Environment & Science

How much rain has SoCal received this year? Here's one way to keep track

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Southern California got drenched Sunday, with some areas getting up to three inches of rain. It's an El Niño year. So does that mean Southern California's rainfall is ahead of normal this year?

Not yet.

As of Monday morning, the region has received 41.1 percent of a normal winter's precipitation. That's behind even the median, or typical year, which by February is at 56.3 percent. (Note: rain gauge readings changed downwards after the day this article ran, so the 2015-2016 percentage was lower one week later.)

And if you look at just recent years with strong El Niños, median precipitation is even further ahead by this point: 78.8 percent.

That's all according to a metric devised by David Pierce of the the Scripps Institution of Oceanography at UC San Diego. "The whole purpose of this was to try to come up with a single number that really is capturing how wet, or dry, the region really is," Pierce said.

To get there, Pierce takes readings from 2o weather stations across greater Los Angeles. They dot the region, including coastal locations from Newport Beach to Santa Barbara, and inland ones in Redlands and Mount Wilson. He weights those stations according to geography and historical weather information.

One other feature Pierce sought out in the stations: a long, reliable record. Many of these stations have been in operation since World War II.

That allows the metric to create a baseline for what a 'normal' year looks like. This chart uses the same data as the gauge above, with median year and median El Niño year for perspective.

You can see that, from January 30 to 31, the orange line representing this year jumped nearly 5 percent. That's a big change for a single day.

"It's a number that continually goes up over the winter," Pierce said. "So it shows you how much you've accumulated, and it's useful because we really care about, at the end of the winter... whether we've accumulated more or less than average." The percent KPCC uses covers the 'water year'—the six months between October and March when the area receives most of its rainfall.

Pierce has also crunched the numbers on the state as a whole, and found that California is slightly ahead of normal in terms of precipitation. Other regions, including the Sierras and San Diego County, are also closer to 100 percent today than in the median year.

Not the Los Angeles area. That's probably random, Pierce says, and difficult to read into. But he notes that the L.A. basin has been an outlier.

Pierce cautioned "it's definitely not time to panic." He added that there's still plenty of time for rainfall to get up to normal levels, or past them.