Weather Watch

Frost’s impact on Central Coast agriculture

The sound of shoes crackling on top of frozen grass is something that resonates in many of us as a symbol that Old Man Winter is headed our way. The frost can sure transform the otherwise soft blades of green under our feet, and it can also have a big impact on the fields that provide produce along the Central Coast.

In fact, George and Beth Kendall of Dos Pasos Ranch near Cambria decided not to grow summer squash this year after experiencing an early October frost over the previous two years. “As soon as the temperature drops to 32 degrees, it burns the leaves and kills the zucchini,” George Kendall said.

So far this year, only the North County has experienced a frost, which has been a blessing for farmers who grow avocados, tomatoes, grapes and other summer crops on the south side of the Cuesta Grade. But as we move toward winter, the chances of the first frost will increase dramatically.

The progressively longer nights of the fall season will allow more of the atmosphere’s heat at the surface to travel out to space, especially during clear, dry and calm conditions.

During these periods, denser cold air flows downward along our mountain slopes and accumulates in the valleys. Often, our valley floors will be much colder than the surrounding hillsides.

The colder weather of winter is essential for many fruit tress.

“In the late 1800s, the western one-third of Santa Rosa Valley was known for its stone fruits — peaches, apricots, cherries and apples; the cold air pockets found in this 3-mile stretch aided in the fruits’ development of rich flavors,” said Dawn Dunlap of Walter Ranch near Cambria.

At night, the temperature of plants is often lower than the surrounding air. The water vapor that comes in contact with these cold surfaces condenses upon them, forming dew. Frost develops when the dew point temperature is at or below freezing.

Most farmers and ranchers are rightfully concerned about frost as an unexpected freeze can destroy their crops.

According to the University of California Agriculture and Natural Resources corroborative extension in San Luis Obispo County, the first frost of the season in Paso Robles historically occurs by Nov. 7 — with the first hard frost by Nov. 15. A hard frost happens when the temperature drops below 28 degrees Fahrenheit. Normally, the last hard frost of the season happens around the first week of March, and the last frost of the year occurs during the first week of April.

Closer to the ocean in the coastal valleys, San Luis Obispo will usually have its first frost Dec. 31 and last frost by Feb. 15. These dates hold true at our beaches as well.

In the far eastern regions of San Luis Obispo County, Cuyama usually has its first frost by Oct. 7 and its first hard frost by Oct. 20. On average, Cuyama has to wait all the way until April 20 for the last frost of the season. However, temperatures in that part of the county have been known to drop below freezing as late as early May.

This week’s forecast

A strong 1,038 millibar Eastern Pacific High will remain nearly stationary about 900 miles to the west-northwest of San Luis Obispo through Wednesday. This is a stable weather pattern, and an extended period of warmer and dry weather with plenty of sunshine is expected for the Central Coast as the jet stream is pushed far to the north in an Omega type of pattern.

Today’s high temperatures will range between low to high 70s throughout San Luis Obispo County. A clear and cool night will once again develop in the North County with temperatures reaching as low as the mid- to high 30s.

A strong north to south surface pressure gradient will continue to give gentle to moderate (8- to 18-mph) northeasterly (offshore) winds, with higher gusts, especially in the eastern regions of San Luis Obispo, Cal Poly and the coastal canyons and passes during the night and morning hours through Tuesday.

Fair weather continues through Thursday as the high pressure system slowly moves east. By the end of the week, a series of low pressure systems will bring rain to Northern California and the chance of a few rain showers to the Central Coast on Saturday.

Surf and sea report

A series of storms will move through the Gulf of Alaska and will continue to produce a series of medium-height wave trains through this week. This morning’s 7- to 9-foot northwesterly (300-degree deep-water) swell (with a 13- to 15-second period) will decrease to 5 to 7 feet this afternoon and will remain at this height but with a gradually shorter period through Monday.

A 5- to 7-foot west-northwesterly (285-degree deep-water) swell (with an 18- to 20-second period) should arrive Tuesday and will increase to 6 to 8 feet but with a gradually shorter period Wednesday.

A 4- to 6-foot west-northwesterly (285-degree deep-water) swell (with a 12- to 14-second period) will develop along our coastline Thursday. Increasing northwesterly winds will generate 3- to 5-foot northwesterly (310-degree deep-water) sea and swell (with a 5- to 13-second period) Friday, increasing to 7 to 9 feet Saturday.

Seawater temperatures will range between 53 and 55 degrees through Friday.

Conservation tip

During the cooler nights of winter, weather-stripping around windows and doors can save a lot of energy. For more energy saving tips, visit John Lindsey is a media relations representative for PG&E. He’s a local weather expert and has lived along the Central Coast for more than 23 years. To subscribe to his daily weather forecast or ask him a question, e-mail