Conditions aligning for Bay Area to expect highest King Tides in 18 years

King tides bring coastal flooding to the Bay Area every winter. But this fall and winter, the region is expected to see the highest tides in 18 years. 

Why does this year have the potential for near all-time high water levels? It's the combination of a few things, not only El Niño. 

First, tides are caused by the sun and moon's gravitational pull on Earth. Earth reaches its closest orbit to the sun in early January, which means the sun has a slightly stronger pull on our tides in the winter. 

During full or new moons — which occur when the Earth, sun, and moon are in alignment — tides are slightly higher. But once about every 28 days, the moon reaches a 'perigee,' its closest point of approach to the Earth. This is the point at which the gravitational pull of the moon is strongest, so a new or full moon in the winter, when the moon is closest to Earth and the Earth is closest to the sun, normally creates the highest tides of the year, also known as King Tides. The Bay Area sees these every year. 

But there's also a slow wobble in the tilt of the moon's orbital plane relative to Earth's equator that cycles every 18.6 years. During the first half of the cycle, the tidal range is stretched out, meaning high tides become higher and low tides become lower. The second half of the cycle features the opposite: Tidal ranges are compressed, with high tides lower than usual and low tides higher than usual. Right now, it's in the stretched-out or amplified stage of that lunar cycle, meaning there are higher high tides. 

Then, there's also the very strong El Niño and ongoing California marine heatwave that will continue to bring warmer-than-normal water temperatures this winter, which causes thermal expansion. As water warms, it literally expands and takes up more room. Plus, baseline sea levels are at least 8" higher due to climate change over the past century. 

In San Francisco, the highest tide on record is 8 feet, which occurred on Jan.27, 1983. Original predictions were 6.8', but a 2' storm surge brought the record height. 

This December's King Tides are forecast to be the highest of the winter, with a predicted height of 7.33 feet. That's higher than 1983's original forecast, and if 2 feet of storm surge arrives during the King Tides, it will definitely break records.