Atmospheric rivers: first of two potent events hits California

• Flood, high wind and heavy surf alerts are up along the coast of California, with winter storm warnings covering the Sierra Nevada Mountain range as the storm rolls in.
• This event, while impactful, may end up being weaker than the next atmospheric river that is likely to roll into Central to Southern California late this weekend into early next week.

The big picture: Atmospheric rivers are highways of concentrated water vapor located in the middle atmosphere, about 10,000 to 20,000 feet above the surface.

• They can stretch for thousands of miles, channeling water from the subtropics to temperate regions.
• Each storm will produce several inches of rain in a state increasingly prone to flash and river flooding along with landslides.
• The first atmospheric river moving into California has ties all the way to Hawaii, which is why meteorologists refer to it as a "Pineapple Express" storm.

Zoom in: The swirling storm nearing the coast of British Columbia is accompanied by a front that stretches far to the south and west, along with the atmospheric river flowing.

• It will slowly move into the West Coast and southward toward Southern California through Thursday.
• The Weather Prediction Center has issued a slight risk for excessive rainfall across Northern, Central and Southern California into Thursday morning.

Meanwhile, another slight risk has been issued for Southern California, including Los Angeles and San Diego, for Thursday into Friday, given that the front will be slow to clear the state even as moisture spreads inland.

• "The associated heavy rain will create mainly localized areas of flash flooding, with urban areas, roads, small streams, and burn scars the most vulnerable," forecasters wrote in an online discussion.
• The associated heavy rain, totaling up to 3 to 5 inches in some spots, will create mainly localized areas of flash flooding, with urban areas, roads, small streams, and burn scars the most vulnerable.

Between the lines: The snowfall in the mountains is likely to be beneficial, since much of the Sierra is running below average for the season.

• This affects water availability during the dry season, so between 1 to 4 feet of snow from the first storm will be welcomed by water managers and skiers alike.

What's next: Confidence is growing that the second, stronger atmospheric river will affect the Golden State late in the weekend into early next week.

• There is the potential, as shown in some computer model projections, for the storm to have a connection not just to the subtropics, but farther south, into the tropics.
• This would enable it to produce torrential rainfall rates not typically seen during West Coast atmospheric river events.
• It also appears likely that this event will target the region from San Francisco Bay southward into San Diego, with the threat of prolonged heavy rains heightened in southern areas.

Of note: Between the first and second storms, some locations in California could see a foot of rain, with five or more feet of snow in higher elevations.

The intrigue: Such a wet forecast is sure to be unwelcome news in San Diego, where a record downpour caused significant damage to the city last week.

• Ventura, California also had a similarly damaging and record-shattering downpour during a storm in December.
• Warmer than average ocean temperatures in much of the tropical and northeastern Pacific, which are only partly tied to El Niño, are likely to add more moisture to both storm systems.
• Over the longer term, human-caused climate change is expected to result in atmospheric rivers that carry more moisture and are capable of dropping heavier amounts of rain and snow.

Go deeper:

What is an atmospheric river? Weather event behind California floods, record snow

Historic rain overwhelms San Diego, causing severe flooding