Weather

Finally, an atmospheric river flows toward California

pineapple-express-dec2014.jpg
From Hawaii to California.

With the drought-causing high pressure zone dubbed the Ridiculously Resilient Ridge pushed aside for now, a big wet Pacific sky river is heading our way. Well, not exactly our way. The so-called Pineapple Express is due to hit the Bay Area later this week and dump many inches of rain. It should then move eastward over the Sierra Nevada and drop several feet of snow. Or it might stall over Northern California and just keep pumping out more free water. It has happened many, many times before and the impacts are well accepted. But that street flooding, levee testing, upper elevation blizzards and hillside rock and mud slides should be especially welcome after three years of severe drought across most of California.

The blizzard warning is for areas of the Sierra above 6,000 feet starting Wednesday night.

At Slate, Eric Holthaus looks ahead at the arriving weather:

This week’s storm will usher in an atmospheric river event, also known as the “pineapple express,” peaking late Wednesday and Thursday. The National Weather Service office in the Bay Area has predicted “the strongest storm of this season to date,” and possibly the strongest since 2008 or 2009, with potential results including “downed trees, power lines, flooding and mudslides.” Wind gusts could exceed 80 mph at higher elevations.


Heavy rainfall could fall at rates up to an inch an hour and could exceed 8 inches in the mountains. The high Sierras are in line for up to 4 feet of fresh snowpack. The National Weather Service notes there’s potential for this storm to stall out over the Bay Area as well, in which case the risk of mudslides and dangerous flooding could quickly increase. Just offshore, ocean temperatures are much warmer than usual, which the NWS says could argue in favor of the more intense rainfall scenario.

Seasonal rains are now technically above average in Northern California, at 112 percent of normal. Despite that, the state’s largest reservoir, Lake Shasta, barely budged higher after last week’s heavy rains. Before the storm, it was at 23 percent of capacity; after, it reached 25 percent of capacity. The rainfall was about 15 or 20 percent of what would be needed to end the drought—but that doesn’t factor in refilling the reservoirs to guard against future dry spells. The state’s snowpack—arguably a more important measure of water storage than the reservoir system—is still running below average for this time in December.

The rain is coming thanks to an eastward shift in the rain-blocking ridge that dominated the Pacific Coast last winter and helped intensify the drought.

We may get some of the rain in the south — for sure we're getting some significantly high surf out of it. But if the rain and snow falls mostly in North California, that's good for us. The Southern California metropolitan area gets most of its water from outside the region, and the Sierra (and farther north) snowpack is the key source of water for Los Angeles and the state's farmers.

In a separate piece, Holthaus discusses the recent coverage of a study that the media played as showing that the California drought is a natural occurrence, not an event caused by human-created climate change. Misses the point, he argues.

I hope that in all my writing on this topic, I’ve never said or implied that any specific weather or climate event was definitively caused by climate change. That fact is almost impossible to know—even more, it’s probably dangerous to even ask it. The atmosphere just doesn’t work in cause and effect. It’s a huge, dynamic system, and everything affects everything else. The research has convinced me that the drought probably had an origin in natural variability of the climate system. But I also still believe—and don’t think that these are mutually exclusive—that global warming nevertheless contributed, perhaps substantially, to California’s current drought….


Even if the “cause” is natural, we can’t ignore the role that climate change likely plays in exacerbating the situation. Warming increases the rate of evaporation, increases the need for irrigation and groundwater pumping, and reduces snowpack. Droughts like this can also be self-sustaining, as drier soils lead to higher temperatures. Monday’s study didn’t address this part of the equation.


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