Sunday, July 1, is the start of the 2018 rainfall year season. In 2015, the National Weather Service forecast offices in California changed from a “rainfall year season” to a “water year” designation.
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, this last rainfall year season that just ended was 2017. On the other hand, the water year is labeled by the calendar year in which it ends, which is understandable because 9 of the 12 months fall in that year. For example, this water year would be referred to as 2018.
According to the National Weather Service, “This change will keep precipitation reports in the daily NWS climate reports consistent with the U.S. Geological Survey, state of California water agencies, and most other weather offices nationwide who utilize the Oct. 1 'water year definition.'”
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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,” Jan Null stated.
So, what were the rainfall totals throughout the Central Coast for the rainfall year season that ended on Saturday? The start of the 2017 rainfall season occurred during a weak La Niña condition, which typically produces below-average rainfall. And sure enough, the 2017 rainfall season started terribly dry. In fact, average rainfall amounts at the Paso Robles Municipal Airport, Cal Poly (home of climatology for San Luis Obispo) and the Santa Maria Public Airport all dropped to approximately 27 percent of average at the end of February. The 2018 rainfall season was on track to be one of the driest on record, and optimism was fading.
Then spring came in like a lion with one of the most significant Atmospheric River events to hit the Central Coast since December 2010, which created moderate to heavy rainfall throughout Central California and produced a ‘Minor March Miracle!’”
Communities in the direct path of the AR saw terrific gains in seasonal averages. Paso Robles, Atascadero, Cayucos, Morro Bay and Los Osos saw their rainfall totals reach near or over 70 percent of average. George Brown of Los Osos recorded 11.45 inches of rain, or about 75 percent of normal. Paso Robles Municipal Airport recorded 9.65 inches of rain, it usually receives 12.78 inches. Atascadero Mutual Water Company reported 12.17 inches, or near 70 percent of average.
Away from the direct path of March’s AR, seasonal totals were less. The Diablo Canyon rain gauge reached 13.45 inches, or 67 percent of average. Cal Poly (home of climatology for San Luis Obispo) reported 14.34 inches, or about 64 percent of average. Arroyo Grande saw 9.01 inches, or about 50 percent of normal. Nipomo reported 8.78 inches, or 63 percent of typical. The Santa Maria Airport recorded 6.48 inches, or just 46 percent of average. Farther south, Santa Barbara had 8.57 inches, when it usually receives about 17.76 inches.
What will this rainfall year season bring?
Since 1950, the National Oceanic and Atmospheric Administration has used sea surface temperatures in a central equatorial region of the Pacific Ocean called Niño 3.4 as the standard for classifying El Niño (warmer-than-normal SST) and La Niña (cooler-than-normal SST) events. The fortunetelling SST cycles in Niño 3.4 are categorized by the amount they deviate from the average SST. In other words, an anomaly. Last year the seawater temperatures in region Niño 3.4 were slightly cooler than usual, hence the classification by NOAA of a weak La Niña condition. The name La Niña, meaning “the girl,” originates from Spanish; it’s analogous to El Niño, meaning “the boy” or “the Christ child.”
This year, The Climate Prediction Center is currently advertising a neutral condition — the infamous “El Nothing” continuing through summer. However, the chance for El Niño will increase to 50 percent during the fall, and 65 percent during winter 2018-19, which is excellent news if you’re hoping for increased amounts of precipitation. The most important aspect of this prediction is how strong of an El Niño event this will eventually become. Typically, along the Central Coast, the higher the classification of the El Niño event, the more rain it will create.
Unfortunately, most of the models and ensemble packages are advertising a weak El Niño condition, which like neutral conditions — or El Nada — typically doesn’t produce any reliable seasonal rainfall predictions along the Central Coast.