In January 2015, the National Weather Service forecast offices in California changed from a “rainfall year season” to a “water or hydrological year” designation.
A hydrological year is defined by hydrologists as the 12-month period that starts Oct. 1 and continues through Sept. 30 the following year. A rainfall year season is the 12-month period from July 1 through June 30 of the next year.
The rainfall year season is designated as the year it started. For example, the rainfall season that ended on June 30 is 2015. 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 2016.
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 ‘hydrological year definition.’ ”
Because we live in a Mediterranean climate with a wet and dry season, most other California organizations with an interest in rainfall totals, such as Jan Null’s Golden Gate Weather Services, Chris Arndt’s SLOweather.com, Cal Poly Irrigation Training and Research Centers — which maintains the university’s rain gauge and records and archives rainfall data back to 1870 — and PG&E’s Diablo Canyon Power Plant 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.
Arndt likes the rainfall year because it is bookended by the dry season. 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 the water year centers on the months in which California receives most of its rainfall.
To add another wrinkle, some groups such as Weather Underground bypass both conventions and simply use the calendar year to archive annual rainfall totals.
So how did the 2015 rainfall year that ended on Thursday do?
Despite the presence of one of the strongest El Niños on record, rainfall totals were a disappointment, especially in the southern part of the county. William Patzert, a respected climatologist with Caltech’s NASA Jet Propulsion Laboratory in Pasadena, told me, “This year’s El Niño was no Godzilla — more of a gecko as far as impacts were concerned.”
Overall, the general pattern that emerged was the farther north you were, the greater the chance that your location would receive near- to above-normal precipitation; this was unlike strong El Niño patterns in the past, where locations farther south exceeded their average rainfall amounts.
One of greatest amounts of precipitation reported in San Luis Obispo County occurred at the SLOCountyWater.org Rocky Butte rain gauge in the Santa Lucia Mountains about 6 miles directly east of Hearst Castle — where 37.2 inches of rain were recorded, or about 95 percent of normal.
Cal Poly reported 19.5 inches, or 87 percent of the historical average.
Arndt’s SLOweather.com tipping-bucket rain gauge in western San Luis Obispo recorded 19.2 inches of precious rain. In the North County, the rain gauge at Union Road and Riverglen Drive in Paso Robles recorded 13.5 inches of rain, or about 108 percent of normal. The above-average rainfall was thanks in part to the unprecedented 3.5 inches of rain that fell just before the opening of the Mid-State Fair in July 2015.
Along the coastline, Cambria reported 17.1 inches, or 78 percent of normal. Diablo Canyon Power Plant logged 13.5 inches, the most since 2010. Oceano saw 11.5 inches.
Farther south, the news was discouraging. Lopez Lake Regional Park received 16.3 inches, or 71 percent of average. The Nipomo Community Services District reported only 9.6 inches, or 60 percent of what that location would normally receive.
So what will this rainfall year bring?
At this time, the latest guidance from the Climate Prediction Center indicates that La Niña (colder than normal ocean water in the equatorial and eastern Pacific) pattern is favored to develop later this summer into winter. This condition often shifts the storm track farther north into the Pacific Northwest, producing below-average rainfall along the Central Coast.
The long-range climate models continue to indicate that droughts in California will become more severe in the future because of climate change. However, just one or two atmospheric rivers or “Pineapple Expresses” can be the difference between a dry and wet year.
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