Prolonged winter storm to batter Pacific Northwest with rain and heavy snow

gfs apr mslp 12z december 2 2023

The Pacific Northwest is bracing for a prolonged winter storm that is forecasted to last through the weekend, threatening the region with a mix of heavy rain in the coastal ranges and lower Cascades and significant snowfall in the mountain passes. This event corresponds with the CW3E’s forecast of several atmospheric rivers, set to affect the area from Saturday, December 2, 2023, through the following week.

Forecasts from both the CW3E and the National Weather Service converge in predicting a series of atmospheric rivers, with the first expected to make landfall on Saturday, December 2. These atmospheric rivers, large bands of moisture originating from the Northeast Pacific, are projected to bring intense precipitation, including heavy rain and snow, to the area.

According to the CW3E’s forecast on November 28, the Pacific Northwest and parts of Northern California will face multiple atmospheric rivers. These systems, spinning out of Asia and across the Pacific Ocean, could bring significant precipitation ranging from heavy rain in northwest California to southern British Columbia, potentially leading to rising river levels.

In northern Washington, freezing levels may initially drop to around 610 m (2 000 feet), and in central California to approximately 1 500 m (5 000 feet) during the initial atmospheric river, before rising with subsequent systems. This fluctuation in freezing levels indicates the possibility of substantial snow accumulation in the mountains.

“The second and third ARs will approach the Pacific Northwest late in the weekend, with landfall
possible early next week. Notably, much of the forecast data at this time suggests the two stronger
ARs will merge prior to landfall,” CW3E forecasters said.

The National Weather Service’s short-range forecast, issued on December 1, further elaborates on the expected conditions. The meteorological winter is set to begin with abundant moisture streaming from the Pacific Ocean, predominantly affecting the Pacific Northwest. The region is forecasted to receive total rainfall amounts of 125 to 250 mm (5 to 10 inches) along the coastal ranges of western Washington and Oregon. Cities like Portland and Seattle are also expected to see several inches of rain.

The Cascades are anticipated to receive heavy snowfall, with accumulations likely reaching several feet, particularly in the mountain passes. This heavy snow, coupled with blowing snow, could make travel extremely difficult, if not hazardous.

Moreover, the combination of heavy rain and subsequent snowmelt on Sunday, December 3, is expected to lead to minor to moderate river flooding. Snow levels in the Cascades are predicted to rise over the weekend, exacerbating the situation with rain falling on freshly fallen snow.

This pattern of heavy precipitation is not limited to the Pacific Northwest; the Great Basin and Rockies are also forecasted to experience heavy snow, extending to mountain ranges from Idaho and Montana down to Colorado. The highest peaks in these regions could see snowfall reaching up to 0.3 to 0.9 m (1 to 3 feet). In response, Winter Storm Warnings and Winter Weather Advisories have been issued for areas most likely to experience impactful and hazardous snowfall.

References:

1 CW3E Atmospheric River Update Outlook – November 28, 2023

2 Short Range Forecast Discussion – NWS Weather Prediction Center College Park MD – 247 AM EST Fri Dec 01 2023

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