Weather Watch

Tropical storm near Japan is creating huge waves. How will it affect SLO County's swell?

Longer-period swell arriving along the San Luis Obispo County coastline.
Longer-period swell arriving along the San Luis Obispo County coastline. Special to The Tribune

Tropical Storm Maliksi, currently off the southeastern coast of Japan, is expected to become an extratropical cyclone as it travels northeastward toward the middle latitudes (between 30 and 40 degrees) of the Earth and away from the tropics. Even though it’s on the other side of the Pacific, we may see the waves it generates along our coastline. Here’s why.

First, what is the difference between a tropical cyclone and an extratropical storm?

A tropical cyclone is a generic term for an organized system of convective clouds that rotate around an area of low pressure over tropical or subtropical waters such as a tropical depression, tropical storm, hurricane, typhoon or cyclone. The term “hurricane” is used in the Atlantic and Northeast Pacific. In the Northwest Pacific, it’s called a “typhoon,” and “cyclones” happen in the South Pacific and the Indian Ocean.

For these storms to strengthen, the ocean temperatures must be at least 80 degrees Fahrenheit or warmer. The eyewall of a hurricane is its engine and warm seawater its fuel. The warmer the water, the faster it will intensify. Tropical cyclones derive most of their energy from the release of “latent heat” when water vapor is condensed into liquid water, which is a warming process.

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Check out the waves at Port San Luis, Avila Beach and Pismo Beach on Saturday, January 21, 2017. Some swells were as high as 20 feet. A storm Sunday is expected to bring high winds and 2 to 4 inches of rain.

You may have noticed that it gets warmer right before it rains. They typically have minimal temperature differences on the horizontal plane across the storm. Physically, tropical cyclones have their strongest winds near the earth’s surface and don’t have fronts (warm, occluded or cold) associated with them.

On the other hand, extratropical storms get most of their energy from the temperature differences across the horizontal plane (baroclinic) when cold- and warm-air masses interact; unlike a tropical cyclone, they have colder air at their core. Naturally, these tempests have associated warm, occluded or cold fronts. Typically, their strongest winds are well above 500 millibars (18,000 feet) near the top of the tropopause, interacting or becoming part of the jet stream.

As tropical cyclones, like Tropical Storm Maliksi, travel northward, they transform into an extratropical or a mid-latitude cyclone as they get caught up in fronts or upper-level low-pressure systems.

Even though they may no longer be a typhoon or hurricane, these mid-latitude storms can be some of the most powerful on Earth as relatively warm air from the south moves northward and cold air from the north travels south, producing a cyclonic wind shear. That’s like your two hands going in opposite directions as they roll over a pencil between them, often creating lower pressure. The lower the air pressure usually the stronger the storm that will result. When pressure drops fast enough, meteorologists refer to this explosive development as a meteorological bomb.

Extraordinarily strong winds from these mid-latitude storms blowing across the ocean can generate unimaginably high seas, well over 50 feet. When these seas move out from under these winds (wind fetch) they became longer-period swells.

Consequently, the ocean waves generated from the forecasted extratropical cyclone that started as Tropical Storm Maliksi should arrive along our coastline from the west-northwest at 2 to 4 feet (with a 19- to 21-second period) on June 16 and 17.

John Lindsey’s column is special to The Tribune. He is PG&E’s Diablo Canyon marine meteorologist and a media relations representative. Email him at pgeweather@pge.com or follow him on Twitter: @PGE_John.
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