What to Expect (or Not) From This Year’s Monsoon

As fires rage in Arizona and New Mexico and drought intensifies and spreads, the questions on everyone’s mind are: Will the monsoon deliver copious rains like it did in 2006, or will it fizzle like 2008? And when will the monsoon arrive this year? Unfortunately, monsoon forecasts don’t provide definitive answers—we’ll have to wait and see.

Forecasting the onset of the monsoon—let alone how much rainfall will occur over the season—is hard, plain and simple. In recent years, however, forecast models have shown some accuracy in projecting the onset, especially when strong climate signals, such as very warm or cold sea surface temperatures (SSTs), occur in the tropical Pacific Ocean, Gulf of California, and/or Atlantic Ocean. When these signals are weak, it’s a coin toss.

Forecasts for the total seasonal precipitation of the Southwest monsoon, however, have not been accurate. This is partially because Arizona and New Mexico lay on the northern fringes of the monsoon region and are therefore influenced by many climate and weather processes, including the position of the monsoon ridge, the rate at which the landscapes heats up in the spring and summer, the amount of snowfall in the preceding winter, and SSTs in the tropical and north Pacific Ocean. All these factors add uncertainty to the forecasts.  Additional questions arise related to our nascent scientific understanding of convection dynamics and the role of vegetation in recycling water back to the atmosphere through evapotranspiration. Both are important components of the monsoon, but cannot be fully represented in most computer models due to the models’ large spatial resolution (typically a 50-km by 50-km grid), which effectively flattens the high elevations of relatively small mountain ranges that are significant to the initiation of monsoon storms.

With respect to this season, most climatologists with whom I have spoken agree that this year is particularly difficult to forecast. The official U.S. seasonal climate outlooks issued by the National Oceanic and Atmospheric Administration’s Climate Prediction Center (NOAA-CPC) call for equal chances of above-, below-, or near-average rainfall for the monsoon season (Figure 1). The primary basis for this forecast is that the moderate-to-strong La Niña event that delivered dry conditions this winter waned rapidly this spring and SSTs in the tropical Pacific are now near average. And while SSTs in the Atlantic Ocean and Gulf of California are above average, they are only slightly so. In other words, there are no strong climate signals to nudge the model one way or the other.

There are several other forecasts worth mentions. The experimental forecasts produced by the North American Monsoon Experiment (NAME) suggest a late onset and below-average rainfall (again, note that forecasts for total precipitation during the monsoon season have not been accurate). Also, a forecast issued by Art Douglass, professor and chair of the Department of Atmospheric Sciences at Creighton University, suggest that the monsoon will be above average for the June−August period. Douglass’s approach, described in more detail on the Mad Weather Blog, does not rely on forecast models. Rather, he selects past years that have similar climate conditions as this year and uses these as a guide for this season.

If I were a gambler, I’d likely stay away from betting on how the monsoon season will unfold. I’m hopeful, however, for an early beginning and constant rains to quell fires and breath life back into parched landscapes.