The University of Arizona

Optimistic Monsoon Forecast

June 29, 2012
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Zack Guido

So far so good: the monsoon in southern Arizona is off to a vigorous start. An early onset had been predicted by several monsoon forecasts, which also call for an above-average July and September and a near-average August. These optimistic outlooks are welcome news for a region in the throes of short-term severe drought for more than 18 months.

The NOAA-Climate Prediction Center (CPC) issues the official U.S. climate outlooks every third Thursday of the month. The seasonal forecasts cover three-month periods spanning 13 months. Usually, the longer the lead time, the less reliable.  Thus in the Southwest, the monsoon forecasts that matter are issued in June.

Figure 1. The July–September forecast issued by the CPC on June 21. Green hues represent regions in which rainfall is expected to be similar to the wettest 10 years in the 1981–2010 record. The numbered contours represent the probabilities for conditions to be similar to the wettest, driest, or middle third. Equal chances reflects a 33 percent chance that rainfall will be in one of the three categories. Image credit: NOAA-CPCOn June 21, the CPC forecast called for slightly enhanced chances for above-average rain in southern Arizona and New Mexico for the July­­–September period (figure 1). I spoke with David Unger, the CPC meteorologist who issued the forecast. Unger stated that an early onset and above-average July precipitation principally drive the seasonal forecast; the fate of August and September is not known.

Details of the CPC forecast include a 33.3­–39.9 percent chance that precipitation will be 0.2 inches above average. But it also calls for a 33.3 percent chance for near-average conditions and a 26.6­–33.3 percent chance for below-average rain. These odds are not heavily weighted, and the total precipitation anomaly of 0.2 inches is also not impressive. However, when two-thirds of the monsoon season—August and September—are unknown, one-sided odds and larger anomalies are less likely. This highlights one limitation of the CPC forecast: it treats the monsoon as one lump sum.

Jon Gottschalck, head of forecast operations at the CPC, elaborated: “Monsoon forecasting over the season is so difficult because July through September is a long period, and a lot can happen. Anything early on could be completely outweighed by the final two months.”

It is also worth highlighting that the CPC forecasts cover the entire U.S. This limits the amount of time a forecaster can spend tuning the models to the particular climate nuances of a region—there are too many climate-unique regions.Figure 2. In the historical record, five summers most resemble current conditions defined by 12 climate variables. Comparing the total June–September precipitation for these years—1984, 1986, 2001, 2006, and 2008—suggests that most of the Southwest may receive rainfall totals exceeding 110 percent of the 1971–2000 average. Image credit: Westmap.

Fortunately, other methods can provide vital and more detailed information about the monsoon. One approach characterizes current conditions and analyzes past summers that most resemble these conditions. Art Douglass, professor and chair of the department of atmospheric sciences at Creighton University, uses this analog approach. He has been forecasting the monsoon since 1977 and his analysis this year selected five summers with conditions similar to those in mid-June. They include 1984, 1986, 2001, 2006, and 2008.

In a nutshell, the forecast issued by Douglass calls for an early monsoon onset, followed by a wet July, a near-average August, and a wetter-than-average September. Taken together, it’s a pretty optimistic forecast for rain, Douglass concluded (Figure 2). I detail Douglass’ forecast in an article in the Southwest Climate Outlook published on June 27.

Will this forecast prove right? I doubt it will be spot on—forecasting the monsoon is no easy task. Arizona and New Mexico sit on the northern fringes of the core North American monsoon region, which is centered over the Sierra Madre Occidental in northwest Mexico. As a result, many climate factors come into play and cause high year-to-year and month-to-month variability. None of the past years deemed most similar to current conditions will be an equal representation, and combined they will smooth out the nuances that make each monsoon unique. However, I’m keeping my fingers crossed that the past is the key to the present. The region needs the rain.

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