US: Adverse weather forecast across northwestern regions through at least Dec. 3

An atmospheric river is forecast to bring a mixture of heavy snowfall and rainfall across the Pacific Northwest through at least Dec. 3. Snowfall accumulations of more than 30 cm (12 inches) are expected in the higher mountain ranges and rainfall totals of 10-20 cm (4-8 inches) are possible in coastal areas. Strong winds may cause blowing snow and hazardous travel conditions are likely, especially in elevated areas, due to reduced visibility and slippery roads. At lower elevations, heavy downpours may trigger flooding in low-lying areas.

As of Nov. 30, the US National Weather Service has issued winter storm warnings across parts of western and southeastern Washington, western and northeastern Oregon, and northern Nevada. Winter storm watches have been issued for parts of southern Oregon, central, northern, and southeastern Idaho, and far western Montana. Winter weather advisories are in effect across most other elevated parts of the affected region. Authorities will likely issue new alerts or update/rescind existing advisories as weather conditions change over the coming days.

Where precipitation falls as snow, strong wind gusts could lead to periods of blowing and drifting snow; decreased visibility is likely in mountainous areas. Sporadic power outages are likely throughout the affected area. Where precipitation falls as rain, flash and areal flooding is possible. Such flooding is possible in low-lying communities near watercourses and other large bodies of water, as well as in urban areas with easily overwhelmed stormwater drainage systems. Sites located downstream of large reservoirs may be subject to flash flooding after relatively short periods of intense rainfall.

Floodwaters and snowfall accumulations may render some bridges, rail networks, or roadways impassable, impacting overland travel in and around the affected area. Flooding in urban areas could also result in significant traffic congestion. Heavy snow will likely make driving hazardous in some areas; authorities could implement temporary road closures or detours in such locations. Mountain passes and tunnels could be closed as a precautionary measure during periods of intense snowfall.

The disruptive weather will likely cause some delays and cancellations at airports in the region. Flooding or snow could block regional rail lines; freight and passenger train delays and cancellations are possible in areas that see heavy rainfall and potential track blockages.