Upcoming El Niño Seasons

Here in Tucson the rain for the past week finally met expectations. But already, even before the monsoon begins to wane, attention is turning to the upcoming El Niño seasons. It will be interesting to see if the consensus outlooks hold for a strong El Niño to develop and persist through the winter. Whatever the strength of El Niño may turn out to be, the evolution of the weather patterns will be fascinating.
I was preparing to update a five-year-old plot and accompanying rant concerning winter precipitation variability here in Tucson, which I had made back when a La Niña was on the way. But a few days ago the local paper had a tantalizing story discussing the prospect of an active tail-end to the monsoon this year, because El Niño.
So I decided to see what the actual numbers look like. Traditionally September rains would be considered the tail-end of the monsoon, and a tropical system in early October would be beyond the monsoon. Actually, that's still the official definition. Instead of trying to separate the two, I totaled the precipitation for the two months of September and October. The precipitation data are from the Western Regional Climate Center. I used the latest, official definition of the calculation of the Oceanic Niño Index by the Climate Prediction Center. There are a total of 65 years on the plot. Selected specific years are labeled.

I hope that nobody tries to fit a straight line through the plotted points. If this year's October ONI is greater than +1.0, as present trends and most projections suggest, what does history tell us about what to expect? On the plot, there are eight years with an October ONI greater than +1.0 (the right one-third of the plot). For seven of those eight years, the combined total September-October precipitation was at least 1.7 inches. In contrast, the left two-thirds of the plot contains 57 years with an October ONI less than +1.0. Of those 57 years, nearly half had less than 1.7 inches, and two of those years had no measurable rain in either month. So this year, bet on more than 1.7 inches in September-October, because you will have a very good chance of not losing that bet. But another way to summarize the right side of the plot would be to say that in six out of those eight years, precipitation was within plus or minus one half inch of the 65-year mean, which is 2.21 inches. On the other hand, one might choose to go with a persistence forecast, imagining that whatever factors contributed to last year's, i.e. 2014, September-October precipitation performance, those factors are only better this year, and thus the decade of the 2010's might join the 1970's as star contributors toward the upper right quadrant of the plot.
Citing 1972's disastrous tropical system Joanne as the kind of thing that could happen this year would at least be the right analogy--both years with oncoming strong El Niños. Citing 1983's September-October disastrous sequence of events as an example of what can happen in an El Niño year, while discussing this year's high and rising ONI's, is mixing apples and oranges. The 1983 data point would support imagining a big September-October next year. That data point is the only big year making the argument for a lagging effect in September-October from the previous winter's El Niño. But then 1972 is the only data point arguing for a big September-October this year. Certainly the possibility of disastrous flooding triggered by tropical systems in September and early October is a concern, but it should be a concern most years. Dangling the notion that that concern should be higher in Tucson this year because a strong El Niño is developing is not supported by the data.