Weather Watch

Rare southwesterly winds can wreak havoc on SLO County trees

Special to The TribuneFebruary 24, 2013 

Correction: The headline on this story initially referred to southeasterly winds; it should have said southwesterly.

Over the decades, I could count on one hand the number of times there has been a big southwesterly wind event. It is pretty rare and seldom seen in San Luis Obispo County.

The vast majority of wind events of more than 50 mph along the Central Coast either blow out of the northwest or southeast. Historically, winds along our coastline blow about 60 percent of the time out of the northwesterly quadrant, and about 23 percent of the time, the winds blow from the southeast. The Santa Lucia winds blow about 12 percent of the time from the northeasterly quadrant. The other 5 percent of the time, the winds are spread evenly across the rest of the cardinal headings.

On those once-in-a-while occurrences when we do experience big southwesterly wind events, the anomalous direction can cause havoc on our trees. Many of our windswept trees along our coastline have grown into a quasi-aerodynamic profile pointed away from the prevailing winds. If the winds blow out of the southwest, they hit the trees perpendicular to a wind direction they have grown into. Like a fully rigged sailing ship, the aerodynamic forces from southwesterly winds exert a greater amount of pressure on the trees and their root systems.

This leads to the question, what causes these unusual southwesterly winds along the Central Coast? On Jan. 2, 2006, an intense Pacific storm took an unusual track and moved eastward over the Big Sur coastline toward the Great Central Valley of California. This unusual track produced fierce southwesterly winds that day. The Diablo Canyon meteorological tower reported wind gusts to 53 mph. Closer to the storm’s center, the winds were even stronger; an anemometer in Cambria reported southwesterly winds of 75 mph. The wet ground from the storm’s heavy rains and southwesterly winds combined to thrash Cambria.

Tribune reporter Kathe Tanner wrote that the wind tumbled more than 100 aging, shallow-rooted Monterey pines onto homes, vehicles and power lines.

“Streets were blocked everywhere along Cambria’s often narrow, winding roadways, and the new year’s first day of school was canceled, because buses couldn’t get through the community to pick up the students,” she told me. “With so many trees down and power outages, it took time to document the path and intensity of the storm’s fury that had damaged 19 homes. Some were so badly damaged, county officials proclaimed them uninhabitable.”

On the other hand, the rare native stand of Monterey pine forest has special value for the Cambria community and ecologists statewide. The license plate frames with the Pines by the Sea logo are sported by many a proud resident.

Greg Saenz, PG&E vegetation program manager, told me that because of pressure from the spread of pine pitch canker, Cambria’s forest is a changing ecosystem. When a large mature tree dies, opportunities for young vigorous trees, which are less susceptible to pine pitch canker, open up.

“Most of these treasured trees have withstood disease, drought and storms over their lifetime, including gale-force winds. Unfortunately, every tree will die. Predicting when and how a tree will fall is a challenge that utilities take very seriously. PG&E patrols every overhead line annually to identify trees which may be a threat to public safety and electric reliability,” Saenz said.

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John Lindsey’s column is special to The Tribune. He is a media relations representative for PG&E and a longtime local meteorologist.

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