The weather phenomena have caused widespread rainfall in California. (AFP Photo)
Thousands of people in California have been left without power due to torrential downpour and damaging winds. In the last few days, the US state has received more than six inches of rain, causing flash floods and muslides in areas where soil is already saturated. Much of the northern two-thirds of California, the most populous state in the nation, is under flood. And people Montecito, one of California's coastal enclaves, have been ordered to evacuate amid a major storm warning.
The storm is powered by two overlapping phenomena - an immense airborne stream of dense moisture from the ocean called an atmospheric river, and a sprawling, hurricane-force low-pressure system known as a bomb cyclone.
What are atmospheric rivers?
According to National Oceanic and Atmospheric Administration (NOAA) of the US government, atmospheric rivers are long, narrow regions in the atmosphere that transport most of the evaporated water. Since this current flows in the sky like rivers flow on land, hence the name.
This column of vapour move with the weather, which NOAA said is equivalent to the average flow of water at the mouth of the Mississippi river. When these rivers make landfall, they release this water vapour in the form of rain.
One of the most well-known atmospheric rivers is the Pineapple Express, said the BBC. It begins in Hawaii and brings moisture from the tropics to the US West Coast.
Bomb cyclone
A bomb cyclone, or bombogenesis, is a quickly intensifying storm that occurs when there is rapid drop in air pressure that creates an explosive effect. The intensification is measured by a drop in millibars, a measurement of pressure.
This usually happens when a warm air mass collides with a cold one, according to the National Oceanic and Atmospheric Administration.