Weather Watch

Living in the Goldilocks Zone is just right

Most of San Luis Obispo County resides in the Goldilocks Zone, not too cold and not too hot but about just right. One way to tell is to look at how often we turn on our air conditioners compared with other parts of the state.

When the mean daily temperature climbs above 65 degrees Fahrenheit, most people in the United States will begin to cool their homes and businesses. I know 65 degrees sounds surprisingly low, but many years of historical temperature data and people’s behavior verifies this temperature. Higher humidity levels in other parts of the country probably played an important part in determining this temperature.

Cooling Degree-Days is used to estimate the energy needed to cool the atmosphere in our homes or businesses to 65 degrees. In other words, CDD is used to relate the day’s mean temperature to the energy demands of air conditioning. CDD is calculated by subtracting 65 degrees from the mean. For example a daily mean temperature in Paso Robles of 80 degrees would produce 15 CDDs (80 degrees minus 65 degrees). The higher the CDD value, the greater amount of energy needed for cooling.

Pacific Gas and Electric Co.’s meteorologists forecast the daily mean temperature throughout its immense service territory, which extends from near the Oregon border, east toward Lake Tahoe and all the way south to the Gaviota Pass in Santa Barbara County. By calculating the CDDs, PG&E can forecast energy demand and generate or buy the correct quantity of energy needed to keep us comfortable.

You can also use this value to compare the heat in different parts of the state.

Along the coastline in Northern California, persistent onshore winds and a cold Pacific Ocean often produce a deep and persistent marine layer. This condition allows a blanket of fog to form over Eureka that greatly diminishes the amount of sunshine that reaches the ground for days on end during the summer. The dreary and cool overcast gives Eureka a CDD of zero. Not a lot of need for air conditioning in Eureka.

As you head farther south down the rugged Northern California coastline, Fort Bragg has a CDD of six, and San Francisco with slightly warmer seawater temperatures has a CDD of 108. As you continue to move toward the more southern latitudes of the Central Coast, occasional offshore wind events produce higher CDD levels. Los Osos has a CDD level of 150 while Pismo Beach has a CDD level of 173. I couldn’t find any official values for Avila Beach or Cayucos, but I suspect they would be higher due to the greater amount of sunshine and warmer temperatures at these locations.

Once you pass Point Conception, the Pacific Ocean abruptly warms. The warmer water and the hot and dry Santa Ana (offshore) winds produce a CDD of 984 at San Diego.

Heading inland toward the interior of San Luis Obispo County, the marine influence is reduced, and a greater amount of the sun’s rays reach the earth’s surface. Those conditions give San Luis Obispo an 894 CDD.

Farther eastward toward the North County, where even more sunshine reaches the surface, the CDD is 956 in Paso Robles. Fortunately, night and morning marine stratus often streams southward from the Salinas Valley and through the Templeton Gap allowing homes to cool naturally at night, which reduces the need for air conditioning.

The same can’t be said for the low desert of far Southern California, which is characterized by extremely hot days and warm nights during the summer where the sun’s rays relentlessly heat the Earth’s surface and cooling is the overwhelming concern. Brawley in Imperial County has the highest CDD that I could find in the state at 6,565 CDDs. Living in Brawley would probably require the air conditioner to be on almost continuously during the summer.

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Today’s weather

The low-pressure trough parked over the state has allowed cool marine air to filter into the interior of San Luis Obispo County, producing one of the coolest Mid State-Fairs in memory.

Not much change in the current weather conditions is expected through Thursday as a 1,032-millibar Eastern Pacific high centered 900 miles to the west of San Luis Obispo and a thermal trough over the Central Valley remain in place.

This morning’s coastal clouds will move back to the coast by the afternoon. The southwesterly (Avila beach and Cayucos) facing beaches will clear, but the northwesterly (Los Osos and Morro Bay) facing beaches will see limited sun.

Today’s temperatures will range between the low 60s along the northwesterly facing beaches and high 60s along the southwesterly facing beaches today.

The coastal valleys will range between the low to mid-70s with the North County hitting the high 80s for the last day of the California Mid-State Fair. To the east, California Valley is forecast to reach the low 100s.

Slight warming is expected Monday and Tuesday with fewer coastal low clouds. However, temperatures will remain below normal at locations closer to the coast.

Potentially further warming is expected into next weekend with near-normal temperatures at most locations and possibly above normal across the interior. Coastal stratus will continue to develop during the night and morning hours but should burn back to the coast by the afternoon.

Some of the models are advertising increasing high pressure moving toward our area from the desert southwest, allowing monsoon moisture to stream over our area next weekend.

Beyond next week, it continues to look like a high-pressure ridge will build across the state with possible above normal temperatures away from the immediate coast for much of the week.

Surf and sea report

Fresh to strong (19 and 31 mph) northwesterly winds along the California coastline will continue to produce a 3- to 5-foot northwesterly (320-degree deep-water) sea and swell (with a 4- to 8-second period) along our coastline all the way through Saturday.

Arriving from the Southern Hemisphere:

The first of two overlapping Southern Hemisphere (195-degree deep-water) swell trains should peak today through Monday at 1 to 2 feet (with a 14- to 16-second period), gradually decreasing on Tuesday.

The other Southern Hemisphere (215-degree deep-water) swell will arrive along our coastline on Wednesday at 1-foot (with a 14-to 18-second period), increasing to 1 to 3 feet (with a 15- to 17-second period) on Thursday. This swell will continue at this height and period through Saturday.

Seawater temperatures will range between 52 and 55 degrees through Saturday.

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