California can’t afford another month like February. Not during its rainy season, not after four years of sustained drought. Sure, the warm weather kickstarted a lovely spring—even Death Valley is blooming—but interrupted what has otherwise been a cathartically wet winter. Luckily, March is starting off with plenty of puddles.

The wetness will begin Saturday, when a tropical weather system called an atmospheric river hits the state. The name is allusory—an atmospheric river isn’t so much a floating canal as it is an invisible mass of water vapor. The allusion becomes apt once you realize that the combined volume of all that vapor can be many times greater than the total outflow of the Mississippi River.

The mass of wetness comes from the humid tropics, and can be invisible as it flows thousands of feet above the Pacific Ocean. “You’ll have this wide mass of moist air that won’t rain,” says Marty Hoerling, a meteorologist with NOAA’s Earth Systems Research Laboratory in Boulder, CO. “But when it hits the shore, the air is forced to rise and lift into colder air, condensing the water vapor into moisture.” This uplift is most dramatic in the coastal highlands north of San Francisco, and further inland in the Sierra Nevadas. Hoerling says the weekend’s storm could bring a half foot of precipitation or more to these areas.

The crucial thing is how much of that precipitation falls above 7,000 or so feet, the predicted snow line. Snow is California’s most important long term water supply. Crucial, because this year’s dry, warm February melted an alarming amount of the promising pack accumulated between last November and January.

Saturday’s atmospheric river is just the first of three visible in the long term forecast. Another is supposed to hit next Tuesday, March 8, and another on March 10. Following the longer trend, NorCal is going to get most of the precipitation, but the three storms could drop up to two or three inches as far south as Santa Barbara. Which is great news—as long as it doesn’t cause any catastrophic mudslides.

Longer term forecasts show this system creating trouble further east. Even though California is supposed to drain most of the system’s moisture, the jet stream is forecast to push the system south and east, towards the Yucatán Peninsula. Replenished with Caribbean waters, Hoerling says the data predicts the storm slicing back to the north to dump 15 to 20 inches of water in the Lower Mississippi River Basin—a region encompassing Louisiana, Arkansas, and western Tennessee.

And if that disaster does come, it’s not like it will be balancing out some sort of karmic debt after quenching California. After four years of drought, the Golden State needs more than a March miracle to balance its water budget.

Original source:

After a Parched February, the West Is About to Get a Deluge