It’s been a dry autumn here in Davis. We’re well behind the average rainfall for the water year. As I’ve discussed before, rainfall in the central valley in the dry season is very episodic. But what about rainfall in the transition from the dry to the moist season? Below are histograms of the first date of rainfall in a water year passing some threshold. X-axis is day of the year. Y-axis is a count of years. The threshold is listed in the title for each panel. For the lowest threshold, just 0.01″, the median first day of rainfall for the water year in Davis is September 25th (the ‘S25’ in the title). This year, my CoCoRaHS gauge saw 0.02″ of rain on October 2nd. So we were a little behind but well within the meaty part of the histogram. For a threshold of 0.10″, the median first date is October 5th. This is probably a more reasonable threshold for a real rain. In 2018, we have not yet seen a 0.10″ rainfall. Our first hope on the long range forecast is for rainfall in excess of 0.10″ on Thanksgiving day or day 326 of the year. That would put us well into the tail of that distribution. In fact it would be the third latest 0.10″ rainfall in the last 50 years. At higher thresholds, the distribution skews more toward later in the calendar year. For a 1″ threshold, 16 of the past 50 years have not seen rain before the end of the year (the ‘+26’ in the title). So, we’re behind for the year in accumulated rainfall, but not necessarily in our first big storm of the year. One big storm could get us right back on track and make our dry autumn unremarkable. That being said, I’m not holding my breath that the storm on Thanksgiving materializes.
