Like a gigantic pinwheel, an intense storm rotated over the Pacific a few hundred miles west of San Luis Obispo on Friday, Feb. 28.
This was one of the most powerful Pacific storms that I've seen off the San Luis Obispo County coastline in my meteorological career. This tempest developed as the southern branch of polar jet stream brought 150-knot-plus upper-level winds from the western Pacific, while relatively warm air at the Earth’s surface from the south moved northward, producing a cyclonic wind shear.
Like two hands going in opposite directions as they roll a pencil between them, these intersecting air masses liberated great amounts of latent heat as water vapor condensed into clouds and precipitation.
This heat caused surface air to expand and quickly rise into the sky, decreasing air pressure. In less than 24 hours, this storm’s surface pressure went from 996 millibars (mb) down to 968 mb. Indeed, these barometer readings were some of the lowest ever seen so close to the Central Coast.
When pressure drops this fast, meteorologists refer to this explosive development as a bomb. Ships that ventured across the storm’s path reported 50-knot west-southwesterly winds on the back side of it. Closer to the storm, southwesterly winds were estimated to be near hurricane-force levels.
These winds blew for more than 24 hours over a wind fetch (distance of water over which the wind blows) that exceeded 150 nautical miles. The greater the wind speed, the higher the waves. The friction between the wind and the ocean’s surface generated 40-foot seas.
When these high seas moved out from under these winds they became longer-period swells. As the storm moved closer to our shoreline at 4 p.m. Feb. 28 it became obvious that this deep low pressure system would produce one of the larger southwesterly wave events that we’ve ever seen.
Consequently, the ocean waves generated from this storm would approach our rugged coastline from the southwest as opposed to the normal northwesterly direction. You see, northwesterly swells bend around Point San Luis and diminish in height due to refraction and diffraction.
This phenomenon creates a shadow zone in San Luis Bay where the Harford, Cal Poly and Avila Beach piers are located. The same condition also occurs at the Cayucos pier.
This is an important reason why these piers were built in their current locations and why they remain standing over the decades. The piers are usually the most sheltered along the Central Coast from damaging waves, but on March 1 no location was immune.
The swells, combined with locally-generated southerly seas, arrived from the southwest that morning and peaked at 21 feet with a 14 second period at the Diablo Canyon Waverider Buoy a few miles north of Avila Beach.
This was one of the highest wave events from this southwesterly direction since March 1, 1983. On that date 31 years ago, an El Niño-driven storm destroyed the 2,700-foot-long wooden Unocal Pier in Port San Luis. It was later replaced with a steel-and-concrete structure in 1984, which became the Cal Poly Pier.
It was estimated that on March 1, 1983, significant swell height (the average height of the waves in the top third of the wave record) reached 27 feet with a 19-second period at the Diablo Canyon Waverider Buoy.
Like the perfect storm, the peak of this recent March 1 morning wave event corresponded to a 5.9-foot morning high tide and perhaps several feet of storm surge generated from the moderate gale-force to fresh-gale force (32- to 46-mph) southeasterly winds that lashed the shoreline.
Harbor Manager Steve McGrath said, “the Port San Luis Harbor Patrol and Maintenance Department folks did an amazing job working with Cal Fire to ensure public safety.”
Harford Pier was mostly fine but the Avila Beach Pier did sustain damage. The Cayucos pier came through the storm intact thanks to about $230,000 worth of above-the-deck reinforcing completed only about six weeks before.
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