Weather Watch

County has a great diversity in temperatures

Flying over San Luis Obispo County this time of the year, you can often see the marine layer to the west, surging into the Los Osos Valley with only the top of Morro Rock visible.

To the east, on the other side of the Cuesta Grade, it is often clear and sunny. The regions around coastal valleys seem to ebb and flow between overcast and mostly clear skies. It’s not uncommon to experience triple-digit temperatures in the North County while the meteorological tower at the Diablo Canyon nuclear power plant by the coast reports temperatures in the high 50s.

Even in much smaller geographic areas of the county, there is a great diversity in temperatures. When conditions are calm, denser cold air flows downward along our mountain slopes and accumulates in the valleys. Chris Arndt of has occasionally reported a 10-degree Fahrenheit difference just 100 feet down the hill from his home in western San Luis Obispo.

This diversity in microclimates can allow farmers in San Luis Obispo County to produce many different crops from spinach to avocados in a relatively small area.

Farmers use an index called Growing Degree-Days as a guide for the types of crops that can be grown on a particular piece of land and for determining when crops will be ready for harvesting.

GDD actually describes the total heat accumulated over a period of time, such as the entire growing season.

A growing degree day for a particular day is defined as a day in which the mean temperature is one degree above the baseline temperature, or the minimum temperature required for the growth of that particular crop. The baseline temperature for snow peas, often grown along the coastline, is 40 degrees Fahrenheit. For grapes, it’s 50 degrees, and for rice grown in Colusa County in the Sacramento Valley, it is 60 degrees.

On a summer day in the North County, the average temperature over 24 hours may reach 80 degrees. On such a day, grapes would accumulate 30 growing degree days. Depending on the variety, grapes usually require 1,700 to more than 3,000 GDDs before they are ready for harvesting.

According to the University of California Division of Agriculture and Natural Resources, Cuyama’s growing season from April through October has the greatest amount of GDDs for grapes at 3,306, followed by Paso Robles at 3,275. San Luis Obispo has 2,632, and Pismo Beach usually receives about 2,080.

Early-ripening grape varieties such as Pinot Noir and Chardonnay can successfully ripen their fruit in the cool coastal areas, whereas late-ripening varieties such as Mourvedre and Petit Verdot can only ripen their fruit reliably in the warm inland areas.

Today’s weather

A 1,030-millibar Eastern Pacific High will remain firmly in place centered about 1,200 miles to the northwest of San Luis Obispo, while a thermal trough remains over the Central Valley. This typical July/August weather pattern will continue through this week.

Saturday’s high at Paso Robles reached 100 degrees, the warmest day of the Mid-State Fair so far and probably the highest temperature the fair will experience this year.

A weak upper-level low pressure system will move toward the Central Coast today, producing cooler inland temperatures and extensive night and morning coastal low clouds and fog.

Temperatures at the Mid-State Fair will reach the low 90s today, while the coastal valleys will range between the low and mid 70s.

The northwesterly-facing (Los Osos and Morro Bay) beaches will range between the high 50s to the low 60s, while temperatures along our southwesterly-facing (Avila Beach and Cayucos) beaches will range between the high 60s and the low 70s under mostly sunny skies during the afternoon hours.

For those venturing to the Sierra Nevada mountains, afternoon thundershowers will persist through most of this week.

For Monday and Tuesday, night and morning low clouds and fog moving back to the beaches during the afternoon hours, light to gentle winds and below-normal temperatures are expected.

The northwesterly (onshore) winds will increase Wednesday through Friday, producing a greater amount of clearing along the coastline, especially along the southwesterly-facing beaches.

Sea and surf report

Today’s 3- to 4-foot northwesterly (310-degree deep-water) swell (with an 8- to 11-second period) will remain at this height and period through today. This swell will decrease to 2 to 3 feet (with a 9- to 11-second period) on Monday and will remain at this height and period through Wednesday morning.

Combined with this northwesterly swell will be 1- to 2-foot northwesterly (320-degree shallow-water) seas this afternoon.

A 2- to 3-foot northwesterly (310-degree deep-water) sea and swell (with a 4- to 9-second period) will develop along our coastline on Wednesday afternoon and continue at this height and period through Friday.

Arriving from the Southern Hemisphere:

Today’s 1- to 2-foot long-period Southern Hemisphere (200-degree deep-water) swell (with a 13- to 15-second period) will gradually diminish through Wednesday.

A 1- to 2-foot Southern Hemisphere (215-degree deep-water) swell will arrive along our coastline Friday, building to 2 to 3 feet next weekend.

Seawater temperatures will range between 55 and 57 degree through Friday.

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John Lindsey is a media relations representative for PG&E. He is also a local weather expert and has lived along the Central Coast for more than 23 years. Send him an e-mail at