(The following description is from the Washington King Tide Program at the University of Washington)
In the simplest terms, king tide is colloquially used to describe an extremely high tide. Ordinary tides are caused by the gravitational pull between the Earth and the moon; king tides happen when astronomical events amplify that pull.
These astronomical events come in a couple different flavors. One type of king tide starts with a spring tide, which takes place when the Earth, moon and sun are aligned. While the moon generally has a bigger influence on Earth’s tides, the sun also has gravitational effects. When the moon and sun are aligned, their gravitational effects compound, and the high tides become a little higher. Spring tides occur twice each lunar cycle (when the moon is either new or full). These already higher-than-average tides are exacerbated when spring tides coincide with the moon in perigee, which means the moon is particularly close to Earth and so has an even greater gravitational pull on the ocean. The perigee happens at the same time as a spring tide three or four times a year in both spring and fall, creating the most common type of king tide known as perigean spring tides.
King tides also happen when the sun is closest to the Earth in its orbit – a position called perihelion. The sun reaches this position in early January each year. Similarly, in early July, the sun is furthest away from Earth – a position called aphelion – and the gravitational pull is weakened, resulting in smaller tides.
In Whatcom County we pay particular attention to King Tides that occur in the late fall / early winter as many times these coincide with, and can be aggravated by, our wind storms. This has resulted in significant impacts in our coastal communities, such as occurred in Birch Bay and Blaine in December of 2018 when over 5 million dollars in damage was caused by a King Tide and wind storm. We define a King Tide as a tide of at least 10.1 at Cherry Point (Whatcom Counties official tide station). For those who are tide watchers, these are pretty impressive tides and fun to go see when they happen. However, when a storm is added to a King Tide as happened in the 2018 storm, the tides were pushed nearly two feet higher from the storm pressure (called storm surge) and then the west wind added another 3-4 feet of waves. It is for these reasons that the Whatcom County Sheriff's Office Division of Emergency Management, along with significant support from the National Oceanic and Atmospheric Agency, keeps a close eye on King Tides.
Over the next couple months, we will have King Tides on the following days:
Anyone mariner who travels our waters knows that the tides in along every shoreline in Whatcom County are slightly different. For example, on December 16th, the highest King Tide of the year (10.8 feet at 0736) occurs. The 10.8 feet is for the NOAA Cherry Point Tide Station. However, at Gooseberry Point the high tide is 10.3 feet at 0720. In Bellingham it is 9.8 feet at 0722, and in Blaine it is 11.0 feet at 0730. The reasons we see a difference has to do with the shape of the coast, depth of water, height of tide gauge and a host of other factors. However, for our planning and baseline we use Cherry Point.
For more information on King Tides, and to help scientists build a library of coastal impacts, you can visit the Washington King Tides program at the University of Washington.