The US west coast is bracing for a “bomb cyclone”, with more than 10 million under flood warnings – just weeks after eastern states were battered by the phenomena, leaving cities buried under snow.
At least 60 people were killed over the Christmas period as a result – and now another freak weather pattern means millions more are advised to stay indoors.
“The storm is coming,” National Weather Service forecasters in the San Francisco Bay Area made clear on Tuesday, with heavy rains and flooding imminent.
Like the snow storms which battered states such as New York in December, it has the same menacing title.
But what is a bomb cyclone?
“Bomb cyclone” is a meteorological term where an area of low pressure in the atmosphere deepens and strengthens quickly as the pressure drops.
For a storm to meet this criteria, the pressure must drop extraordinarily quickly – at least 24 millibars in 24 hours.
This rapid drop can dramatically increase the winds – something known as bombogenesis – and cause a serious storm.
Although the storm which hit the US over Christmas killing more than 60 people wasn’t particularly rare, it was unusually powerful.
A front of cold air moved down from the Arctic, sending temperatures plunging.
It all started farther north, as frigid air collected over the snow-covered ground in the Arctic, said Ryan Maue, a private meteorologist in the Atlanta area.
Then the jet stream – wobbling air currents in the middle and upper parts of the atmosphere – began pushing this cold pool down into the US.
As this arctic air is pushed into the warmer, moister air ahead of it, the system can quickly develop into serious weather – including what’s known as a “bomb cyclone”.
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These severe weather events usually form over bodies of water, which have lots of warmth and moisture to feed the storm, said Maue.
But with the huge amount of cold air coming through, a rare overland “bomb cyclone” formed.
It devastated parts of the US and could offer a preview of the intense storm California faces over the next few days.
Of weather bearing down on California, the NWS said: “Major wind and rain impacts are EXPECTED tomorrow into Thursday. Prepare now for flooding, downed trees, and power outages. NEVER drive through flooded roadways! Remember turn around, don’t drown. Stay safe everyone!”
The rain expected would usually not lead to flooding, but with recent downpours over the past weeks already saturating, and even flooding, parts of the state – it is likely more flooding will occur.
At least one person died as a result of a storm over the weekend, which dumped snow and rain over much of the Golden State, prompting evacuations and avalanche warnings.
