Weather Watch

Could rain — and big waves — be headed toward fire-ravaged California?

Big waves swamp both breakwaters at Port San Luis near the lighthouse in March 2014.
Big waves swamp both breakwaters at Port San Luis near the lighthouse in March 2014. Special to The Tribune

The two numerical models most trusted by weather forecasters — the European Center for Medium-Range Weather Forecasts and NOAA’s Global Forecast System — are advertising as of Saturday morning that an intense, 966-millibar storm with 70 mph winds will develop in the Gulf of Alaska and move eastward Monday into Tuesday.

If these models verify, the associated cold front will move through California on Thursday into Friday with some much-needed rain that would lower the fire-risk indexes and provide relief for our first responders battling fires throughout the state.

We would all love to see some rain Thursday and Friday, but since it’s predicted about 108 hours in the future, there’s less confidence that the models will verify. Model predictions more than 72 hours in the future are suspect; beyond 72 hours, you can either shake your fist at the sky in frustration or take the most reliable strategy — WAS, or wait and see.

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However, since this storm is expected to intensify in the Gulf of Alaska on Monday into Tuesday, or about 48 hours in the future, confidence is higher that eventually it will produce large waves along the California coastline Friday, and here’s why:

The four most important factors in the formation of waves that eventually arrive along the Central Coast are wind speed, wind duration, length of wind fetch and air and water temperature differences.

The stronger the wind speed, the higher the waves.

For example, a 20-knot wind can generate a 5-foot wave, while a 70-knot wind can create a 56-foot wave with the same wind duration and length of the wind fetch. The longer the wind blows, the more of its energy is transferred to the ocean, resulting in bigger waves.

The wind fetch is the area that winds blow over the ocean. If you blow over a hot cup of soup, the distance between the edges of the bowl equals the length of the wind fetches. Wind fetches in the Pacific can exceed 500 miles.

Air temperature is the final factor. Cold air is denser (heavier) than warm air and can transfer its energy to the ocean more readily than the less dense (lighter) warm air.

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So, what is the difference between seas and swell? They are both ocean waves.

Seas are usually shorter-period waves generated by the local winds. When the seas move out from the wind fetch, they become swells or longer-period waves. The longer the wave length of the swell, the faster it will travel across the ocean. As a general rule, the average speed of wave trains across the deep waters of the Pacific is about 25 mph or approximately 600 miles per day.

Some of these swells have traveled more than 7,000 miles across the Pacific before reaching our coastline. The expected swells from Monday and Tuesday’s Gulf of Alaska storm will take about 72 hours to arrive along the Central Coast.

The swell events in October through May mostly approach California from the northwest directions from storms in the North Pacific and the Gulf of Alaska. June through September, swell events often come from cyclones in the South Pacific. These swells travel along the Great Circle Route and can take as long as 11 days to reach the Central Coast.

Many of us visit the beaches to enjoy the waves along the Central Coast. But ask anyone who has spent significant time at sea, and I’m sure they will have their own stories of vexing experiences with waves.

Waves are powerful and full of energy, so regardless of whether they appear small or large, never turn your back to them and always be respectful of the ocean.

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I will be speaking at “Sharks After Dark” fundraising event at the Central Coast Aquarium in Avila Beach on climate change and its consequences for California on Thursday, Oct. 19, at 7 p.m. I would love to see you there.

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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