Rare double weather-whammy threatens California

The combination of a ‘bomb cyclone’ and an ‘atmospheric river’ have placed a larger number of Californian homes and businesses typically untroubled by rising waters at risk of flood, according to US underwriting agency reThought Insurance.

 “An atmospheric river and a bomb cyclone have arrived on the California coast together, presenting a double weather-whammy for residents and business owners,” said Cyrena Arnold, meteorologist and chief sales officer at reThought. “Places will flood that are not in traditional flood zones.” 

A US National Weather Service (NWS) alert over the weekend warned that the cumulative effect of successive heavy rain storms since late December could bring rivers to record high levels and cause flooding across much of Central California.

Arnold explained the distinct, concurrent weather phenomenon which are likely to lead to “unprecedented flooding”:

A bomb cyclone is a rapidly intensifying area of low pressure that occurs when atmospheric pressure drops 24 millibars or more in 24 hours. A handful of these severe events occurs in the US each year, she said, bringing weather phenomena such as Nor’easters. 

An atmospheric river is a funnel of water vapour from the intertropical convergence zone, the stormy ring of clouds around the equator where the trade winds of the northern and southern hemispheres converge. They siphon water vapour northwards to the west coast of North America, where it condenses over terrain to become liquid. 

“They release rain like a firehose,” Arnold said. “An earlier atmospheric river delivered 15 inches of rain in Northern California in a week, and another last year dumped 15 feet of snow in the same area.” 

reThought uses several flood modelling technologies to produce a granular, proprietary view of flood risk for every building in the United States. The map indicates congregations of buildings at high risk of flood from the current climatic events in red, at medium risk in yellow, and at low risk in green. Black outlines indicate areas currently experiencing or expecting high rainfall from the one-two climate hit.