California

Tarantulas are appearing across the Central Coast. What does that mean for the rainy season?

It’s tarantula season in SLO County

It may be pumpkin spice season for some, but fall is more like Valentine's season for tarantulas. Males are out seeking females. Ron Ruppert, the Cuesta College Biology Division chair, explains.
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It may be pumpkin spice season for some, but fall is more like Valentine's season for tarantulas. Males are out seeking females. Ron Ruppert, the Cuesta College Biology Division chair, explains.

This time of the year, most folks are yearning for the cold and rain of winter. It reminds me of the time frame between the end of February into the beginning of March where everyone can’t wait for the sun and heat of summer.

With that said, I can hardly believe that we are already in September, which on average loses more than two minutes of daylight each day at our latitude.

Further north the loss of sunlight is much faster as the Northern Hemisphere speeds into winter. At the North Pole, the sun will set at the fall equinox on Sept. 22 and will not rise till the spring equinox in March. The polar night will spread southward until the winter solstice on Dec. 22 when it will cover the entire Arctic Circle.

This profoundly uneven loss of sunlight between the North Pole and the equator due to the Earth’s 23.5-degree axle tilt results in colossal air temperature differentials between the north and south during the later fall, winter and early spring.

This condition gives rise to the polar jet stream which often forms the border between the frigid air to the north and the warmer air to the south. As a rule of thumb, the higher the temperature differential between these two air masses the faster the upper-level winds will blow.

The faster the jet stream, the more direct route it takes. Like the momentum of a fast-moving aircraft carrier, it takes a greater amount of time to change direction if at all. The slower the jet stream, the more likely it will tend to change course.

In California, the most advantageous pattern for precipitation is for the upper-level winds to travel in a direct path across the Pacific to the California coast, smashing through the Eastern Pacific High and dragging low-pressure systems and associated cold fronts through the Central Coast. In other words, as the mid-latitude westerly winds increase, the higher the probability of rainy days.

This condition is the main reason for our dry and wet seasons or Mediterranean climate.

According to the website https://weatherspark.com, the wettest part of the year for the Central Coast is from Nov. 19 through April 3, with about a 12 percent chance of rain on any given day with the probability of a wet day peaking at 24 percent on Feb. 20.

Which leads to the question of what will this rain season bring?

So far, I’ve been noticing a marked increase in storm activity in the northern Pacific. This winter, the Climate Prediction Center is advertising that a weak or even perhaps a moderate El Niño could develop. The most important aspect of this prediction is how strong of an El Niño event this will eventually become. Typically, along the Central Coast, the higher the classification of the El Niño event, the more rain it will create.

Unfortunately, a weak El Niño condition, which like neutral conditions — or El Nada — typically doesn’t produce any reliable seasonal rainfall predictions along the Central Coast, but a moderate El Niño if it develops does. As of this week, the Climate Prediction Center is forecasting a 33 percent probability of the above average precipitation during the months of September, October and November.

Many Central Coast residents have reported a great emergence of tarantulas, the big and beefy hairy prognosticator of weather. According to longtime residents, you can expect rain in about three weeks.

It may be pumpkin spice season for some, but fall is more like Valentine's season for tarantulas. Males are out seeking females. Ron Ruppert, the Cuesta College Biology Division chair, explains.

Unfortunately, the relentless warming of the oceans and atmosphere have slowed the average speeds of the polar jet and increased the strength of high-pressure systems along the West Coast. Some climate scientists believe this is having a more significant effect than the El Niño Southern Oscillation. On the other hand, a warmer atmosphere can hold more water vapor, meaning we could see more intense rainfall events. If nothing else, it certainly has made long-range weather forecasting much more difficult.

In 2017, nearly 80 percent of the electricity that PG&E provided to our customers came from sources that are renewable and/or emit no greenhouse gases.

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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