US: Severe weather forecast across northwestern and western regions through at least early Dec. 18

Severe weather is forecast across parts of the northwestern and western US through at least early Dec. 18. Successive weather systems associated with an Atmospheric River are expected to bring heavy rainfall to coastal areas and heavy mountain snowfall to inland areas of the Pacific Northwest, northern California and into the Great Basin and northern Rockies. Snowfall accumulations of 10-20 cm (4-8 inches) are expected in elevated parts of the region. Flooding is possible in areas where sustained heavy rainfall occurs and hazardous travel conditions are likely in areas where more wintry precipitation falls due to slippery roads and reduced visibility caused by blowing snow. Strong winds may cause property damage and power outages.

As of early Dec. 16, the US National Weather Service (NWS) has issued winter storm warnings for parts of central Oregon and western Wyoming and winter weather advisories across inland areas in much of the rest of the affected area. Coastal flood advisories and other hydrological warnings are in place for parts of western Washington and around San Fransico Bay in California. Wind advisories are in place for parts of southern Oregon and northern California and gale warnings and small craft advisories have been issued across affected coastal areas. The NWS's Weather Prediction Center has forecast a slight risk of excessive rainfall (the second lowest level on a four-tier scale) across coastal areas of northern California north of San Francisco Dec. 16. Officials could update and extend the coverage of weather alerts over the coming hours and days.

The storm systems could produce further rounds of heavy precipitation, including rain and snow, and strong winds across 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. Precipitation could fall as snow in elevated parts of the affected area over the coming days. Wind gusts could cause blowing and drifting snow; decreased visibility is likely in these areas.

Floodwaters and related debris 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 mountainous areas; authorities could implement temporary road closures or detours in such locations.

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.

Disruptions triggered by inclement weather and resultant hazards, such as flooding or strong winds, could persist well after conditions have improved - it could take days before any floodwaters recede and/or officials clear debris. If there is severe damage to infrastructure, repair or reconstruction efforts may result in residual disruptions.