Is this the driest Central Coast rain season on record? Here’s what history shows

Jim from Avila Beach asked, “Do you know what the record is for the driest rain season that we have experienced? I have a feeling we may challenge it this year.”

To find out, I looked at the longest continuous rainfall observation database along the Central Coast, which is maintained by the Cal Poly’s Irrigation Training and Research Center, available online at www.itrc.org.

The first monthly recording in this data set occurred in October 1869 with a reading of 0.84 of an inch of rain.

After review, the driest year on record was 1897, when only 7.2 inches of rain was recorded. The next driest year was 1923, when 8.19 inches of the wet stuff was measured.

The third most parched rain season happened in 1893, when 9.81 inches of rain fell to the Earth. It is interesting to note that all these years were preceded or followed by typical or above-average rainfall amounts.

That could not be said for the worst extended drought which occurred from 2011 through 2015.

2013 only saw 10.61 inches of rain, followed by 11.52 inches in 2014.

This rainfall season, which runs July 1, 2020, through June 30, 2021, we are already passed the one-third mark of the highest probability of rain, and rainfall at all Central Coast locations is less than 10% of normal. In fact, the last time the Diablo Canyon rain gauge recorded one inch or more occurred in March, or nine months ago.

After a dry October and a nearly parched November and December, a storm was expected to produce much-needed precipitation Sunday night into Monday morning. A stretch of mostly dry weather started Tuesday and is forecast to continue through New Year’s Day.

So here’s the big question on many people’s minds: “What will the rest of the rain season be like?”

I decided to check John Neil’s observations.

Neil loves the sciences of hydrology and meteorology and their potential impact on people’s lives. The Atascadero Mutual Water Company, which he manages, has collected rainfall data since 1914 at a rain gauge at the Salinas River and Atascadero Creek’s confluence.

After reviewing the historical data from this rain gauge, Neil discovered that, when the area received more than 2 inches of precipitation during October, the rest of the rain season was higher than average. Neil calls this the “2 inches of rain in October rule.”

However, in the years when we had no measurable rainfall from July 1 through Oct. 31, annual totals surprisingly weren’t too bad. The average is 16.13 inches or about 1.5 inches below the historical average.

With that said, the current La Nina condition, combined with the cool phase of the basin-wide Pacific Decadal Oscillation (PDO), points to below-average rainfall.

I would predict this rain season will be between 60 and 70% of average, but I don’t think it will break the record for the driest year on the books. Only time will tell.

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