Weather Watch

Mechanical air mixers help growers fend off frost fiend

Growing up in Sonoma County, I often woke up to the noise of wind machines in the vineyards late at night and early in the morning. They came on during the late winter and early spring to provide frost protection.

The wind machines mixed the cold air at the surface with the warmer air above, which can prevent frost from developing. I took a job in high school lighting kerosene-fueled orchard heaters when the temperature dropped near freezing. Suffice it to say, I didn’t get a lot of sleep that March.

Years later, a friend in the Navy Reserve who owned a helicopter company got a contract to fly helicopters up and down the rows of vineyards during the dark hours of late winter and early spring.

Helicopters hovering above the vineyard can mix out the temperature inversion layer, thus raising the temperature near the vines.

During winter in San Luis Obispo County, it is not uncommon to experience frost during late-night and early-morning hours. The longer nights allow more of the atmosphere’s heat at the surface to radiate out in space, especially during clear, dry and calm conditions.

When conditions are calm, denser cold air flows downward alongour mountain slopes and accumulates in the valleys. Even when the land is only gently contoured, the cold air will accumulate in the low-lying areas. Often our valley floorswill be much colder than the surrounding hillsides. This condition can leadto below-freezing temperatures.

Below-freezing temperatures at this time of the year can be a major problem for our local grape growers. Some of our vineyards are already reporting bud break. If bud break occurs too early, the young shoots may be vulnerable to frost damage.

John Salisbury of Salisbury Vineyards in Avila Valley said below-freezing temperatures can ruin an entire year’s crop of grapes and can even kill the vines. Many growers use sprinklers for frost protection, Salisbury told me.

“The resulting ice protects the grapes by holding the temperature of the vine at 32 degrees,” he said.

To save water, Salisbury discs the rows between his vines.

“The resulting dark soil absorbs the heat fromthe sun during the day and releases it at night. Often this is all the difference we need becausethis can give us an extra two degrees of protection,” he said.

Ironically, this year, the ground is too saturated to disc and many growers may need to use water for frost protection.

This week’s forecast

A 1,030 millibar surface high, centered about 500 miles off the Central Coast, will keep the storm track north of our area through this week.

Another surface high over the Great Basin should produce an offshore flow during the night and morning hours.

After a clear and frosty morning, the first morning of Pacific Daylight Saving Time, the north to northeasterly (offshore) winds will warm up the temperatures nicely by this afternoon.

Temperatures will reach the high 60s in the interior to the low 70s in the coastal valleys. The afternoon northwesterly winds will keep most of our beaches cooler, except for Avila Beach and Cayucos, which should also reach the low 70s.

Tonight’s overnight lows shouldn’t be as cold as this morning’s, but a few areas may drop below the freezing level.

Monday and Tuesday will be sunny and warmer, with highs reaching the mid-70s throughout the Central Coast.

Dry and mostly sunny weather will continue throughout this upcoming week with near- to above-normal temperatures.

Look for another wet weather pattern to develop late the following week as the storm track shifts southward.

Surf and sea report

This morning’s 9- to 11-foot northwesterly (315-degree deep-water) sea/swell (with a 7- to 14-second period) will decrease to 7 to 9 feet by this afternoon and tonight.

A 5- to 7-foot northwesterly (295-degree deep-water) swell (with an 11- to 13-second period) will develop on Monday.

A 963 millibar Gulf of Alaska storm will produce an 8- to 10-foot west-northwesterly (295-degree deep-water) swell (with a 16- to 18-second period) along our coastline on Tuesday, increasing to 11 to 13 feet on Wednesday.

Combined with this northwesterly swell will be 3- to 5-foot northwesterly (320-degree shallow-water) seas on Tuesdayafternoon through Wednesday.

This swell will decrease to 9 to 11 feet (with an11- to 15-second period) on Thursday, further lowering to 5 to 7 feet on Friday.

Recent gale-force northwesterly winds have produced a great amount of upwelling along our coastline. Consequently, seawater temperatures have decreased.

Seawater temperatures will range between 50 and 53 degrees through today, increasing to 51 to 54 degrees on Monday through Wednesday.

Preliminary analysis long-range forecast:

A high west-northwesterly swell could arrive on our coastline on March 24.

Conservation tip

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John Lindsey is a media relations representative for Pacific Gas and Electric Co. He is also a local weather expert and has lived along the Central Coast for more than 23 years.