Did Copious Rains in September Save the Monsoon?

After a bang-up start around July 4, the 2011 monsoon ended up being pretty lackluster. Most of the Southwest received below-average rainfall in part because the monsoon high parked over Texas and Oklahoma, too far east to continuously funnel moisture into most of New Mexico and Arizona. In addition, weak winds aloft and the lack of a large temperature gradient between the surface and high altitudes created conditions that quelled storm formation even though ample moisture was in the air. But in the second week in September, seemingly out of nowhere, storms flared up across Arizona. Several cells produced microbursts with winds exceeding 70 mph and intense rain doused areas with more than two inches of rain. In a seven-day period, most of Arizona and parts of northern New Mexico were soaked.  By mid-month, Tucson (or at least the airport, where the weather is officially recorded) had experienced its wettest September on record, and the total monsoon precipitation, which began on June 15, ranked as the 10^th wettest.

Aspects of the regional atmospheric context that helped generate the mid-September rain, however, were not monsoon-like. In fact, the monsoon circulation had ended a few days prior to the copious rains, according to experts.

So did the mid-September storms save the monsoon from being labeled drab in many parts of Arizona? Yes, if your interests are not dependent on a strict definition of the monsoon, and no if they are (more on that in a bit).

The ambiguity is driven by winter-like winds that created conditions favorable to exploit monsoon moisture that was lingering in the region. What occurred was a cut-off low pressure system off the coast of southern California that helped drive cool temperatures into the region at mid to upper levels in the atmosphere. This increased the temperature gradient between the hot land surface and cooler higher altitudes, enabling moisture to rise, cool, and then condense in explosive storms. But the prevailing winds were from the west, not the southwest where monsoon winds originate.

For most people, it’s not important when the monsoon began or ended. What matters is the quantity of rain and its character. I doubt reservoir managers care if rain falls a few days before or after the monsoon ended, nor will ranchers mind if grasses green up before or after the monsoon.

For the National Weather Service (NWS), which has a charge of helping inform the region of weather-related events that can cause harm, it is easier to define a static start and end date, which is June 15 through September 30. However, it is still a worthy exercise to analyze the start and end dates from more physically-based perspectives because a lot of interpretation and analysis depends on these definitions. For example, when assessing how and why the monsoon has changed, perhaps due to global climate change, a more complete analysis will go beyond simply quantifying precipitation and also look at wind directions and the origin of moisture, among other factors. And when assessing how well climate models simulate the monsoon, part of the model performance is based on non-precipitation variables. If climate models replicate the wind patterns well, for example, but over-estimate precipitation, the models still provide useful information.  

From a technical perspective, the monsoon likely ended around the 5^th of September (see Bob Maddox’s blog), but when you read or hear about the 2011 monsoon, it will include all of September’s rainfall in order to conform with the NWS definition. For most people, a static season is a good way to characterize the monsoon. It is worth keeping in mind, however, that the monsoon is nuanced and the physical processes that create it are never stationary.