‘Potent’ atmospheric river to bombard central California on Friday and Saturday then onto the Colorado Mountains ~ The Washington Post

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American model simulation of atmospheric moisture (precipitable water) showing atmospheric river pointed at central California on Friday night. (WeatherBell.com)

 

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.

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