US: Adverse weather forecast across parts of California through at least Jan. 6

Event

A series of weather fronts are forecast to draw in a plume of moisture over parts of the west coast over the coming days and will lead to excessive rainfall and heavy mountain snow across parts of California Jan. 4-6. Northern and central coastal areas will likely see rainfall totals of 2.5-10 cm (1-4 inches) late Jan. 4-late Jan. 5, with isolated higher totals, and snowfall accumulations of up to 90 cm (3 feet) are possible in mountainous inland areas. Scattered flash flooding is possible, especially near recent burn scars and in low-lying areas where river levels remain high following previous heavy downpours. Heavy rainfall across parts of northern and central California Jan. 1-2 caused two fatalities and led to evacuations, landslides, and widespread power outages. Hazardous travel conditions are likely across the affected area, especially over mountain passes due to snowfall accumulations.

As of early Jan. 3, the National Weather Service (NWS) has issued flood watches for parts of western and central California and winter storm warnings for parts of eastern and north-central California. The NWS's Weather Prediction Center (WPC) has warned of a "Moderate Risk" (Level 3 on a four-tier scale) of excessive rainfall over parts of the northern and central coasts of California Jan. 4-5, as well as a "Slight" and "Marginal" risk (Levels 2 and 1) for surrounding areas of western and central California. The "Slight" and "Marginal" risk continues over much of coastal California as well as inland central and north-central regions Jan. 5-6. Authorities will likely update and possibly extend the coverage of weather alerts over the coming days as the storm progresses and weather conditions change.

Sustained heavy rainfall could trigger flooding in low-lying communities near rivers, streams, and creeks. Urban flooding is also possible in developed areas with easily overwhelmed stormwater drainage systems. Sites located downstream from large reservoirs or rivers may be subject to flash flooding after relatively short periods of intense rainfall. Landslides are possible in hilly or mountainous areas, especially where heavy rainfall has saturated the soil. Authorities could issue mandatory evacuation orders for flood-prone communities over the coming days. Disruptions to electricity and telecommunications services are possible where severe weather impacts utility networks.

Precipitation will likely fall as snow in the higher elevations over the coming days. Wind gusts could cause blowing and drifting snow; decreased visibility is likely in mountainous areas. Rain-induced landslides cannot be discounted in areas of elevated terrain; there is also the possibility of avalanches in mountainous areas where the snowpack has become unstable due to heavy snowfall.

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 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.

Severe weather will also likely trigger flight delays and cancellations at airports across the affected region. Flooding could block regional rail lines; freight and passenger train delays and cancellations are likely in areas that see heavy rainfall and potential track inundation.

Disruptions triggered by inclement weather and resultant hazards, such as flooding or avalanches, 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.

Advice

Monitor local media for weather-related updates and advisories. Confirm all transport reservations and business arrangements before traveling in the affected area. Seek updated information on road conditions before driving or routing shipments through areas where severe weather is forecast; plan for possible supply chain disruptions throughout the affected areas. Stay away from elevated streams, creeks, and other watercourses that are prone to flash flooding. Do not attempt to navigate flooded roadways. Exercise caution in elevated terrain due to the threat of landslides, as well as mountainous regions where avalanches pose a threat. Charge battery-powered devices in the case of prolonged electricity outages.

Resources

National Weather Service