in Tucson. Last October I posted about the oncoming La Niña winter in Tucson. Included with that post was a plot meant to illustrate that, based on the previous sixty years, the then oncoming La Niña winter had a range of possibilities for total rainfall, from well-below average to just-below average. The vertical scale in that plot is the rainfall total for a five month period extending from November through March. It is now close to the end of March, so this 61st five-month winter period is almost over. We aren't going to get any more rain in the next two days. Tucson airport will have had a total of 0.73 inches for November 2010 through March 2011, making it the second lowest November-March period in the last 61 years. The lowest was just five years ago, and that winter did not quite qualify officially as a La Niña winter. This past winter's nearest dry neighbors in the moderate-to-strong La Niña category were 1970-1971, 1998-1999, and 1999-2000. You can identify those three previous winters in the plot by noting the color-coding for the decades. (Although the March sea surface temperature anomaly in the Niño 3.4 region is not yet available, the average over the five-month period should work out to about -1.25 °C.)
My rainfall total for this most recent five-month winter period was 1.64 inches, more than twice that of the airport. Five years ago, when the airport had it's driest November through March, I also had about twice what the airport had. (I don't get twice as much as the airport on average.) To some extent the driest winters at the airport are just dumb luck, with rare showers in an already dry year happening to miss the airport.
Fortunately, within a day's drive both west and north of Tucson it has been a wet winter. Colorado River water inflow to Arizona is expected above normal. The fact that demand exceeds long-term supply in Arizona won't have to be reckoned with for a few more years.
