Sunday, June 30, marks the end of the 2018 rainfall year season, while Monday, July 1, is the start of the 2019 rainfall year season. The National Weather Service forecast offices in California changed from a “rainfall year season” to a “water year” designation in 2015.
Hydrologists define a water year as the 12-month period that starts Oct. 1 and continues through Sept. 30 the following year. A rainfall-year season is defined as the 12-month period beginning July 1 that extends through June 30 of the subsequent year. The rainfall year season is designated as the year it started.
For example, the rainfall year season that just ended was 2018. On the other hand, the water year is labeled by the calendar year in which it ends, which is understandable because nine of the 12 months fall in that year. For example, this water year would be referred to as 2019.
The state’s water managers and hydrologists tend to like the water year designation because October usually has the least amount of stream and river flows and tends to center on the months in which California receives most of its rainfall. Santa Barbara County Water Resources utilizes a water year, while the Water Resources Division of San Luis Obispo County Public Works Department has remained with a rainfall year season.
Since we live in a Mediterranean climate with a wet and dry season, many other California organizations with an interest in rainfall totals, such as Jan Null’s Golden Gate Weather Services in the Bay Area, Chris Arndt’s SLOweather.com, Cal Poly Irrigation Training and Research Center (which maintains the university’s rain gauge and records and archives rainfall data back to 1870) and PG&E’s Diablo Canyon Power Plant will continue to use the historical rainfall year season (July to June) designation.
“This convention is based on over 100 years of sound meteorological/climatological practice in California,” Null stated.
Last year, I predicted that San Luis Obispo County would receive around 110 percent of above-average rainfall, and northern Santa Barbara County would see normal amounts due to the predicted El Niño and the positive phase of the Pacific Decadal Oscillation (PDO).
With that said, here were my rain season’s predicted rainfall totals last year:
▪ Cambria, 24 inches;
▪ Paso Robles, 14 inches;
▪ San Luis Obispo, 25 inches;
▪ Nipomo, 20 inches;
▪ Santa Maria, 14 inches;
▪ Lompoc, 16 inches;
▪ Santa Ynez, 14 inches.
Here are the actual rainfall totals throughout the Central Coast for the 2018 rainfall year season that just ended:
▪ Cambria, 26.2 inches or 119 percent of average;
▪ Paso Robles, 15 inches or 119 percent;
▪ Cal Poly (home of climatology for San Luis Obispo), 29.48 or 132 percent;
▪ Nipomo, 19.58 or 109 percent;
▪ Santa Maria, 15.68 or 121 percent;
▪ Lompoc, 15.62 or 106 percent;
▪ Santa Ynez, 14.43 or 104 percent.
Some of the other rainfall totals:
Rocky Butte above San Simeon recorded 52.40 inches of rain or 131 percent of average annual rainfall. SLOWearther.com in western San Luis Obispo finished with 32.38 inches or 132 percent of typical.
The rain gauge at Diablo Canyon saw 20.08 inches or 107 percent of normal. Overall, this was a near-ideal rainfall season. I wish we picked up more precipitation, but with few “gully washers,” little flooding was reported along the Central Coast. The highest 24-hour rainfall total at Diablo Canyon was 1.5 inches recorded on Nov. 29, 2018.
However, we have seen 76 days of measurable rainfall at the power plant. Last year, there were only 37 days of measurable rain for the entire rain season (July 1, 2017, through June 30, 2018) with over 4 inches of rain recorded on one day, March 22, 2018. Even during the terrible dry years of 2013, ‘14 and ‘15 when seasonal rainfall totals didn’t even reach 10 inches, we had occurrences of 24 rainfall totals reaching over 1 inch; nearly 2 inches was recorded on Dec. 12, 2014, when that rain season only saw a total of 7.5 inches at the power plant.
The first rain of the season occurred on Oct. 4 when nearly three-quarters of an inch was recorded at Diablo Canyon, then it turned dry until Nov. 22 when a cold front followed by an upper-level trough produced heavy rain throughout the Central Coast. Rain was reported nearly every day afterward through the first week of December.
The ranchers loved the November rains as they promoted an early start to the grasses that transformed our hills to emerald green. Later in December, the Eastern Pacific High anchored itself off the California coastline and combined with transitory high-pressure systems over the Great Basin — the area between the Sierra Nevada range to the west and the Rocky Mountains to the east — created persistent Santa Lucia northeasterly (offshore) winds.
These winds brought in a cold air mass that created frosty mornings and bone-dry weather after Christmas to the Central Coast. Then a significant change occurred in the first part of 2019; the Eastern Pacific High weakened and shifted, allowing the upper-level winds (jet stream) and the storms it carries and nourishes to travel at high speeds across the Pacific from west to east toward the Golden State and produce rain nearly early day in January and February.
All this rain significantly increased the lake and reservoir levels throughout the Central Coast and officially ended the drought for California, according to the U.S. Drought Monitor, a partnership between the National Drought Mitigation Center at the University of Nebraska-Lincoln, the United States Department of Agriculture and the National Oceanic and Atmospheric Administration (NOAA).
So why did the Eastern Pacific High weaken and open the storm door to an impressive series of storms that marched across the Pacific into California through February. At this time, nobody knows for sure, but it could be the following oceanographic and atmospheric phenomena, and here is why.
The first possible cause is El Niño (warmer than average sea surface temperatures in eastern Equatorial Pacific). Another conceivable reason, the Madden-Julian Oscillation (MJO). Unlike El Niño and La Niña — both of which are a standing pattern where they stay fixed in the same geographic area, The MJO is a large traveling pattern of increased rainfall and thunderstorm activity that propagates eastward at approximately 8 to 18 mph across the tropical parts of the Indian and Pacific oceans.
Another cause for above-average rain is another great oceanographic cycle that can orchestrate changes in our weather is a longer lasting cycle called the Pacific Decadal Oscillation or PDO. While the ENSO phase typically lasts from eight to 13 months, the PDO alternates between a warm phase (positive) and a cooler phase (negative) that can last a few years to decades. Unlike El Niño, which focuses on SST in the central equatorial region of the Pacific Ocean, the PDO is classified by seawater temperatures throughout the northern Pacific Ocean.
According to Josh Willis, oceanographer and climate scientist at NASA’s Jet Propulsion Laboratory in Pasadena, the PDO shifted to the positive phase. Historically, the positive phase of the PDO typically enhances the effects of El Niño events.
Last but perhaps not least is the Arctic vortex, which split into two sections and created a blocking high in the Gulf of Alaska that helped to steer storms into California. All of these or just one or two of these coupled conditions between the ocean and atmosphere may have helped to produce above-average rainfall and snowpack for California.
Since this week’s column is running long, I will give my long-range prediction for this upcoming rainfall season in next week’s column.
This story was originally published June 29, 2019 2:54 PM.