US: Adverse weather forecast across parts of California through at least Dec. 22

Severe weather is forecast across parts of California through at least Dec. 22. Successive weather fronts associated with an atmospheric river bringing moisture in from the Pacific will bring rainfall across the region over the coming days and snowfall over elevated areas. The heaviest rainfall is expected in parts of southwestern California from late Dec. 19 through early Dec. 22. Rainfall totals of 5-10 cm (2-4 inches) are likely across the affected area, with localized heavier amounts possible. The heavy downpours may lead to flash flooding in places, with urban areas, roads, small streams, narrow crayons or gullies, and burn scars the most vulnerable. Snowfall accumulations of more than 20 cm (8 inches) are possible over parts of the Sierra Nevada mountains through Dec. 20.

As of Dec. 19, the US National Weather Service has issued flood watches across Los Angeles, Santa Barbara, San Luis Obispo, and Ventura counties. Winter storm warnings are in place across elevated areas of central California. 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 slight risk (level 2 on a four-tier scale) of excessive rainfall for coastal and adjacent inland areas from Monterry southwards to Long Beach early Dec. 20- early Dec. 21. There is a moderate risk (level 3) of excessive rainfall for coastal and inland adjacent areas from the western Santa Ynez mountains to Long Beach Dec. 21-22, as well as a slight risk for surrounding areas from Santa Maria southward to the Mexico border.

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.

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.

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.