“I have spoken of the rich years when the rainfall was plentiful. But there were dry years too, and they put a terror on the valley. The water came in a thirty-year cycle. There would be five or six wet and wonderful years when there might be nineteen to twenty-five inches of rain, and the land would shout with grass.”
― John Steinbeck, East of Eden
As Steinbeck would say, the rain was plentiful this season. In fact, coastal mountain ranges and the Sierra Nevada saw near- to record-breaking amounts of precipitation. Most of us joyously welcomed these rains after five years of drought turned many of our Central Coast hills to shades of gray as vegetation wilted away.
Weak to moderate low-pressure systems created persistent (gentle to fresh 8 to 24 mph) southerly winds that brought in massive amounts of subtropical moisture associated with atmospheric rivers that stretched across the Pacific past the Hawaiian Islands this rain season (July 1, 2016, through June 30, 2017). This condition caused periods of heavy drizzle/light rain that produced impressive amounts of precipitation over time, especially along the southwesterly facing mountain slopes. In other words, most of the weather systems lacked convective dynamics.
Yes, there were a few exceptions, like the notorious 983 millibar storm that slammed into the Central Coast on Friday, Feb. 17 at the start of the Presidents Day weekend. It brought 70-mph-plus southerly winds that toppled numerous trees that caused power outages. Severe convective dynamics produced thunderstorms and heavy rain throughout the region. The Santa Ynez Airport, which was in a rain shadow for much of year, reported 4.5 inches of rain in less than 24 hours.
Otherwise, it was a season of lower-level subtropical moisture that created enormous amounts of orthographic enhancement. In this process, the southerly winds lifted the low-level subtropical moisture over the coastal mountains and cooled it 2.7 degrees Fahrenheit for every 1,000 feet of elevation change; this is called the saturated adiabatic lapse rate, which rings out the moisture from the heavens like squeezing a wet sponge or mop.
This condition created vast differences in percent of average rainfall this season throughout the Central Coast. For example, the Paso Robles Municipal Airport on the east side of the Santa Lucia Mountains recorded 117 percent of normal rainfall. On the ocean side of these mountains, Cal Poly reported 174 percent of average, while SLOweather.com in western San Luis Obispo reported 167 percent.
Along the coastline, San Simeon at slightly above sea level, saw 143 percent of average. However, farther up in the Santa Lucia Mountains, Rocky Butte recorded 218 percent. Another example: The Diablo Canyon rain gauge at 85 feet above sea level reported 154 percent of normal, but just a short distance away but at nearly 1,800 feet, Davis Peak came in at 212 percent. The rainfall amounts on the higher reaches of the Santa Lucia Mountains were breathtaking. Dawn Dunlap at the Walter Ranch above Cambria recorded more than 92 inches, while Debby Mix of the Circle 3 Ranch, at 2,150 feet of elevation in the San Simeon Watershed between Vulture Rock and Rocky Butte, logged 102 inches.
In these conditions, just a few hundred feet can cause significant changes in rainfall amounts. The Nipomo South rain gauge at 305 feet above sea level reported 166 percent of average, while the Nipomo East rain gauge at 545 feet reported 185 percent.
Santa Maria Airport finished the season with 18.6 inches of precious rain, or 143 percent of average. The Santa Ynez Airport reported only 75 percent of average rainfall.
This year’s pattern of persistent southerly winds created a rain shadow on the northern downward slopes of the Santa Ynez Mountains. As the air mass descended the northerly facing slopes, it warmed approximately 5.5 degrees for every 1,000 feet of dissent, which in turn increased the dew point temperature spread. This process is called the dry adiabatic lapse rate, which often causes the low coastal stratus clouds to evaporate on the leeward or downward slopes of the coastal mountain range throughout California during summer. Once the moist subtropical air reached Santa Ynez, it was warmer and much drier versus Channel Island side of the mountains resulting in less rainfall.
▪ ▪ ▪
If you would like to participate in a “Weather Watchers” tour of Diablo Canyon Power Plant and lands, which will include atmospheric and oceanographic instrumentation used for weather forecasting and other interesting weather information, please email me at email@example.com to register. The tour will be offered Thursday, July 20. The tour will start at 9 a.m. at the PG&E Energy Education Center, 6588 Ontario Road in San Luis Obispo, and will finish by noon.