[californiadisasters] A Tropical Cyclone Threat in Southern California?

A Tropical Cyclone Threat in Southern California?

By Mark Moede, NWS San Diego
Coast to Cactus Weather Examiner
Volume 17 Number 3 - July 2010

Tropical storms and hurricanes form over the eastern Pacific each summer. Many form off the west coast of Mexico near 15 degrees N latitude, and then propagate northwestward with the prevailing trade winds. The cyclones weaken as they move northward over the cooler waters of the California Current, a river of cold water originating in the Gulf of Alaska that flows southward along the West Coast and down the length of Mexico's Baja Peninsula. It is this cool ocean current that marks the demise of Eastern
Pacific tropical cyclones.

However, during El Niño years, the warmer ocean waters prolong the life of Eastern Pacific Tropical cyclones.

Many of the strongest storms have occurred during El Niño years. Hurricane Linda, a category 5 hurricane, is the strongest Pacific hurricane on record and occurred during the strong El Niño of 1997.

El Nino Factoid #1: El Niños extend the ocean water 80* F isotherm about 5 degrees latitude farther north than the normal.

El Nino Factoid #2: El Niño decreases easterly trades (trade winds) which decrease mid-level wind shear. This extends the life of hurricanes.

Let's take a look at some Tropical Cyclones during past El Niño episodes.

Hurricane, San Diego, October 1858 (El Niño undetermined)

* This category 1 hurricane is the only documented hurricane to strike the U.S. west coast.

* Strong winds followed by heavy rain caused extensive damage to the tiny town of San Diego.

Tropical Storm, Long Beach, September 1939

* 5.66 inches of rain in Los Angeles, 11.64 at Mt. Wilson.

* People were caught unprepared by the storm. 45 died at sea and 48 died on land.

* In response, the Weather Bureau (nee National Weather Service) established a forecast office for southern California, which began operations in February 1940.

Tropical Depression Kathleen, Southern California desert, September 1976

* Most of Ocotillo was destroyed by half-mile wide wall of water, causing 13 deaths.

* A section of Interstate 8 was washed out, and major damage hit railroads.

* 57 mph sustained winds (gusts to 76) at Yuma are the highest ever recorded from an Eastern Pacific Tropical Cyclone.

Tropical Depression Doreen, Southern California desert, August 1977

* Most areas of southern California from Los Angeles south received at least 2 inches of rainfall with up to 8 inches in the mountains.

* Flooding was widespread with extensive crop damage.

Hurricane Linda (Category 5, no landfall), September 1997

* Strongest Hurricane recorded over the Eastern Pacific Basin.

* Actually forecast briefly to make landfall in California as a hurricane before turning west into open ocean.

* Remnants produced thunderstorms and a deadly, damaging flash flood in Forest Falls.

* Surf was 15 to 20 feet high.

Tropical Depression Nora, September 1997

* $150-200 million in agricultural losses.

* 5.5 inches fell at Mt. San Jacinto, two in the Coachella Valley.

* More rain in Yuma (three inches) in a few hours than the yearly normal.

Could it Happen Here?

Many years with tropical cyclone remnants reaching California are El Niño years, during which sea surface temperatures rise along the west coast of North America, allowing tropical cyclones heading to California to maintain intensity longer than usual. Under the right circumstances a tropical cyclone will hit again. A tropical cyclone impacting Southern California is a rare event, but one that would have high impact. A hurricane or tropical storm, like those that that struck southern California in the past, would cause hundreds of millions of dollars in damage and enormous socioeconomic problems today. And it most likely would occur during a strong El Niño year.

Source: http://www.wrh.noaa.gov/sgx/newsletter/current-newsletter.pdf

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