Fall Festival of Rain

Tucsonans don't complain when it rains. Rain makes the actual festivals more fun, more reason to celebrate while packed under the tents. Tucson so far this fall has escaped any serious flooding such as has affected points south, west and north of here.

In a post two months ago, I noted the Climate Prediction Center's odds for El Niño during the upcoming fall and winter, and revisited my rant from three years ago regarding chatter about fall rain in Tucson related to El Niño. Since this fall's rains have been so good, I decided to present a fresh scatterplot for fall. For more explanation about the reasoning behind the scatterplot and for additional sources, follow the links in the previous post.

The Oceanic Niño Index (ONI) is defined and calculated by the Climate Prediction Center [CPC], and won't be available for October until the end of November. For now I'm forecasting 2018's October ONI will be approximated by the latest (mid-October) weekly Niño 3.4 region SST departure. As for this year's fall (September plus October) rain, so far for the period from 1 September through the morning of 16 October the airport has had 3.44 inches; that running total will probably increase by the end of the month. (I've had 4.76 inches for the same period, including 0.84 overnight last night.)

Considering just the last six years (2013-2018), there is a trend toward higher precipitation totals when moving from colder to warmer ONI. The decade of the 2010's has joined the decade of the 1970's in favoring above normal fall precipitation when the ONI is in positive territory. The exceptional years (fall precip well below normal but positive ONI) for those two decades were 2012 and 1979. Fall of 1976 was close to normal.

Considering the wet years appearing in the negative territory of October ONI, with the exception of the decade of the 1950's, in each decade there is one unusually wet fall. These wet falls have followed El Niño fall/winters by one (1964, 1970 and 1983) two (1996 and 2011) or three (2000) years. Certainly it's possible that the climate system sometimes stores somewhere a lingering effect of a previous El Niño, maybe even for a few years, and that that lingering effect influences Tucson precipitation in a specific La Niña fall. But most often any lingering effect does not appear to have an influence. As a specific example, most recently the fall of 2016 does not appear to have been influenced by a lingering effect of the previous fall/winter's strong El Niño. It is as much nonsense to call the fall of 1983 an El Niño year as it would be to apply the same label to the fall of 2016.