Terri and Bob Blanchard of Old Creek Ranch, nestled in the hills near Cayucos, have been ranching for decades, growing certified organic avocados and oranges and raising livestock.
As agriculturists, they depend on winter rains to water their orchards and irrigate hillside grasses that their cattle eat. So far this winter, rainfall has been inconsistent.
“The steady and penetrating rains of December kept the ranch in pretty good shape through the dry spell of January,” Bob Blanchard said. “The ground is still wet from the December rains.”
The deluge of rain in December followed by the parched and clear skies of January and this first week of February have shown that the formation of high- and low-pressure areas in the upper atmosphere can be the driving force behind the Central Coast’s weather.
High-pressure areas produce clear, sunny conditions and can block storms. Generally speaking, low-pressure systems are what make storms.
In early December, a deep area of low pressure developed in the upper atmosphere over the Gulf of Alaska, while a blocking high took a position west of the Aleutian Islands. At the Earth’s surface, the eastern Pacific high was forced southward, which left the storm door wide open.
There was another factor at work as well. The jet stream is a ribbon of high-speed winds some 18,000 to 40,000 feet up.
It flows like a river from the west to the east for thousands of miles. As the holidays approached, the jet stream moved over the Central Coast and directed a series of weather systems toward our area.
The result was the second-wettest December on record.
The Cal Poly rain gauge (the official home for climatology records in San Luis Obispo) recorded 9.66 inches of rain in December.
That’s the wettest December since 1996, when 10.88 inches of rain were measured. On average, San Luis Obispo receives 3.79 inches during December.
Actually, the rain events of December weren’t especially unusual until Dec. 18 to 19, when a nearly stationary, upper-level, low-pressure system took a position off the Oregon coast, and along with a persistent west-southwestern jet stream, steered moist, subtropical air toward the Central Coast.
The surface charts indicated what meteorologists call an occluded front stalled over San Luis Obispo that stretched westward thousands of miles over the Pacific Ocean.
The skies opened up that weekend and produced more than 6 inches of rain over a two-day period at Diablo Canyon nuclear power plant. That was the greatest two-day total at the plant since March 9 to 10, 1995, which produced 8.54 inches.
Things changed after New Year’s Day. In early January, a strong ridge of high pressure formed in the upper atmosphere over California and forced the storm track far to the north into British Columbia.
At the Earth’s surface, the eastern Pacific high parked itself off the California coast and another area of high pressure developed over Nevada and Utah, in a region known as the Great Basin.
This condition produced persistent night and morning northeasterly offshore winds, unseasonably warm temperatures and one of the longest dry spells during the month of January.
From Jan. 3 through Jan. 28, the Central Coast did not get any rain. That was the longest dry spell since January of 1984. This January, the Cal Poly rain gauge recorded 2.56 inches of rain, or about 50 percent of normal.
January is the wettest month of the rain season, so last month’s below-normal total is significant. February is the second-wettest month of the season, but so far it, too, has been dry.
The strong high-pressure ridge responsible for this mild and dry weather will gradually weaken and migrate westward late next week.
At this time, it looks like our dry weather pattern could change by about mid-to late February, with increasing chances for wet weather.
John Lindsey is a media relations representative for PG&E. He is also a local weather expert and has lived along the Central Coast for more than 23 years. To subscribe to his daily weather forecast or ask him a question, e-mail firstname.lastname@example.org.
New Paso robles rain gauge
In December, there was some controversy surrounding the rain gauge at the Paso Robles Airport. Upset Paso Robles residents said it was not showing proper readings from storms.
Paso Robles Municipal Airport manager Roger Oxborrow said the National Weather Service has installed a completely new and calibrated tipping-bucket gauge. On Jan. 30, the gauge reported .07 inches of rain at the airport. The other rain gauge, a standard rain gauge, read .06 inches.