8 Reasons Why Rain Is a Big Deal in Southern California

Jon Erdman
Published: February 16, 2017

A rainy day may elicit a shrug of the shoulders in other cities, but in Southern California, rain is almost always a big story.

I learned this quickly in my first job out of college, providing forecasts and consultation for several of the state's largest newspapers, including the L.A. Times. When even a small chance of rain appeared in the forecast, there would be a call from the Times weather desk, often multiple times per shift, when new forecast guidance rolled in.

I had never been asked before, nor since, the type of granular questions about rainfall. It was an eye-opening experience for a young meteorologist fascinated by severe thunderstorms, snowstorms and hurricanes.

With that in mind, here are eight reasons why even a light rain event really matters to Southern Californians.

Average annual rainfall at the Los Angeles Civic Center, based on 1981-2010 data.
(National Weather Service)

1. It Doesn't Rain Often

As you'd expect for sunny Southern California, nine out of 10 days are dry.

Downtown L.A. only averages 36 days a year with measurable precipitation.

Both Chicago and New York typically have 3 to 4 times more wet days per year than Los Angeles.

2. Usually, It Doesn't Rain for Months

California has one of the most well-defined wet and dry seasons anywhere in the Lower 48 states.

It is typical for L.A. to go months without a single drop of rain from late spring into early fall.

During this dry season, high pressure over the West and a weakening, northward-migrating jet stream pushes the Pacific storm track well north of the Golden State.

Over 90 percent of the Southland's average rain falls in just six months' time from November through April.

A wildfire burns in the Hollywood Hills on July 19, 2016 in Los Angeles, California.
(Kevin Winter/Getty Images)

3. Rain Can Help Snuff Out Wildfires

Once that first decent rain arrives after the dry season, it can be a huge blessing in one regard.

While wildfires can occur in California virtually any time of year, early fall is typically the peak in Southern California, particularly when Santa Ana winds howl through canyons and passes with vegetation dried out from months without rain.

There is no bigger friend to firefighters than a steady, soaking rain.

4. But It Can Be a Menace For Burn Areas

Paradoxically, long after a wildfire is out, the charred, barren strip of land left behind is very susceptible to flooding.

"After years of drought and wildfires, the vegetation has been stripped away along many slopes," said Crystal Egger, meteorologist at NBC4 News. "Those slopes can’t handle a deluge."

A burn scar's top layer of soil acts like pavement, which keeps rain from soaking into the ground. Lacking vegetation, nothing remains to hold the soil in place, and a flow of debris – including mud, rocks and tree stumps – rushes downhill, taking out anything in its path, including homes and roads.

Debris flows can occur with relatively light rain rates and can be a threat for years until the slope's vegetation recovers.

This is a particularly cruel fact of life for those homes that survived wildfires near burned-out hillsides. The mere threat of rain can put them on edge.

A home sits on the edge of a cliff Jan. 13, 2017 in the Hollywood Hills after its concrete patio was washed away in a mudslide caused by heavy rains in Hollywood, California.
(ROBYN BECK/AFP/Getty Images)

5. It Can Also Trigger Landslides Elsewhere

It isn't just burn scars that can experience landslides.

If you've seen coverage of a strong California storm, you've probably seen video of a home either teetering or tumbling down a hillside.

The cumulative effect of repeated Pacific storms – or one particularly wet, long-lived storm – can trigger rockslides that block major roads, such as the Pacific Coast Highway, or claim expensive hillside homes.

(MORE: Atmospheric Rivers: a Blessing and a Curse)

6. It Worsens L.A.'s Notorious Traffic

Combine the legendary bad traffic of Southern California with infrequent rain, and you can imagine the results.

After months of dry weather, motorists may be inexperienced when it comes to driving in the rain. Following a long dry period, the initial coating of rain mixes with deposits of oil from vehicles, which makes roads slick.

"We can have less than a tenth of an inch of rain, and motorists drive like there was an inch per hour rate," said Curt Kaplan, meteorologist with the National Weather Service in Oxnard, California. "It's embarrassing."

Given the concrete jungle of Southern California, even moderate rain tends to run off quickly rather than soak into the ground, flooding streets and, occasionally, parts of freeways and on/off-ramps.

Bitelio Ramirez, 52, looks out over tons of trash and debris that piled up near the mouth of the Los Angeles River after two days of heavy rain fell in Southern California in January 2016.
(Rick Loomis/Los Angeles Times via Getty Images)

7. It Can Also Foul Beaches

Some runoff from heavy rain eventually makes it to the ocean, funneled through such concrete channels as the Los Angeles River.

This fast-rushing water is not only dangerous for anyone nearby; it also contains lots of trash and debris that eventually makes its way to some beaches in the Southland.

This stormwater runoff can negatively impact water quality for days after a heavy rain event, prompting closures of some popular beaches.

8. But California Needs It

Despite the problems with runoff and flooding in the L.A. Basin, these Pacific storms are vital to the area's water supply.

Heavy snowpack deposited by these storms in the Sierra and high country of California melts in spring, recharging reservoirs ahead of the long dry season.

(MORE: California Reservoir Status)

A wet season without these storms almost always leads to drought in Southern California, as was the case the past several years.

Time series of drought severity areal coverage in California from 2000 (at left) through mid-February 2017 (at right). The more severe drought is indicated by the darker orange, red and brown colors.

It's a love-hate relationship L.A. has with rain – an annoyance for those who value the perfection of sunny Southern California, yet a necessity for life in this dry climate.

Jonathan Erdman is a senior meteorologist at weather.com and has been an incurable weather geek since a tornado narrowly missed his childhood home in Wisconsin at age 7.


 

MORE: Near-Disaster at the Oroville Dam


The Weather Company’s primary journalistic mission is to report on breaking weather news, the environment and the importance of science to our lives. This story does not necessarily represent the position of our parent company, IBM.

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