Weather Watch

Above-normal snowpacks make for increased hydroelectric power

Out over the Pacific Ocean, the sun’s light evaporates enormous amounts of seawater into water vapor in the atmosphere.

The sun, along with the Earth’s rotation, is also the driving force behind the jet stream, the upper-atmosphere river of air that drives all this water vapor in an easterly direction toward California.

Low-pressure centers, or storms, lift the water vapor in the sky, where it condenses on microscopic particles of dust and salts and changes back to liquid form as clouds.

When the conditions are right, the small water particles in the clouds increase in size and fall to the surface as precipitation, usually in the form of rain or snow.

During winter, much of this precipitation falls as snow over the Sierra Nevada.

This snow pack in the Sierra acts as a storage reservoir for water. As this reservoir melts, it slowly releases water for the needs of forests and agriculture, households and endangered species and provides the energy for hydroelectric power used during the longer days of spring and summer.

Most of the hydroelectric power delivered to Pacific Gas and Electric Co. customers comes from a network of tunnels, canals and conduits that direct 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, creating tremendous pressure to spin hydroelectric turbines connected to electrical generators that produce electricity.

After the water is used to produce clean and renewable energy, it’s returned to streams and rivers.

Electricity from other power plants, such as Diablo Canyon nuclear power plant, is used to pump some of this water back uphill during the night at the Helms Pumped Storage Project in the Sierra Nevada when demand for electricity is lower. This water is then allowed to run downhill to produce electricity during periods of higher demand in the afternoon hours.

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 dropped to about 16 percent in 2009.

Careful measurements by PG&E hydrographers and many other agency hydrographers in the Sierra this March indicate the snowpack is above normal.

Surveys are conducted on a monthly basis from January through April.

PG&E hydrographers fly in by helicopter, drive in by snow cat or hike in utilizing snowshoes, and take snow depth and water content measurements over a wide expanse of the watershed.

This month’s survey indicates the water content of the snow is about 110 percent of average statewide — or about twice as much as at this time last year.

The emerald-green hills and the flowing streams of San Luis Obispo County this March tell the story. Many locations throughout the county have recorded near- to above-normal rainfall amounts. Rocky Butte, near San Simeon, has recorded about 43 inches of rainfall. in western San Luis Obispo has recorded nearly 31 inches of rainfall or about 150 percent of normal. As of this week, 17 inches of rain had fallen at Diablo Canyon compared to 8 inches as of March 20, 2009.

All this rain has increased the level of local lakes and reservoirs. Back in late September, Nacimiento Lake was less than 10 percent of capacity, but has since increased to 61 percent.

Lopez Lake has reached 62 percent. Salinas Reservoir in Santa Margarita actually spilled this month.

The National Climate Prediction Center is still predicting above-normal rainfall for April and May, but historically, rainfall rates dramatically decrease during these months as the wet season comes to an end.

This week’s forecast

A 1,025-millibar area of high pressure over the eastern Pacific combined with a thermal low over the great Central Valley of California will produce a classic Central Coast spring weather pattern: wind, fog and cooler temperatures along the coastline, while the interior remains mostly clear and warmer.

Today, the first full day of spring, will usher in increasing northwesterly (onshore) winds late this afternoon along with marine low clouds and fog and cooler temperatures along the beaches and in a few of our coastal valleys.

The interior will remain mostly clear and warmer.

Today’s high temperatures will range from the mid-70s in the interior to the high 60s in the coastal valleys. The beaches will be cooler, only reaching the high 50s to the low 60s.

A steep pressure gradient will develop along the coastline Monday and Tuesday producing strong to gale force (25 to 38 mph) afternoon northwesterly winds along the shoreline and a greater amount of sunshine along the beaches.

The interior and coastal valleys will reach the low 70s while the beaches will mostly be in the low 60s except for Avila Beach and Cayucos, which could reach the mid-70s.

The rest of next week shows little change; but over the last few days, the longer-range charts and models have been fairly consistent in advertising an unsettled weather pattern developing for the Central Coast by next Sunday and beyond. Only time will tell.

Surf and sea report

This morning’s 3- to 4-foot northwesterly (300-degree deep-water) swell (with an 11- to 15-second period) will continue at this height and period through this afternoon.

Increasing northwesterly winds will produce a 4- to 6-foot northwesterly (300-degree deep-water) sea/swell (with a 5- to 15-second period) this evening, increasing to 6 to 8 feet on Monday through Tuesday.

A 4- to 6-foot northwesterly (290-degree deep-water) swell (with an 8- to 18-second period) is forecast along our coastline on Wednesday.

Another round of increasing northwesterly winds will produce a 6- to 8-foot northwesterly (300-degree deep-water) sea/swell (with a 5- to 15-second period) on Thursday, increasing to 7 to 9 feet on Friday.

Arriving from the southern hemisphere:

A 1- to 2-foot southern hemisphere (230-degree deep-water) swell (with a 20- to 22-second period) will arrive along our coastline Monday, increasing to 2 to 3 feet (with an 18- to 20-second period) on Tuesday through Wednesday.

Preliminary Analysis long-range forecast:

A high, long-period west-northwesterly swell could arrive on our coastline March 31.

Seawater temperatures will range between 51 and 53 degrees through Sunday, decreasing to 49 to 52 degrees Monday and Tuesday.

Conservation tip

For information about Pacific Gas and Electric Co. hydroelectric facilities, visit

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.