US: Adverse weather forecast across northwestern regions through at least early Dec. 6 /update 1

A system is forecast to bring adverse weather across the Northwest Region and into parts of the Northern Rockies and Plains and Southwest regions through at least early Dec. 6. Heavy rainfall is likely over the Pacific Northwest through early Dec. 6. As the system tracks eastwards, heavy snow will develop over parts of the northern and central Rockies through Dec. 4. 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 late Dec. 3, the US National Weather Service has issued winter storm warnings across parts of north central Colorado, central and southern Idaho, northeastern Nevada, northern Utah, and western Wyoming. Winter weather advisories are in effect across northern Idaho, western Montana, and northern and central Washington. Flood and flash flood watches and warnings are in place across most of the Pacific Northwest. Authorities will likely issue new alerts or update/rescind existing advisories as weather conditions change over the coming days.

The NWS's Weather Prediction Center has warned of a moderate risk (level 3 on a four-tier scale) of excessive rainfall for the west and southwest facing slopes of the Olympic Mountains in western Washington and a slight risk across far northwestern Oregon and the rest of western and central Washington Dec. 4-early Dec. 5. A slight risk of excessive rainfall is in place across much of the Oregon coast ranges and portions of the Washington and northern and central Oregon Cascades Dec. 5-early Dec. 6.

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 will likely 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.