Weather Watch

Central Coast headed for a big weather transition; what it means for the rain season

The jetstream produced standing wave clouds, also referred to as mountain clouds, Oct. 19 before a cold front produced rain the following morning. The jetstream is expected to shift southward again over the Central Coast on Friday into the following weekend.
The jetstream produced standing wave clouds, also referred to as mountain clouds, Oct. 19 before a cold front produced rain the following morning. The jetstream is expected to shift southward again over the Central Coast on Friday into the following weekend. Special to The Tribune

If you don’t like the weather along the Central Coast, wait a few days and it will probably change. That’s certainly the case with this week’s whiplash weather pattern as we transition from T-shirts and shorts to long pants and jackets.

Last week saw a large area of high pressure develop into the Great Basin, while at the same time, a trough of low pressure developed along the coastline to produce a steep pressure gradient between the land and ocean. This condition created 50-plus mph Santa Lucia (northwesterly/offshore) wind gusts at Diablo Canyon with even stronger winds in other coastal canyons and passes.

Not only did these winds create bone-dry relative humidity levels, but they also resulted in record-breaking temperatures throughout the region. The Diablo Canyon meteorological towers have been tracking air temperatures since 1976, and Tuesday reached 99 degrees, which tied the record for that site. Last Monday through Wednesday, both San Luis Obispo and Santa Maria airports broke their daily high-temperature records with most Central Coast locations reaching triple digits. The Paso Robles airport set new daily temperature records Wednesday and Thursday.

By Friday, the Santa Lucia winds and high-pressure ridge that has brought the record-breaking temperatures across the Central Coast weakened, and the marine layer with areas of dense fog and mist redeveloped along the coastline, sending temperatures plummeting along with the beaches. In fact, Saturday’s high air temperature at Diablo Canyon was only expected to reach 58 degrees under overcast skies.

Even colder temperatures and significant rain is forecast later Friday into next weekend, and here’s why. The Eastern Pacific high will weaken and move northward, which will allow the upper-level (jetstream) winds to track farther southward toward California. A low-pressure system is forecast move southward out of the Gulf of Alaska into Central California on Friday into next weekend. This potential storm will continue the cooling trend, but more importantly, it will bring increasing southerly winds and rain across the Central Coast with snow in the Sierra Nevada.

Please remember, these are long-range charts, and the timing and intensity of the system will probably change. In any event, it appears a significant weather pattern change will evolve later next week.

This leads to the question of what will the rest of the rain season be like? Since 1950, the National Oceanic and Atmospheric Administration has used sea surface temperatures (SST) in a central equatorial region of the Pacific Ocean called Niño 3.4 as the standard for classifying El Niño (warmer-than- normal SST) and La Niña (cooler-than-normal SST) events.

The fortune-telling SST cycles in Niño 3.4 are categorized by the amount they deviate from the average SST. A weak El Niño is classified as an SST deviation of 0.5 to 0.9 degrees Celsius, while a strong El Niño ranges from 1.5 to 1.9 degrees Celsius. A very strong El Niño anomaly is anything above 2 degrees Celsius (or 3.6 degrees Fahrenheit). On the other side of the coin, the cooler La Niña is categorized in the same manner but with minus degrees from normal.

This year, seawater temperatures in this region are forecast to range between -0.5 and 0.5 degrees Celsius, not warm enough to be classified El Niño or cold enough to be La Niña. In other words, it’s the El Nothing — or, officially, a neutral condition through December. Then it is expected to decrease to a weak La Niña for the first half of 2018.

However, another tremendous oceanographic cycle that can orchestrate changes in our weather is a longer lasting cycle called the Pacific Decadal Oscillation or PDO. According to climatologist Bill Patzert of NASA’s Jet Propulsion Laboratory in Pasadena, the PDO shifted to the positive in 2014. Historically, the positive phase of the PDO typically enhances the effects of El Niño and La Niña events. Traditionally along the Central Coast, El Nothing combined with a positive PDO has produced near average amounts of rainfall.

I still think we will receive near average amounts of precipitation this rain season, which started July 1 and continues through June 30, 2018.

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John Lindsey’s column is special to The Tribune. He is PG&E’s Diablo Canyon marine meteorologist and a media relations representative. Email him at pgeweather@pge.com or follow him on Twitter: @PGE_John.

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