US: Adverse weather forecast across most of California through at least Jan. 7 /update 1

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 most of California through at least Jan. 7. Widespread rainfall of 7.5-15 cm (3-6 inches) within a relatively short period of time is likely to result in flooding across the already saturated region. The highest amounts of precipitation are forecast over the coastal ranges of California. Rapid runoff and landslides are possible over recent burn scars. 0.9 meters (3 feet) of precipitation, mostly as snow, is likely in the higher elevations of the Sierra Nevada. Gusty winds, together with hazardous heavy snow, are likely to result in potential tree damage and whiteout conditions. Potentially damaging winds could spread into the central valley. High winds combined with saturated ground conditions could lead to additional downed trees and resulting power outages. Hazardous travel conditions are likely across the affected area, especially over mountain passes due to snowfall accumulations. Another system is forecast to impact California by late Jan. 6, spreading heavy rain into northern California, and leading to additional flooding concerns.

Heavy rainfall across parts of northern and central California Jan. 1-2 caused four fatalities. Authorities declared a state of emergency across California Jan. 4. Authorities have warned that water would overtop the spillway at North Fork Dam by early Jan. 5; residents in low-lying areas beneath the dam should move to higher ground immediately. Evacuation orders have been issued over portions of Sacramento, Santa Barbara, and Santa Cruz counties due to the risk of flooding and landslides. Classes across northern California are canceled Jan. 5.

Dozens of flights at San Francisco International Airport (SFO) and several flights at Oakland International Airport (OAK) and San Jose International Airport (SJC) were canceled Jan. 4. Hundreds of flights at the airports were also delayed. Further flight disruptions are likely as airlines work to clear backlogs. Further severe weather is also likely to cause additional disruptions. As of late Jan. 4, more than 154,000 people across California are without power, with the northern and central coasts the worst affected.

As of late Jan. 4, the National Weather Service (NWS) has issued flood and flash flood watches, high wind warnings, and winter storm warnings for most of 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 most of the coast of California through early Jan. 5, as well as a "Slight" and "Marginal" risk (Levels 2 and 1) for surrounding areas of western and northern California through early Jan. 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