Weather Watch

Windy spring really blew meteorologist away

Persistent and strong northwesterly winds have allowed for fog-free moonrises along the beaches of San Luis Obispo County, this one south of The Abalone Farm in Cayucos.
Persistent and strong northwesterly winds have allowed for fog-free moonrises along the beaches of San Luis Obispo County, this one south of The Abalone Farm in Cayucos. Special to The Tribune

Correction: The photo captions in an earlier version of this story had been inadvertently switched, mislabeling a sunset as a moonrise and vice versa.

Summer is 10 days away, a new rain season will begin on July 1, and with the California Mid-State Fair just around the corner (July 18-29), I thought it would be interesting to look at the long-range forecast.

The La Niña condition (cooler than normal ocean water in the equatorial and eastern Pacific) has dissipated. La Niña shifted the position of the jet stream farther north, thereby directing more Pacific storms toward the Pacific Northwest this winter and leaving the Central Coast with below-normal rainfall. In its place, an El Niño/Southern Oscillation (ENSO)-neutral condition developed in May.

This condition may have helped to create some of the strongest and persistent springtime northwesterly winds that I have seen along the coastline.

From May 16 through May 31 peak afternoon northwesterly winds reached at least 40 mph every day. A few days saw the winds reach more than 50 mph at the Diablo Canyon nuclear power plant meteorological tower. On May 23, the northwesterly winds reached 60 mph at Point Buchon. Very windy, indeed!

Why does air move? In general, winds develop because of uneven heating of Earth’s surface by the sun, which produces pressure differences.

Air will flow from areas of high pressure to areas of low pressure. Think of an air leak from a bicycle tire (high pressure) flowing out to the atmosphere (low pressure).

In my daily weather forecast, I often refer to these differences in pressure as gradients. The steeper the pressure gradients, the stronger the winds are. These pressure gradients were especially steep this past May.

These relentlessly persistent winds produced a lot of fog-free days along the coast. You may find this unusual, but I love the fog. Personally, I’ll take June gloom any day over the winds. Of course, a lot of wind and kite surfers would passionately disagree.

Not only have these winds created plenty of fog-free days, but combined with an ENSO-neutral condition, they produced a tremendous amount of upwelling. This upwelling brought cold water up from the ocean depths along the immediate coastline.

Over the past few months, ocean water conditions have been bone-chilling. Normally, the seawater temperature at Diablo Canyon averages 52.1 degrees during the month of May. This past May, the average was near 50 degrees.

The seawater temperature recorders that keep track of these temperatures are exchanged, downloaded and calibrated on a continuous basis. Over the decades, numerous marine biologists have journeyed to the tide pools along the Pecho Coast and swapped these recorders that measure seawater temperatures.

If the conditions are right at night, bioluminescence from plankton blooms and/or the deep scattering layer can mark their track across the cool, wet and slippery rocks of the littoral.

These below-normal seawater temperatures may be coming to an end. The National Oceanic and Atmospheric Administration’s Climate Prediction Center in Camp Springs, Md., has issued an El Niño watch (warmer-than-normal ocean water in the equatorial and eastern Pacific) for late summer into the fall and winter as ocean water is expected to continue to warm.

This shift can have profound ramifications all around the world.

There is a respectable chance that the Central Coast will experience above-normal temperatures this summer. The average maximum temperature at the Paso Robles airport during July is 94 degrees, while San Luis Obispo is cooler, with an average July maximum temperature of 78 degrees at Cal Poly.

Along with the higher temperatures, an El Niño condition can often produce above-normal rainfall. However, another large-scale ocean water temperature cycle continues to lurk in the North Pacific, and it could produce lower-than-average winter rainfall.

This other large-scale ocean water temperature cycle is the Pacific Decadal Oscillation (PDO), which can increase or decrease the El Niño effect. The PDO is found primarily in the North Pacific. The phases of the PDO are called warm phases or cool phases.

Unlike El Niño and La Niña, the PDO stays in one phase for an extended period — between 10 and 40 years.

The El Niño and La Niña phases usually last for about a year or so. It appears that we are still in the cool phase of the PDO, which means it could cancel out the El Niño effect and produce below-normal rainfall. These are long-range forecasts and should be taken with a grain of salt. Only time will tell the story.

An El Niño condition can also help to produce a less active hurricane season as it increases the hurricane-killing wind shear in the upper atmosphere that often shuts down any tropical cyclones that try to form in the Western Atlantic and Eastern Pacific oceans.

This week’s forecast

A ridge of high pressure along the Central Coast will shift the wind fields to the north. San Luis Obispo County residents will wake up to clear and sunny skies this morning.

Today’s maximum temperatures will reach the low to high 60s along the northwesterly facing beaches (Morro Bay, Los Osos, Montaña De Oro and the Nipomo Mesa); the low 70s along the westerly facing beaches (Pismo Beach); and the mid-70s along the southwesterly facing beaches (Cayucos, Avila and Shell beaches).

Maximum temperatures should reach the high 70s in the coastal valleys and the low 90s in the North County. Temperatures are expected to climb even higher Monday. Relative humidity levels will continue to drop over inland areas; thus, fire danger is high.

Marine low clouds and fog are expected to return along the coastline Monday night and move into the coastal valleys by Tuesday morning. The temperature inversion layer will remain shallow; consequently, the North County will remain clear and warm, while the coastal valleys and beaches will began to cool.

At this time, it appears that strong to gale-force (25- to 38-mph) northwesterly winds will return along the coastline Wednesday and continue through Friday.

There is some uncertainty in the long-range forecast as models begin to diverge, but the current thinking is that temperatures will gradually cool into the following weekend.

Surf and sea report

This morning’s 6- to 8-foot northwesterly (290-degree deep-water) swell (with an 8- to 11-second period) will decrease to 5 to 7 feet along the Pecho Coast this afternoon through tonight. This northwesterly swell will further lower to 4 to 6 feet (with an 8- to 11-second period) Monday through Tuesday.

Strong to gale-force (25- to 38-mph) northwesterly winds will develop along the Pecho Coast on Wednesday and continue through Saturday.

These winds will generate a 6- to 8-foot northwesterly (310-degree deep-water) sea and swell (with a 5- to 9-second period along our coastline Wednesday, further increasing to 7 to 9 feet Thursday and remaining at this height and period through Friday.

An extratropical storm off the northern Japanese coastline will move northwestward. A 2- to 4-foot west-northwesterly (285-degree deep-water) swell (with a 16- to 18-second period) from this storm will arrive along our coastline Wednesday about the same time as the locally generated northwesterly sea and swell builds.

Today’s seawater temperatures will range between 49 and 51 degrees, increasing to 51 and 53 degrees Monday and Tuesday.

Another round the northwesterly winds will produce cooler seawater temperatures Wednesday through Friday.

Learn about Diablo

Planned refueling outages at Diablo Canyon provide a great amount of economic benefit for San Luis Obispo County. To learn more, please log into www.pgecurrents.com.

John Lindsey, media relations representative for PG&E and local meteorologist, has lived along the Central Coast for more than 25 years. To subscribe to his daily weather forecast or ask him a question, email him at pgeweather@pge.com.

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