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‘Potent’ atmospheric river to bombard central California on Friday and Saturday

The term “Pineapple Express” sounds like the tropical cousin of famous Warner Bros. Christmas movie “The Polar Express.” In the world of weather, it’s a narrow plume of deep tropical moisture that snakes from Hawaii all the way to the West Coast.

Others call these phenomena “atmospheric rivers,” because they are ribbons of warm, humid air drawn into the subtropical air current. Although they sometimes stretch for thousands of miles in length, they oftentimes are less than a few hundred miles wide.

Though it’s late in the season, another one is brewing — one that the National Weather Service is calling “potent.” It will take aim at the California coast Friday into Saturday, unleashing heavy rainfall, gusty winds and flash-flood concerns in the higher elevations.

The heaviest precipitation looks to come down in the Santa Clara Valley, along the Coastal Range and the Santa Cruz mountains. While Los Angeles will be too far south and will barely skirt the moisture plume, San Francisco will sit smack dab in the middle of the fire hose. While 2 to 3 inches of rain is possible there, the mountains farther inland will poke into the jet stream and snatch the water right out of the sky. Some places along the Cascades and the Sierra Nevada could see up to 6 to 8 inches.

Now that’s no small potatoes, but even more astonishing is that this could be the wettest air mass on record in central California. Meteorologists measure atmospheric moisture content with an index called PWATs — short for precipitable water. It’s a measure of how much moisture is stored in a column from the ground to the top of the atmosphere. Most of the time, it’s impossible to tap into all of that moisture. But sometimes, storm systems can squeeze it all out of the air just as one would wring out a washcloth.

Computer models are hinting that PWATs could top 2 inches on Saturday, which would slash the highest weather balloon-
measured value of 1.79 inches. Records date all the way to 1948. Even more staggering is that we’re in the midst of California’s dry season —  normally, one has to wait until August or the fall before the sweaty weather arrives!

Atmospheric rivers aren’t all bad. In fact, for the typically dry state, this can be a significant contributor to its annual rainfall during El Niño years. Thirty to 50 percent of the rain and snow that falls along portions of the Sierra Nevada comes from these type of events. Last year, they nearly wiped out California’s long-standing drought.

Right now, 77 percent of California is experiencing some level of drought. That is down from 88 percent last week. This weekend’s deluge will probably slash those numbers further.

Marty Ralph is a research scientist studying the atmosphere and oceans at the Scripps Institution of Oceanography. He is one of the nation’s foremost experts in atmospheric rivers. This one, he says, is a biggie.

“It will be the strongest atmospheric river by far this winter for the region,” he said. “Its total water vapor transport will be greater than roughly 25 Mississippi Rivers’ worth of water, though, of course, only a fraction of this will fall from the sky as rain or snow.”

He did note that forecasting on atmospheric rivers has a long way to go. Even though we’re two days out from the event, it’s still impossible to say whether it will stall and bring higher precipitation totals than expected. “There are signs that could happen, but the details remain highly uncertain.”

Earlier this year, the National Oceanic and Atmospheric Administration called in the Hurricane Hunter jets to probe inside atmospheric rivers to help refine forecasts. While they weren’t available now, confidence is still up there for a high-impact rain event.

After all, it’s going to rain cats and dogs, and maybe even pineapples.