualmie Pass could remain snow until 10 AM at which point low-level flow should switch W and snow levels rise to around 3500 ft.

Expect mostly light rain and snow showers to linger Thursday evening as steadier rain and snow move into the southern Cascades through the overnight hours associated with the next system. This system won't be as intense, but will still deliver moderate to locally heavy snowfall. Precipitation spreads northward as the package lifts NE during the day Friday. It remains unclear how much moisture will reach as far north as the Canadian border.