California weather: flooding and possible tornadoes explained, storms in San Francisco and Los Angeles - forecast

Five deaths have been linked to the latest storm surge in California’s Bay Area, with another “atmospheric river” - the 12th since late December - drenching the already-saturated state with more rain and snow as thrashing winds felled trees.

The storm that hit California on Tuesday (21 March), the first day of spring after an unusually long winter, was brought on by an interaction between a Pacific low pressure system and an atmospheric river, according to the National Weather Service

Thousands were left in the dark across the state owing to severe power outages, as the National Weather Service issued a warning that flooding risks would continue into Wednesday (22 March).

But what is an atmospheric river? What is the situation in California, and how long is the bad weather likely to last? Did the storm surges bring relatively rare tornadoes to the state? Here is everything you need to know about it.

What is an atmospheric river?

An atmospheric river is a narrow and elongated corridor of concentrated moisture in the Earth’s atmosphere, which can extend thousands of kilometres in length. These rivers of water vapour transport large amounts of water from the tropics to other regions of the world, such as the western coasts of North America and Europe.

Atmospheric rivers are important for global weather patterns, as they can cause heavy precipitation events, such as rain and snow, when they make landfall. However, they can also lead to severe flooding and landslides in affected areas. In fact, some of the most destructive and costly natural disasters, such as the 2017 California floods, have been associated with atmospheric rivers.

Scientists study atmospheric rivers using a combination of satellite observations, ground-based measurements, and numerical models to better understand their formation, behaviour, and impacts on weather and climate. Flooding and severe storm weather can occur in California, especially during the state’s rainy season, which typically lasts from November to April. The frequency and severity of such events can vary from year to year and region to region within the state.

Coastal and mountainous areas of California are particularly vulnerable to flooding and landslides, especially during intense precipitation events associated with atmospheric rivers. To prepare for and mitigate the impacts of severe weather events, California has implemented various measures, such as building levees and dams, improving stormwater management systems, and providing emergency response and evacuation plans for affected communities.

How many people have died?

The wind and rain mayhem from San Francisco Bay south to Monterey Bay on Tuesday was caused by an extraordinary drop in barometric pressure over the eastern Pacific that meteorologists described as “explosive cyclogenesis”.

Trees and power lines were blown down. Windows were blown out from two San Francisco high-rise buildings, NBC Bay Area reported. Ferry services were disrupted because conditions were too rough. Three barges got loose and damaged a bridge.

An Amtrak commuter train carrying 55 passengers struck a downed tree and derailed near the East Bay village of Porta Costa. The train remained upright and nobody was injured, Amtrak and fire officials said.

Five deaths have been linked to the latest bout of extreme storm weather in California, with most attributed to fallen trees.

They include a man driving a sewer truck in the Bay Area community of Portola Valley, who was killed when a tree fell onto the vehicle, and a driver was injured and a passenger died after a large tree fell onto a car in the community of Rossmoor.

Were there tornadoes?

A possible tornado also left a line of damage across roofs of commercial buildings in the Los Angeles suburb of Montebello a day after two other possible tornadoes occurred in Southern California.

The National Weather Service tweeted that it was sending teams to assess the damage in Montebello and the southern Santa Barbara County city of Carpinteria. Montebello city spokesman Alex Gillman said information was still being gathered.

A tornado warning based on radar was also issued on Tuesday night for the Point Mugu area west of Malibu. The warning was later cancelled and the Ventura County Sheriff’s Office tweeted there was no evidence a tornado touched down.

Tornadoes are relatively rare in California compared to other states in the United States. Its climate and geography make it less conducive to tornado formation, with the conditions required for tornado formation, such as strong vertical wind shear and instability, not frequently present.

According to the National Oceanic and Atmospheric Administration (NOAA), California averages only six tornadoes per year, with most of them being weak and short-lived.

What is the weather forecast for California?

The extreme weather in California was predicted to subside on Wednesday (22 March) as the storm moved inland, according to projections. Those predictions did mean that on Tuesday, some residents of north-central Arizona were told to prepare to evacuate because of rising water levels in rivers and basins.