Rainy Day History

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This is Kevin Roderick with LA Observed for KCRW.

After a week of tornado damage, power outages, evacuations, hail and snarled traffic, it's time to pronounce all this rain…a welcome success.

Southern California has so far escaped the disastrous mudslides and debris flows that many feared after the big Station Fire. And we've gotten a nice reminder of how much our lives are tied to the Pacific and the jet stream.

The jet stream is the fast moving river of air that blows west to east around the earth. It's a storm generator since, as we all no doubt remember about Bernoulli's Principle, where air flows the fastest the pressure is lowest.

When the jet stream dips down into our latitude, the storms blow in right over us -– after slurping up moisture from way out in the Pacific.

Add in a strong El Niño effect, or Southern Oscillation -- a reference to warm ocean temperatures in the equatorial Pacific -- and we here in L.A. get one of our inevitable and necessary rainy years.

These years that shape so much of what we think of as normal in Southern California. The canyons, foothills and the fans of debris on which places like Sierra Madre and Sunland-Tujunga exist are there because of eons of this cycle repeating.

Same with our beaches, which historically are replenished with sand that originated high up in the San Gabriel and San Bernardino Mountains – washed down to the sea in the big rain events like we're having this week.

It's these monster years, where boulders tumble down onto suburban streets and the snow piles up 20 and 30 feet deep in the Sierra Nevada, that makes California as we know it even possible.

We don't have such a thing as a "normal" rainfall year in Southern California. Only long stretches of dry sunshine and morning low clouds punctuated by memorably wet seasons that skew the so-called average.

The big rain events refill the reservoirs and underground aquifers with water that we use in the drier years.

The reason so many of our rivers and arroyos are encased in ugly, nature-defying concrete is because of these big El Niño years.

In particular, 1938.

That March, the rain fell so fast that nearly half of the San Fernando Valley was inundated. The Arroyo Seco and other major canyons funneled crashing waves of runoff toward the Los Angeles River, which overflowed onto large swaths of land and city streets.

The Oscar ceremony that year had to be postponed because the stars and directors and producers couldn't get out of the Valley, where many of them had horse ranches and suburban estates.

Demands for flood control reached critical mass politically that year. Hansen Dam was built to catch the flood waters out of Tujunga Canyon.

Sepulveda Dam went up to intercept flash flooding on what the engineers call the upper LA River. The river's banks were gouged out and cemented in, upstream and downstream.

Most of the time, the dry concrete channels look absurdly wide and pointless, especially to new arrivals. But in weeks like this, when even those ridiculous channels nearly overflow, a tamed L.A. River makes more sense.

It's been about five years since we had a monster wet year – that one was the biggest since the 1880s. So we're due for this soaking. It's as "normal" as anything else.

Forecasts are for the jet stream to hang around awhile, and for the Pacific's Southern Oscillation to remain strong.

In the dry prose of the National Weather Service, El Niño is expected to exert a significant influence on global weather for the next few months.

Enjoy the ride, and hope the hillsides hold.

For KCRW, this has been Kevin Roderick with LA Observed.