08 Mar 2023 | 10:12 AM UTC
US: Adverse weather forecast over western regions through at least March 11 /update 2
Heavy rainfall and snowfall forecast across parts of the western US through at least March 11. Possible flooding and associated disruptions.
Event
Heavy rainfall and higher elevation snowfall are forecast across parts of the western US through at least March 11. A lingering storm is forecast to continue to impact the region March 8 before a more impactful atmospheric river begins to affect the area from March 9. Rainfall totals of 10-20 cm (4-8 inches) are possible across parts of central California in the coming days and the heavy downpours could trigger flooding in low-lying areas and landslides on unstable slopes. Snowfall accumulations of up to 30 cm (12 inches) are possible in elevated parts of the affected region; however, snow levels are expected to rise to areas above around 2,300 meters (7,500 feet) and some areas that have recently been receiving snow could experience heavy rainfall. These downpours over areas of heavy snowpack will pose a threat of both flooding and avalanches due to snow melt. Strong winds could also combine with snowfall to cause blizzard conditions in areas of higher elevation.
Authorities in California declared a state of emergency March 1 for 13 counties affected by a series of winter storms that have struck the state over recent days. Some elevated areas have received more than 250 cm (100 inches) of snowfall over the past week.
Government Advisories
As of early March 8, the National Weather Service (NWS) has issued winter storm warnings for parts of northern and north-central California and southeastern Oregon. Winter storm watches have been issued for parts of eastern California and winter weather advisories are in place across parts of northern California and southeastern Oregon. Flood watches have been issued for parts of northern and north-central California and western Nevada. The NWS's Weather Prediction Center has warned of a moderate and slight risk (levels 3 and 2 on a four-tier scale) of excessive rainfall across parts of central and north-central California March 9-11. Officials could update and possibly extend the coverage of weather alerts over the coming days.
Hazardous Conditions
The storms could produce rounds of heavy precipitation (including rain and snow), strong winds, and isolated thunderstorm activity in 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 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. Power outages could occur throughout the affected area.
Transport
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.
The disruptive weather will likely cause some delays and cancellations at airports in the region. Authorities may temporarily suspend port operations along the Pacific coast if strong winds trigger hazardous sea conditions, impacting freight and passenger maritime traffic. 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 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.
