Weather Watch

Forecasters calling for more hurricanes this season. What it means for SLO County

Subtropical moisture Saturday morning at San Simeon.
Subtropical moisture Saturday morning at San Simeon.

El Niño typically reduces hurricane activity in the Atlantic Basin. As the jet stream tracks further south, it creates wind shear in the upper atmosphere, which can disrupt tropical cyclone formation. This may have been the reason behind the lower number of hurricanes than anticipated in the Atlantic basin over the past few years. However, this condition may be changing, and here’s why.

The Climate Prediction Center is calling for neutral conditions — the infamous El Nothing — that will continue into late fall.

With the “Spring Predictability Barrier” behind us, the chances are good that this forecast will verify. In a neutral condition, the amount of wind shear in the upper atmosphere tends to be diminished, aiding in the development of hurricanes. To make matters worse, near- or above-average sea-surface temperatures across the tropical Atlantic Ocean and the Gulf of Mexico will provide the energy needed for tropical cyclone genesis and intensification.

The Atlantic hurricane season runs from June 1 to Nov. 30 each year, with the peak on or around Sept. 10. The Eastern Pacific hurricane season runs a little longer, from May 15 to Nov. 30. Even though the Atlantic hurricane season is less than few weeks old, it’s already produced a greater number of tropical storms that we’ve seen in years.

The National Oceanic and Atmospheric Administration predicts 11 to 17 named storms this season; as many as nine of those may become hurricanes, of which four could be major hurricanes.

“Tropical cyclone” is the generic term for an organized system of convective clouds that rotate around an area of low pressure over tropical or subtropical waters. For these storms to strengthen, the seawater temperatures must be at least 80 degrees Fahrenheit or greater. That’s why it’s rare for tropical storms to reach the California coastline.

Nevertheless, it does happen. In 1939, a tropical storm slammed into Long Beach with gale-force winds and torrential rains. Mount Wilson reported 11.6 inches of rain in just 24 hours. In the Los Angeles area, 45 people drowned in the resulting flood, and high winds took 48 souls at sea.

Once these systems reach a sustained wind speed of 74 mph or greater, it’s classified as a hurricane, typhoon or cyclone. The only difference between a cyclone, hurricane or typhoon is the location in which the storm is formed. The term “hurricane” is used in the Atlantic and Northeast Pacific. In the Northwest Pacific, it’s called a “typhoon.” “Cyclones” happen in the South Pacific and Indian Ocean.

By the way, the first person to name tropical cyclones was Clement Lindley Wragge of Queensland, Australia, in the late 1800s. Wragge had an inclement temper, and he took to naming cyclones after politicians whom he disliked. A forecaster could publicly describe a storm named after a politician as “wandering about the Pacific with no aim or purpose.”

Last week, Tropical Storm Cindy produced huge amounts of rainfall and flooding as it pushed northward into Texas and Louisiana. High-level moisture from this tropical system made it to the Central Coast on Saturday morning with high-level clouds, a few sprinkles and lightning strikes west of Point Conception.

So what does an active hurricane season mean for the Central Coast with our Mediterranean climate, which normally means dry summers and wet winters? It could perhaps spell a more active North American monsoon this year. Sure, it may not be as glamorous as its Asian counterpart, but nevertheless, it can produce large amounts of rain in northern Mexico, much of Texas, New Mexico and southern Arizona. On rare occurrences, it can produce heavy rain in Southern and Central California also.

Typically, when the desert Southwest heats up in summer, it creates a thermal low. This low-pressure zone can change the direction of the jet stream, steering subtropical moisture northward toward the Golden State. When this pattern occurs, we often experience higher relative humidity levels and plenty of virga — observable streaks of rain that falls from a cloud but evaporates before reaching the ground.

If the subtropical moisture source is plentiful — especially from remnants of hurricanes and tropical storms from the eastern Pacific Ocean and the Gulf of Mexico — periods of rain and thunderstorms can interrupt our dry summers. As the summer progresses, the chance of receiving this type of rainfall becomes greater until it peaks during the month of September.

As the atmosphere and oceans continue to warm, long-range climate models are predicting the North American monsoon will become more common in our state in summer. Of course, the increased amount of rain is welcome; however, lightning accompanying these events can spark wildfires.

PG&E is serious about working with Cal Fire and other state and local agencies to do our part to reduce that risk — including daily aerial fire detection patrols across hundreds of miles of its service area. To learn more about safety, please visit pgecurrents.com.

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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