From San Francisco to Los Angeles and for most areas in between, 2013 will go down as the driest calendar year on record.
San Francisco rain records stretch back to the gold rush era of 1849. This year, only 5.6 inches of rain has fallen. That's an absurd 3.4 inches less than the previous dry record of 9 inches of rain recorded in San Francisco in 1917.
Downtown Los Angeles has only seen 3.6 inches, making it the driest year on record since 1877 when their rain records started.
From January through December, only a skimpy 4.5 inches of rain has 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. The previous driest year on record was in 1898, when nearly 7 inches of rain were recorded at Cal Poly, smashing the previous record by 2 ½ inches!
Normally, San Luis Obispo receives 22.4 inches of rainfall per year.
Paso Robles has had only 1.9 inches of rain this year, or about 15 percent of normal.
Santa Maria has had 3 inches of rainfall, their driest on record since the early 1900s.
PG&E’s Diablo Canyon Power Plant has recorded just 2.9 inches of precipitation.
Santa Barbara avoided their driest calendar year on record when a storm produced nearly an inch of rain the day after Thanksgiving, but it didn’t produce much rain in San Luis Obispo County.
So why are we seeing this record-smashing dry weather?
For most of 2013 the southern branch of the polar jet stream has remained north of our latitude out over the eastern Pacific Ocean due to a persistent ridge of high pressure along the west coast. The polar jet stream travels in a wavy or even looping pattern at great speeds around the northern hemisphere in a west-to-east pattern at elevations of 6 to 9 miles.
As the wave amplitude of the jet stream increases, the air flow may transform from a wavelike structure to that of a counter-clockwise circulation, producing storms. These mid-latitude Pacific storms and the southerly winds and rain they bring have mostly stayed north of the California-Oregon border.
Historically, winds along our coastline at the Diablo Canyon meteorological tower blow about 60 percent of the time out of the northwesterly quadrant, and about 23 percent of the time, the winds blow from the southeast. These southeasterly winds are also called pre-frontal winds and often bring rain.
This year, the southeasterly winds have only blown 8 percent of the time. To make matters worse, this December has seen a much greater occurrence of Santa Lucia (northeasterly or offshore) winds.
These winds often produce low relative humidity levels, cold nights and warm afternoons. In fact, the coastal valleys and beaches of the Central Coast have experienced record-breaking heat this last week with temperatures reaching the mid-80s in the coastal valleys day after day.
Without any rains, this offshore flow is freeze-drying our vegetation. Robert Lewin, Cal Fire chief for San Luis Obispo County, said, "Since June the Central Coast fuels (vegetation) have been at critically low moisture; fortunately we have kept the fires from becoming more destructive. This long ongoing fire season has forced our men and women to spend many weeks away from their loved ones; we appreciate their commitment and understanding.”
Bill Tietje is an Area Natural Resources Specialist with the University of California at Berkeley, stationed at the UC Cooperative Extension office in San Luis Obispo. He has recently received numerous reports about oaks appearing in bad shape.
Ranchers and farmers throughout the county of have told me that they have never seen anything like this. Many of these fine agriculturalists have been working the land for more than 75 years.
So what does January through March hold in store for us?
The latest guidance from the U.S. Climate Prediction Center (CPD) is now calling for “below normal precipitation” for Central and Southern California during the heart of our rainfall season. You might recall, last month the CPC was forecasting “equal chances of equal, above- and below-normal precipitation.”
After reviewing all the rainfall data records stretching back for hundreds of years, even the driest years had at least a few significant rain events. Some the longer-range models indicate an unsettled weather pattern with a good chance of rain by mid-January. I hope and pray they verify.