Mystery of Southern California’s intense winter rains may be solved

Cold-frontal rainbands are incredibly long, extremely narrow, and lead to devastating flash floods and landslides. Accurately predicting these events is essential to protect communities.

A new study conducted by the American Geophysical Union (AGU) may finally answer one of the biggest ecological questions in the US. Specifically, what makes winter downpours in the country’s far south west so extreme. 

Storms can last just a few minutes, but result in severe damage to property and infrastructure, and the loss of life. As a result, these brief but catastrophic weather anomalies regularly lead to widespread evacuation alerts, largely because it is impossible to ascertain when storms will hit and where, beyond very broad areas. 

By building a catalogue of rainbands, scientists at AGU aim to improve the accuracy and detail of predictions, reducing evacuation zones and public fatigue caused by unnecessary alerts. In turn, this will bolster the public’s trust in such warning systems. 

Focusing on the years between 1995 and 2020, the team found that 60% of all cold-frontal events resulted in at least one National Weather Service warning. From a total of 94, 15 of these led to 10 or more warnings, with some causing so-called atmospheric rivers,  particularly between January and March, when the region is already at its wettest. 

Over time, it is hoped that by monitoring weather patterns more closely it will become easier to ascertain when a system presents danger to communities. Eventually, the goal is to ‘now-cast’ cold-frontal rainbands, which can be hundreds of kilometres in length but rarely more than 2KM wide. This would mean being able to issue accurate warnings hours before disaster strikes. 

“Our current predictive models struggle with location, timing and intensity of these events, and we need to improve that for evacuation warnings. … If we issue evacuation warnings for what turns out not to be a high-impact event, you start to run into evacuation fatigue, where people might not trust the warnings in the future,” says Nina Oakley, atmospheric scientist at the Scripps Institution of Oceanography and co-author of the new research.  

In related news, atmospheric rivers are increasingly threatening East Asia. Elsewhere, the think tank Bright Blue has warned that the UK is not prepared for the increase in flooding predicted to take place in the coming years. 

Image credit: Inge Maria