Clarification of Monsoon 2024 Outlook

This is not a clarification of my own outlook, which I haven't comitted to (yet), but a clarification of reporting about other outlooks. On the weekend of June 1-2 the Tucson paper published online a story headlined, 2024 Monsoon outlook: Hotter, drier summer in Tucson.

The story repeatedly refers to something it calls the National Weather Service 2024 Monsoon Outlook, which the story says was newly released last week. I'm not familiar with such a product, but stand ready to be educated. I'm guessing that most of the time the reporter is summarizing and interpreting Climate Prediction Center three-month outlooks for Jun-Jul-Aug and Jul-Aug-Sep. I'll comment more about this later.

The fourth paragraph of the Tucson story reads,
Last year was the 17th-driest monsoon across Arizona since 1895, according to the National Weather Service 2024 Monsoon Outlook. Tucson received only 4.73 inches of rain in 2023 or .96 inches less than average, National Weather Service records show.
There's that mysterious (to me) Outlook reference again. Last year's 4.73 inches was the 39th driest in Tucson since 1895 (NWS Tucson -> Monsoon -> Monsoon Stats -> scroll to "Haywood plot"), a far cry from 17th driest. My impression of last year's monsoon in Tucson is in line with how it was summarized by azcentral. After they reported how dry Sky Harbor airport had been during the 2023 monsoon, they wrote:
As a whole, the deviation from the norm for Tucson is not that negative. A typical season usually produces around 5.7 inches of rain for Tucson's airport, coming mainly in July and August. This was mirrored in 2023, as the prime months brought 2 and 2.39 inches, respectively, making up for a zero in the June column and a lackluster September.

It's important to remember that the Climate Prediction Center issues outlooks that provide probabilities for three categories: bottom 1/3, middle 1/3, and upper 1/3. Last year's Tucson monsoon coming in at 39th driest since 1895 puts it in the bottom 1/3 (i.e., 1895-2023 = 129 years, bottom 1/3 = 43 driest years). The current CPC Jun-Jul-Aug outlook tilts the odds just slightly toward the bottom 1/3 (33-40% chance), which leaves 60-66% chance for either middle one-third or upper one-third. The CPC outlook for Jul-Aug-Sep is a little more pessimistic, but still leaves a 50-60% chance for either middle one-third or upper one-third.

I agree with two quotes in the Tucson news story from Michael Crimmins: ... forecasting the monsoon is incredibly hard and ... the summer impact of La Niña on the monsoon is actually quite weak.

My bet, or maybe it's just wishful thinking, is that above normal temperatures that are expected to persist over New Mexico will instead by July have shifted a bit toward north Texas. The circulation around the southwest side of the associated upper-air area of high pressure (what media have taken to calling a heat dome) would favor squall lines on several days sweeping across Tucson during the afternoon and early evening, effectively squeezing out available moisture. That's a pattern that has been lacking in recent years, but it's due.

Tucson Winter Precipitation Update

Starting almost 14 years ago I have been periodically posting here a figure similar to the one below. Last year at this time we had just come out of three straight La Niña winters. Now it's time for the scatterplot to incorporate this past winter, which was dominated by a strong El Niño.

Last year's update was the first in a few years, so in that post I reviewed my reasoning about what is plotted in the figure. Please see last year's update for a more complete discussion. The reason I use a 5-month winter for Tucson is that even in a wet winter an entire month can be well below average, and conversely for a dry winter. It's better in El Niño winters to take note of 4 out of 5 wet months, instead of focusing on mid-winter and so sometimes discounting a precipitation total that results from combining only 2 wet months with 1 dry. As it turns out for this past 5-month winter it was November that was the dry month. A wet March made up for November. So for this past season in particular the net impact of broadening the definition of winter from 3 to 5 months was a wash.

On the horizontal axis, the ONI is a number calculated monthly as an objective way to summarize the status of ocean temperatures in a critical part of the tropical Pacific. Since the calculation is a running mean, the most recent value lags by a month. There is also a persistence requirement, as explained by the Climate Prediction Center in their weekly updates. Generally a value greater than 0.5 means El Niño conditions are present, while less than -0.5 is La Niña. This past winter the ONI edged up to 2.0 in December before dropping back to 1.8 in January. See my last update a year ago for more discussion about what I have plotted here. Though the ONI for this month (April) will probably still be in El Niño territory, model outlooks have it falling rapidly to below -1.0 by this October.

The TUS 5-month total precipitation for this past winter was 6.16 inches, which makes it the wettest winter in 26 years, and ranks it 11th wettest of the last 75 years (i.e., of the 5-month winters 1949-1950 through 2023-2024). Last fall, even though an El Niño of moderate (ONI > 1.0) or strong (ONI >1.5) intensity was expected for the 2023-2024 winter, for the West a minority contrary forecast had a near normal outlook for winter precipitation, apparently based on giving maximum weight to recent trends (subsequent to the 1997-1998 winter). Now that the winter of 2023-2024 is a fact, the underperforming winter of 2015-2016 looks even more like an outlier, not part of a new trend.