MoonSet

The supermoon is shown in December above Morro Rock. January will see two supermoons, on New Year's night and Jan. 31.

Contributed photo

There was only one supermoon event for all of 2017 and that occurred in early December.

January will see two supermoon occurrences. The first one, nicknamed a "wolf moon," will happen New Year’s night, followed by another supermoon Jan. 31 -- a rare red and blue moon combination, thanks to a partial lunar eclipse. A blue moon is the second full moon of the month. A red, blood or copper moon is the color of the moon during a lunar eclipse.

All these colored moons of January correspond to the highest tides of 2018. In fact, we won’t see predicted tides this high until December 2020.

In early December, I wrote about the 2017 king tide that occurred Dec. 4, when the sea reached 6.8 feet at the Port San Luis tide gauge.

On New Year’s Day and again Tuesday, the king tides will be even higher than December’s high tide or late January’s expected tides and here's why.

Unlike the king tides of early December and late January, the Earth will be at perihelion, when we will reach our closest point to the sun for all of 2018 on Jan. 2. Perihelion occurs during winter in the Northern Hemisphere and summer in the Southern Hemisphere. Aphelion is when Earth is farthest from the sun.

You see, our planet's orbit around the sun is an ellipse, a shape that can be thought of as a “stretched out” circle or oval. Not only is the Earth’s orbit around the sun an oval, but also the moon’s orbit around the globe. The moon is about 30,000 miles closer to Earth at perigee than apogee, when it’s farther away.

During the time between New Year’s Day and Tuesday, the sun, moon and Earth will line up in what is known as syzygy, but the moon will also be at perigee; and when this happens, astronomers called it perigee-syzygy! The gravitational forces exerted by the moon and sun create the timeless tides. These “tidal forces” are not the total gravitational forces exerted by the sun and moon on Earth, but the difference between these gravitational forces over the surface of the planet.

On Monday, the predicted tide will reach 6.9 feet at 8:29 a.m., followed by a minus low tide of -1.7 feet at 3:47 p.m. at the Port San Luis tide gauge. In other words, the level of the sea will shift 8.6 feet in a little over seven hours, followed by another 6.9 high tide at 9:16 a.m. Tuesday and a low tide at of -1.7 4:35 p.m. later that day. These flood and ebb tides will create ripping currents in California’s bays and estuaries.

On Monday, and especially on Tuesday, the predicted high tides could be higher than expected. Seawater temperatures are warmer than typical for this time of the year. When the water warms, it causes thermal expansion in the upper levels of the ocean. Consequently, seawater levels can be several inches higher than those predicted in the tide tables.

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Also, a vigorous upper-level low-pressure system off our coastline will produce increasing southerly winds and decreasing atmospheric pressure. Lower atmospheric pressure can create higher sea levels. The southerly winds can also produce storm surge along our shores.

All these factors together -- warmer seawater, southerly winds that can change the tides and low pressure -- could yield even higher water levels. To view the real-time Port San Luis tide and sea-level data, visit https://www.portsanluis.com

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Pacific Gas and Electric Co.'s commitment to clean energy took a big step forward in 2017. To learn more, visit http://www.pgecurrents.com.

John Lindsey 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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