20 Mar 2023 | 01:12 PM UTC
US: Adverse weather forecast over western regions through at least March 22
Heavy rainfall and snowfall forecast across parts of the western US through at least March 22. Possible flooding and associated disruptions.
Event
Successive storm systems are forecast to bring a mixture of heavy rainfall and snowfall across parts of the western US through at least March 22. The first system is expected to spread rain and snow across the Rockies on March 20 before moving into the northern plains late March 20. The second system is expected to move from the Pacficic over southern California on March 21, bringing heavy rainfall to southern and central parts of the state before spreading rainfall and snowfall across the Great Basin and into the Four Corners region late March 21-early March 22. Snowfall accumulations of up to around 15 cm (6 inches) are possible in elevated parts of the affected region over the coming days, with isolated heavier amounts likely over some of the higher peaks. Strong winds could also combine with snowfall to cause blizzard conditions in parts of the affected area. Where sustained heavy rainfall occurs, the heavy downpours could trigger urban and flash flooding across parts of the affected region. Winds gusting up to 80 kph (50 mph) are possible in parts of southern California as the second storm comes ashore. The strong winds could cause power outages and property damage. Hazardous driving conditions are likely due to slippery roads and reduced visibility caused by blowing snow.
Government Advisories
As of early March 20, the National Weather Service (NWS) has issued winter storm warnings for parts of central and southern California, central and southern Nevada, central, eastern, northern, and southern Utah, northern Arizona, northern and northwestern New Mexico, and central and southwestern Colorado. Winter storm watches have been issued for parts of central and southern California and central Nevada and winter weather advisories have been issued for parts of eastern and southern Califronia, northern Arizona, eastern Utah, central and southeastern Idaho, southwestern Montana, southern and western Wyoming, northern and western Colorado, and northern New Mexico. High wind warnings, watches, and advisories are also in place across parts of the affected area.
Flood warnings, watches, and advisories have been issued for parts of central Nevada and central Arizona. The NWS's Weather Prediction Center has warned of a Slight Risk (level 2 on a four-tier scale) across parts of southwestern California and central Arizona March 21-22. There is a marginal risk of excessive rainfall for surrounding areas of central and southern California and central Arizona.
Officials could update and possibly extend the coverage of weather alerts over the coming days.
Hazardous Conditions
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 and 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, especially in 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. Flooding or snow could block regional rail lines; freight and passenger train delays and cancelations 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. Repair or reconstruction efforts may result in residual disruptions if there is severe damage to infrastructure.
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.
