Weather Watch

Weather’s major role in PG&E power production

During a presentation at the Arroyo Grande Valley Kiwanis Club, I was asked why Pacific Gas and Electric Company is concerned about climate and weather.

Weather and climate can affect the delivery of energy to homes and businesses. The weather can also affect electrical generation, especially clean and renewable sources of energy.

If a large storm is forecast to hit a certain part of the state, PG&E line crews and trucks from other parts of California can be prestaged or brought in to the impacted area to help restore power as safely and quickly as possible.

If a cold snap is expected, the utility can acquire additional natural gas for heating. Conversely, if a heat wave is forecast, more electrical generation can be brought online for cooling.

During a normal rain and snow season, a little more than 20 percent of the electricity delivered to PG&E customers is from hydroelectric power. After three seasons of below-normal precipitation, this percentage has dropped to about 16 percent in 2009. Hopefully this season, with a forecast for normal snowfall in the mountains, the percentage will increase. A warmer climate could mean a diminished snow pack in the Sierra and less energy from hydroelectric in the future.

Most of the hydroelectric energy delivered to PG&E customers comes from a network of tunnels, canals and conduits that directs a portion of streams and rivers in the mountains of California toward penstocks. Penstocks are massive, near-vertical pipes that can be more than 1,000 feet long. Earth’s gravity accelerates the water in the penstock as it flows downhill, gathering tremendous pressure to spin turbines connected to electrical generators, producing electricity.

As Bob Dylan wrote, “you don’t need a weatherman to tell which way the wind blows,” but a good meteorologist who can forecast the winds can determine approximately how much electricity will be generated.

During the spring and summer months, a thermal low often develops over the Central Valley, while at the same time, the Eastern Pacific high parks itself off the coast. This difference in pressure produces onshore winds, especially in the coastal passes and gaps. During a normal fall and winter, storm systems move over our state, preceded by increasing prefrontal southerly winds, followed by postfrontal northwesterly winds. Modern wind towers convert the kinetic energy of wind into electrical energy.

Weather can also affect the output of solar panels — when clouds pass overhead, the power output of panels declines. Climate can tell you how much sunlight a specific area receives on average. In the future, ocean waves from weather systems in the Pacific Ocean may play an increasingly important role in generating clean energy for California. PG&E signed an agreement with the U.S. Air Force that will let the utility investigate the feasibility of a wave energy project off the coast of Vandenberg Air Force Base.

If you think about it, most of these clean energy sources are driven by the sun in one way or another. The sun is also the driving force behind the weather, producing storms, winds and waves. With the state striving to reach a greater percentage of its energy from clean and renewable sources each year, weather and climate will play an even more important part in reaching this goal.

This week’s forecast

A weak front moving past our area will produce widespread light rain showers this morning.

A weak upper-level trough will follow this afternoon and tonight, producing partly cloudy weather with a chance of a few afternoon and evening rain showers. Rain fall accumulations will be light, with snow levels around 5,000 feet.

A ridge of high pressure is forecast to move over the Great Basin on Monday, producing gentle to moderate northeasterly (offshore) winds, partly cloudy skies and dry weather through Tuesday. Ground fog will likely return across the interior during this period.

Another weak cold front will pass our area on Wednesday morning with increasing mid- to high-level clouds along with a few light rain showers.

Early indications are that this weak system will impact only the northern half of San Luis Obispo County.

Then dry and mostly clear weather will return Thursday, continuing through New Year’s Day.

Surf and sea report

This morning’s 11- to 13-foot west-northwesterly (285-degree deep-water) swell (with a 16- to 18-second period) will decrease to 10 to 12 feet on Monday. This west-northwesterly (285-degree deep-water) swell will continue to lower to 7 to 9 feet (with a 12- to 14-second period) on Tuesday.

Combined with Tuesday’s swell will be 1- to 3-foot northwesterly (320-degree shallow-water) seas on Tuesday afternoon.

A 4- to 6-foot northwesterly (290-degree deep-water) swell (with a 12- to 17-second period) is forecast along our coastline on Wednesday, increasing to 6 to 8 feet (with a 13-to 15-second period) on Thursday through Friday.

Preliminary analysis

This morning’s longer-range surface charts are indicating a large storm developing along the northeastern coast of Japan on Wednesday.

This intense storm is forecast to track across the North Pacific into the Gulf of Alaska. If this storm develops as advertised, a large west-northwesterly swell should arrive on our coastline on January 5.

Note: The minus low tides have shifted from the morning hours to the afternoon and evening hours as we move into winter.

A predicted minus 1.7-foot low tide will occur on Thursday afternoon, accompanied by a 6.9-foot high tide that morning.

John Lindsey is a media relations representative for Pacific Gas and Electric Company. He is also a local weather expert and has lived along the Central Coast for more than 22 years.

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