A 'bomb cyclone' bringing heavy downpours, flooding, and landslides is set to hammer the West Coast on Christmas Eve and further endanger holiday travelers. 

The entire California coast is under a severe storm warning this morning, with major coastal cities from San Francisco to Los Angeles expected to see at least two to four inches of rain and near hurricane-force winds reaching 70mph in certain parts of the Bay Area and Northern California.

Some areas inland have been warned that an extreme 'firehose' of rainfall could deliver as much as eight to 16 inches of rain throughout California, including in the Los Angeles basin and the Sierra Nevada and Transverse mountain ranges.

A bomb cyclone is an extremely intense storm that forms from a low-pressure system that rapidly strengthens, usually in a day or so. It typically forms over the ocean near the West Coast, bringing extreme weather in just hours.

Between Tuesday and Christmas Day, over 1,300 flights are expected to be cancelled at airports in San Francisco, Los Angeles, San Diego, and dozens of smaller travel hubs throughout the region. 

The National Weather Service (NWS) has issued a winter storm warning for much of Northern California and the state's mountainous areas along the Nevada border.

Meteorologists with AccuWeather have predicted that heavy snow, likely exceeding four feet, could be dumped on the entire region; however, even more is expected at the highest elevations between Wednesday and Friday.

AccuWeather chief on-air meteorologist Bernie Rayno said: 'We're probably looking at at least 10 feet of snow this week over the ridges and peaks of the Sierra Nevada.'

The heavy rain hitting California right now has been fueled by a strong 'atmospheric river' slamming into the West Coast. 

This giant river in the sky acts like a long, narrow band of air loaded with a huge amount of water vapor that flows through the atmosphere. The Christmas storm is actually the second atmospheric river to hit California in a week.

A first river brought deadly flooding to Northern California on Sunday and Monday, killing at least one person in Redding, about 160 miles north of Sacramento.

Heavy rains pelted the region with nearly 10 inches of water in just 24 hours during the first storm.

On Christmas Eve, the heaviest rain from this second atmospheric river is expected to pound California, bringing one to three-inch downpours of rain along the coastal plains through midday.

Northern areas such as San Francisco and Sacramento could see ongoing rain adding up to five inches through the end of the week.

Meanwhile, Southern California, including Los Angeles and San Diego, faces a high risk of flash flooding and landslides, with some spots getting between four and six inches of rain on Wednesday, making this one of the wettest Christmas Eves on record.

Strong wind gusts measuring between 40 and 60mph could cause power outages, fallen trees, and dangerous driving conditions throughout the state. Flooding is expected in urban areas, along rivers, and in burn scars from this year's wildfires.

Forecasters said this monster storm will officially turn into a bomb cyclone if the air pressure at its center drops really fast over 24 hours this week.

Air pressure is basically how heavy the air is pushing down on everything. In storms, the center has lower pressure, lighter air pushing down, which pulls in winds and makes the storm stronger. 

For this second atmospheric river to develop into a bomb cyclone, that pressure has to fall by at least 24 millibars in one day. Millibars are a unit experts use to measure air pressure.

This would be the equivalent of the storm suddenly sucking in large amounts of air quickly, which makes it explode in power, bringing hurricane-force winds and heavier rain to the West Coast this week.

AccuWeather meteorologist Brandon Buckingham warned: 'Regardless of whether the storm rapidly strengthens and experiences the required 24 millibars of pressure drop in 24 hours (0.71 of an inch of mercury), the storm will intensify enough to create strong winds along the coast of Northern and Central California.'

Over 1,500 flights had already been delayed globally as of Tuesday morning, with Flight Aware's Misery Map revealing that flights between New York and Los Angeles were the most affected domestic trips so far.