Starting almost 13 years ago I have been periodically posting here a figure similar to the one below. Newer versions of the figure incorporate recent years and occasional refinements. The last update was three years ago, so it's now time to add three more winters.
Since it has been awhile, I'll review my reasoning about what is plotted here. Why November through March? In any winter there are always periods of both wet and dry weather patterns. Though some patterns may be fleeting, others may persist for the better part of a month. A three-month winter could equally end up wet-dry-wet, or dry-wet-dry. I think that five months is a better window for capturing the overall winter. Since on average the months of November and March in Tucson are each drier than any of the other three months, in most years it matters little. But when it does rain in Tucson in those edge months, it is basically a winter pattern. Whatever those two months produce, I think their results deserve to contribute to the winter as a whole. So the vertical axis is Tucson Airport precipitation totaled for five months. Before turning to the horizontal axis, notice that the data point for this past winter of 2022-2023 is labeled 23, corresponding to the end of the five-month period, also to the year in January, the middle of the five-month period. That is what I use to categorize the winters by decade. Selected years are also labeled.
The Climate Prediction Center (CPC) issues a weekly update presentation on ENSO, with each update providing, among many other things, an explanation of and discussion about the Oceanic Niño Index (ONI). Summarizing, the calculation of ONI starts with a climate-adjusted dataset of monthly ocean surface temperature anomalies for a key area of the tropical Pacific. These monthly anomalies are averaged over three months (i.e., the January ONI is an average of the anomalies for the months of December, January and February), and then the ONI is defined to be that average rounded to one decimal place. I've repeated the three-month averaging calculation, but since I've rounded to two decimal places, same as the input dataset, technically what I have plotted is not ONI. The difference amounts to no more than the width of a plotted marker. Notice for the dry winters of 2020-2021 and 2021-2022 (unlabeled, cyan-diamond) the horizontal positions of their markers, plotted here with their (pseudo) ONI value rounded to two decimal places. For both years the January (DJF) official ONI rounds to -1.0.
The expectation that La Niña would rapidly diminish toward the end of this past winter was already well forecast at the beginning of last fall by a consensus of dynamical models. Back then it was already clear that the upcoming winter's La Niña was not going to be the same as the previous two winters. This year's ONI for January (DJF) was down to -0.7. The ONI for February (JFM) is not yet available, but will probably be close to the -0.5 threshold. Barring significant amounts of precipitation during the last two days of this month, the five-month winter of 2022-2023 ranks 23rd wettest among the last 74 winters. The decade of the 2020's, even with the two dry La Niña years, is/will be off to a good start (compared to, for example, the decade of the 2000's). There's every reason to expect that next winter's precipitation will be at least near normal, and maybe even above normal again.
