Weather Watch

Why has it been so dry? A few reasons

A view of Nacimiento Lake Dam on Friday. The lake is currently at 23 percent capacity.
A view of Nacimiento Lake Dam on Friday. The lake is currently at 23 percent capacity.

It will come as no surprise that so far this rain season — which runs from July 1, 2013, through June 30, 2014 — has seen the smallest amount of precipitation on record.

Both the Paso Robles airport and Diablo Canyon rain gauges have only recorded about a half-inch of rain, while Cal Poly (home of climatology for San Luis Obispo) has logged an inch of rain.

Since 2011, we’ve seen well-below-normal rainfall. John Neil of the Atascadero Mutual Water Co. sent me some frightful rainfall data. His company has collected rainfall information over the past 100 years at a rain gauge at the confluence of the Salinas River and Atascadero Creek. After he carefully reviewed the historical data from their rain gauge, Neil discovered that the “three-year running rainfall totals” this decade compared with the drought years of 1976 and 1990 are considerable drier.

“They’re nothing compared to what’s going on presently,” Neil told me.

The Central Coast is in the heart of its rain season, and this week, like so many other weeks, is looking bone dry with low relative humidity levels and high fire danger. Ironically, this year, the beaches have received more precipitation during the summer months because of marine low clouds and drizzle.

So why has it been so dry?

For one reason, a large-scale, high-pressure ridge has continued to dominate our weather, never fluctuating far from the western edge of North America. This condition hasn’t allowed the normal wintertime storm systems to progress southward into California.

An area of high pressure about 3,700 miles away as the crow flies may be the culprit. This year’s drought pattern across California is related to a strong and persistent area of high pressure in the upper atmosphere near Greenland. The “Greenland Block” is a high-pressure ridge that has forced the polar jet stream south over the Midwest and East Coast.

There’s a growing amount of evidence that climate change may be altering the path of the jet stream, keeping storms farther to the north of California’s longitude.

Another possible contributing factor is that conditions don’t exist for either an El Niño or its sister, La Niña. The U.S. Climate Prediction Center is expecting that the neutral conditions — the infamous El Nothing or El Nada — will continue through spring.

The other ocean water temperature cycle that plays a role in rainfall is called the Pacific Decadal Oscillation, or PDO. Unfortunately, we’re still in the cool phase of the PDO, meaning we could continue to see below-average rainfall.

Both of these oceanographic phases are considered to be a standing pattern. In other words, they stay fixed in the same geographic area. However, another phase called the Madden-Julian Oscillation is a large traveling pattern of increased rainfall and thunderstorm activity that moves eastward at 8 to 18 mph across the tropical parts of the Indian and Pacific oceans.

In 1971, Roland Madden and Paul Julian stumbled upon this pattern when analyzing wind anomalies in the tropical Pacific. But little attention was paid to the oscillation until the strong 1982-83 El Niño event, which led researchers to believe that this pattern may have enhanced the amount of rain in California.

As the Madden-Julian Oscillation moves across the Pacific Ocean, a split in the polar jet can develop. The southern branch of the polar jet can extend far out over the Pacific toward the coast of California. This river of air in the upper atmosphere can steer moist subtropical air toward California and can bring several days of heavy rain.

This phase is believed to have produced heavy rain along the Central Coast in December 2010, when Cal Poly reported 12 inches of rain, much of it in one weekend. That was the second-wettest December on record; San Luis Obispo normally gets 4 inches for the month.

At this time, some of the very long-range models are indicating that this scenario could develop by early to mid-February. Nevertheless, when trying to predict the weather, especially more than a month away, there are no guarantees.

That being said, you may want to keep your umbrella close by for the time being.

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