Weather Watch

December will make or break this year's title as driest on record

California has seen record-breaking low-rainfall amounts so far this year.

From January through November, only a meager 4.2 inches of rain have fallen in San Luis Obispo, the smallest amount of the wet stuff to date since record keeping began in 1870 at Cal Poly, home of climatology in the city. Normally by Dec. 1, San Luis Obispo should have received 18.4 inches of precipitation.

The previous driest January-to-November period on record was in 1898, when 6.3 inches of rain were recorded at Cal Poly. Then, just a little more than 0.6 of an inch was recorded the following month, December 1898.

The next driest January-to-November period was 1984, with a recorded 7.9 inches. That December, an additional 3.6 inches of rain fell.

To prevent 2013 from becoming the driest calendar year on record, at least 2.7 inches of rain must fall this month.

Other areas along the Central Coast are even drier. Paso Robles has had only 1.8 inches of rain this year, or about 17 percent of normal. Dry, indeed! PG&E’s Diablo Canyon Power Plant has recorded 2.6 inches of precipitation.

Farther south and north, Santa Barbara reported 4.6 inches since Jan. 1, or 31 percent of normal. Fresno has seen 2.9 inches of rain, or about 30 percent of normal.

Since January 2011, almost every month has experienced below-normal precipitation. This extended period of dry weather is extending the fire season.

Robert Lewin, Cal Fire chief for San Luis Obispo County, said, “Until we get a couple of inches of rain and a forecast for more, we are going to be forced to continue staffing all of our fire season engines, dozers, crews and aircraft. This is going to strain our budget.”

Former San Luis Obispo County Supervisor Bill Coy, who grows avocados on his Cottontail Ranch near Cayucos, told me, “We can stand most cold spells with our wind machines. However, everything is compounded by the dry, windy weather, and the lack of any moisture. Trees and crops that are well-hydrated usually fare pretty good; this year, they are not. I’ve checked all of our weather records, and we’ve never seen anything like this.”

So what does the heart of the rain season — December through March — hold in store for us? As I’ve written before, the U.S. Climate Prediction Center (CPC) is still predicting that the neutral conditions — the infamous El Nothing or El Nada — will continue through spring 2014. Neutral conditions generally don’t produce any reliable seasonal rainfall predictions along the Central Coast. The CPC is forecasting “equal chances of equal, above- and below-normal precipitation.”

In other words, a forecast of normal rainfall.

Jan Null, a meteorologist with Golden Gate Weather Service and a former PG&E meteorologist, put together an interesting review of the CPC Winter Outlook rain prediction versus what actually transpired based upon the categories: Above Normal, Normal and Below Normal precipitation. Null discovered that over the past 18 years, the CPC had the correct category eight times. Seven times, it was off by one category, and three times it was off by two categories.

The other ocean water temperature cycle that seems to be playing a larger role is called the Pacific Decadal Oscillation, or PDO. The phases of the PDO are called warm or cool phases.

There’s a growing amount of evidence that the warm phase of the PDO produces above-normal levels of rainfall, while the cool phase produces below-normal levels. Unfortunately, we’re still in the cool phase of the PDO, meaning we could continue to see below-

average rainfall.

So far, this fall has been difficult to predict. It reminds me of the movie “Groundhog Day,” where each day is repeated again and again. Hardly a week goes by that the numerical models have forecast significant rain for the Central Coast seven or 10 days in the future, only to reverse themselves a few days before the predicted storm was to reach us. My updated prediction for this rain season: between 13 inches and 18 inches as measured at Cal Poly.