It's Monsoonal Season

The monsoonal season has arrived in Tucson. No, not the two months or so of near daily thunderstorms; that's still about a month away. What's already in full force is the silly season of talk about the monsoon. For the next month Tucsonans will endure media and meteorologists making pronouncements regarding the monsoon in general and the upcoming season in particular.

My peeve is not with the general public. Anyone who has lived in Tucson for awhile understands what the monsoon is about, and when it arrives. It's always fun to talk about the monsoon, and to speculate about the exact date when it will start. Most years the monsoon can be relied on to start within a week of, and usually just after the 4th of July. It's fun to plan on a dry 4th, anticipating a wet 6th, 8th or 10th.

More often than not at least once before the 4th it rains enough to wet the ground in Tucson. Obviously that will be the case when the monsoon arrives early. But there are rare rain days in June or very early July caused by mid-latitude westerly troughs. Traditionally meteorologists in Tucson would emphasize that those days were not the beginning of the monsoon. Their emphasis would be based not only on a diagnosis—that the winds aloft did not come close to meeting the definition of a seasonal wind shift, but also on a forecast—that the rains would not persist right on into the monsoon.

How does the National Weather Service currently define the beginning of the monsoon? I've lost track of the changes. I think that the NWS missed an opportunity a few years ago to reestablish a distinction between the monsoon, when thunderstorms occur almost every day, versus the encompassing summer thunderstorm season, the early part of which is mostly dry but with infrequent and especially hazardous days of high-based thunderstorms.

Although it goes against tradition, there is a case to be made for counting non-persistent June thunderstorms, perhaps racing swiftly off to the east-northeast, as part of the monsoon. The argument would go like this. The associated westerly trough managed to temporarily capture part of the moisture that was pooling south of the subtropical ridge. Climatologically that ridge is in the process of moving north to it's July-August position in Arizona. Parts of Sonora, Mexico, may already be experiencing the effects of a monsoonal flow. Thunderstorms in Tucson during the mid- to late-June period are less rare than earlier in June, suggesting that the northward advance of the subtropical ridge is more relevant than the obstinacy of the westerlies. The case against: those storms are followed by extended periods with no rain.

Tucson is very different from Phoenix in a number of ways. With regard to temporary surges of moisture before the monsoon, the difference lies in the trajectories of air flowing from the west-southwest toward each city. Phoenix is more susceptible to dry air from above the cold waters of the Pacific, say just south of San Diego. Tucson has more exposure to moist air channelled through the central Gulf of California. Temporary surges of high humidity at Phoenix get scoured away within a few days. In contrast, after the initial surge, high daily-average dewpoints may linger for several days in Tucson. It might be argued that persistently high daily-average dewpoints in June, albeit not quite high enough for afternoon/evening rain in the city, should define the beginning of the monsoon. But that definition clashes with what would have been the traditional call (based in part, I re-emphasize, on forecasters making forecasts) as well as my impression of what most Tucsonans think of as "the monsoon". If the monsoon starts this year on July 6, it will not be late. It will be right on time.