Weather Watch

Dry La Niña didn’t show in California

This rain and snow season serves as a reminder that all La Niña events are not the same.

According to the long-term rainfall averages, La Niña events produce about 87 percent of normal rainfall on the Central Coast. These generate cooler-than-normal sea surface temperatures in the central and eastern equatorial Pacific, and tend to shift the storm track northward.

As many local surfers and scuba divers will tell you, seawater temperatures along our coast have been cold. Seawater temperatures at Diablo Canyon nuclear power plant have been running about 2 degrees cooler than normal this winter.

However, the emerald green hills, flowing streams and the promise of a remarkable wildflower bloom in San Luis Obispo County tell a different story.

Many locations throughout the county have recorded above-normal rainfall amounts.

Rocky Butte near San Simeon has recorded nearly 51 inches. SLOweather.com in western San Luis Obispo has recorded nearly 34 inches of rain or about 150 percent of normal.

So far this rain season, Diablo Canyon has recorded 25.8 inches. Even if it didn’t rain again until the end of the season in June, Diablo Canyon would finish at 111 percent of normal.

The lake and reservoir levels of San Luis Obispo County have significantly increased over the past few months.

As of last Friday, Lake San Antonio is at 76 percent of normal and Nacimiento Lake is at 100 percent of capacity and for only the fourth time in its 55-year history is spilling, according to Robert Johnson, chief of water resources planning for the Monterey County Water Resources Agency.

Whale Rock Reservoir was at 88 percent, Lopez Lake reached 91 percent and the Salinas Reservoir near Santa Margarita was over 102 percent and also spilling. Even Twitchell Reservoir near Santa Maria, normally dry, is at 45 percent.

Over the Pacific, the sun evaporates enormous amounts of seawater into water vapor in the atmosphere. The jet stream drives all this water vapor 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, usually in the form of rain or snow.

During winter, a lot of this precipitation falls as snow in the Sierra Nevada.

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

Most of the hydroelectric power delivered to PG&E customers comes from a network of tunnels, canals and conduits that direct a portion of streams and rivers in the mountains toward penstocks.

Penstocks are typically large diameter pipes that can be more than 1,000 feet long and move water to the powerhouse.

The potential energy developed is the difference in elevation of the penstock and the quantity (pressure and flow) of water transported, converted to kinetic energy that in turn drives the turbine/ generators to produce electricity.

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

Electricity from other power plants such as Diablo Canyon is used to pump some of this water uphill at the Helms Pumped Storage Project in the Sierra, 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. Careful measurements by PG&E hydrographers and many other agency hydrographers in the Sierra last month indicate the snowpack is above normal. In fact, measurements indicate that we have not seen a snowpack like this in years.

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 wearing snowshoes. They 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 165 percent of average statewide. March storms added a lot of depth to this year’s snowpack. At Donner Summit, site of one of the oldest continuous snow records in North America, this year’s pack is the third deepest in 89 years of record-keeping at 179 inches.

There has been so much rain and snow this year and last, Gov. Jerry Brown declared an end to the three-year drought. Dam officials are releasing water from lakes and reservoirs to make room for all the water from the snow melt that will come from the Sierra.

This week’s forecast

In April, the more the weather changes, the more it stays the same.

This morning’s deep marine layer will produce a few areas of drizzle along the coast.

Fresh to strong (19 to 31 mph) northwesterly winds will develop along the coast with partly cloudy skies by this afternoon.

Temperatures are expected to be 15 to 20 degrees cooler than Friday — much closer to normal for this time of year.

A high-pressure ridge is expected to build over the state Monday and Tuesday, bringing back northeasterly (offshore) winds and sunny skies and warm weather. Temperatures during this period should be in the mid-70s.

However, these warmer conditions will be short- lived, as a cold front is expected to sweep southeastward over the Central Coast on Wednesday.

No precipitation is expected from this system, but it will cause partly cloudy skies and cooler temperatures.The main effect of this cold front will be to produce strong to gale force (25 to 38 mph) northwesterly winds Wednesday through Friday.

A return to more fair and mild conditions is expected for next weekend.

Surf and sea report

This morning’s 9- to 11-foot northwesterly (300-degree deep-water) sea and swell (with a 7- to 14-second period) will decrease to 6 to 8 feet by tonight.

A 5- to 7-foot northwesterly (300-degree deep-water) swell (with an 11- to 13-second period) is forecast along the coast Monday, decreasing to 3 to 5 feet Tuesday morning.

Strong to gale force (25 to 38 mph) northwesterly winds will generate 4- to 6-foot northwesterly (300-degree deep-water) sea and swell with a 4- to 11-second period Tuesday afternoon and night.

These winds combined with a Gulf of Alaska swell will produce a 10- to 12-foot northwesterly (310-degree deep-water) sea and swell (with a 7- to 17-second period) Wednesday through Friday. Arriving from the Southern Hemisphere:

A 1- to 2-foot Southern Hemisphere (225-degree deep-water) swell (with a 20- to 22-second period) will arrive along the coast Wednesday, increasing to 2 to 4 feet (with an 18- to 20-second period) Thursday through Friday.

Seawater temperatures will range between 52 and 54 degrees through Tuesday, decreasing to 50 and 52 degrees Thursday through Friday.

Earth Day event

Join PG&E employees April 16 to celebrate Earth Day at Montaña de Oro State Park.

The event is one of a number of service projects sponsored by PG&E and the California State Parks Foundation.

Please register at the California State Parks Foundation website, www.calparks.org.

Be sure to dress for outdoor work with long pants, long-sleeved shirt, sturdy shoes, hat, gloves and sunscreen.

Snacks and a light lunch will be provided. Bring your own refillable water bottle. Rangers will provide tools and supervision.

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