Like a gigantic pinwheel, an intense 965-millibar storm with hurricane-force winds in the Gulf of Alaska last week moved into the Pacific Northwest. These winds blew for more than 36 hours over a wind fetch — the distance the wind blew over water — that exceeded 400 nautical miles.
The friction between the wind and the ocean’s surface generated 48-foot seas. The greater the wind speed, the higher the waves.
When these high seas moved out from under the winds, they became longer-period swells. By late Wednesday night, the National Oceanic and Atmospheric Administration West Oregon marine buoy, floating 275 nautical miles west of Coos Bay, reported a 42-foot significant swell height with a 17-second period.
These huge waves continued to travel in a southeasterly direction and reached the NOAA West California buoy, 357 nautical miles northwest of San Francisco, at 35 feet in height with a 17-second period and remained at this height for hours afterward.
The West California buoy is sometimes called the 24-hour buoy, because it takes about a day for swell trains from the Gulf of Alaska to reach the Central Coast. Like clockwork, the biggest waves of 2015 slammed into our coastline Friday morning. The offshore buoys along our coastline reached nearly 25 feet in height that morning. Because of refraction and diffraction as these waves bent around Point Buchon near Montaña de Oro State Park, wave heights at the Diablo Canyon Waverider buoy were lower, reaching about 17 feet.
Like the perfect storm, the peak of Friday morning’s wave event corresponded to a 6-foot morning high tide and flooded many coastal areas, including the parking lot at Spooner’s Cove in Montaña de Oro State Park.
A powerful Pacific storm is expected to develop along the Aleutian Islands on Sunday. The storm is too far away to produce any unsettled weather along the Central Coast, but nevertheless, we will see the effects of it along our coastline in the form of ocean waves.
This storm will develop as the polar jet stream brings glacial air from Siberia west-southward toward the western Pacific, while relatively warm air at the Earth’s surface from the south moves northward, producing a cyclonic wind shear. Like two hands going in opposite directions as they roll a pencil between them, these intersecting air masses will liberate unfathomable amounts of latent heat as water vapor condenses into clouds and precipitation. This heat causes surface air to expand and quickly rise into the sky, which decreases air pressure.
When pressure drops fast enough, meteorologists refer to this explosive development as a bomb. This storm is expected to become a bomb and produce hurricane-force winds. The storm’s surface pressure should drop to 936 millibars. These barometer readings rival that of hurricanes.
These longer-period waves, also called forecast forerunners, from this storm with intervals of more than 27 seconds will arrive along our coastline Tuesday night, peaking Wednesday at 9 to 11 feet with a 20-second period. These extraordinarily long-interval waves will travel across the vast expanses of the Pacific at more than 30 knots.
Even though the wave height is not particularly significant, the extremely long period of this swell will create strong rip currents and large individual sets of waves between intervals of relatively calm ocean conditions. Unfortunately, these relatively calm conditions can lull beachgoers into a false sense of security. If you’re venturing to the coast, please remember to never turn your back to the ocean and watch these waves from a safe distance.
John Lindsey’s column is special to The Tribune. He is the PG&E Diablo Canyon meteorologist and media relations representative. Email him at email@example.com.