Annual precipitation in southern Arizona follows a bimodal distribution. That means most rainfall occurs in two distinct periods, with little rainfall during the remainder of the year. Winter precipitation results from Pacific frontal storms and occurs between December and mid-April. A summer rainy season results from tropical moisture and convective heating during July and August. Locals commonly refer to the summer rainy season as the “Monsoon”.
While the North American Monsoon is a significant weather event, the name “monsoon” is somewhat of a misnomer. People generally associate a monsoon with a widespread, torrential, damaging rain. The thunderstorms associated with the North American Monsoon can be locally severe, but widespread rainfall and damage is uncommon. These storms produce incredible lightning and wind, but not the widespread damage associated with hurricanes, tornadoes, or flooding as seen in central or eastern North America. Locally, an Arizona Monsoon storm may drop 1-2 inches of rain, whereas nearby areas remain completely dry. An Arizona Monsoon storm is a local headline, not a national one.
From the perspective of water managers and the overall water budget, the Arizona Monsoon is insignificant. Far more important is the widespread winter precipitation and mountain snowfall that is responsible for filling reservoirs and recharging aquifers. The ecological and cultural impacts of the Monsoon are more significant. First, Monsoon rain falls when plants are active and growing. Second, indigenous cultures coordinate crop planting with the arrival of summer rains. And perhaps less recognized is the the annual community dialogue regarding the rain or lack thereof. Discussing weather seems old fashioned, but “whether one got rain at their house” or “when is it going to rain again” are significant conversation topics for Arizona residents. The longing for a cooling summer thunderstorm becomes a human commonality.
A few years ago, the National Weather Service established June 15 to September 30 as “monsoon season”, creating a local controversy. Before then, the “monsoon season” began after a few consecutive days of average dew points above a certain threshold value. In Phoenix, the Monsoon officially began after three consecutive days where the average dew point was 55 degrees or higher (Tucson used 54 degrees, and other areas of Arizona may have had different criteria). The National Weather Service recognized the following inadequacies of the old method, and opted to change:
1) The goal of the Weather Service is to predict and warn people about weather hazards. Declaring something has begun 3 days after it has already begun fails at this task. The old method did not typically declare the start of the Monsoon season until the first few storms had hit.
2) A dew point system sounds scientific, but is arbitrary. A 55 degree dew point is not scientifically significant, as it is possible to have storms when the dew point is lower than 55, and a dew point above 55 does not guarantee rain. A “three consecutive days” period is also arbitrary. Why not two days or four days? Using the old system, Phoenix could have a 60 degree dew point on the first day, 54 degree dew point on the second day, and a 60 degree dew point on the third day, yet the “Monsoon” had not begun despite the average dew point of 58 for the three days combined.
3) Other areas of Arizona had different criteria, leading to confusion. Tucson used a dew point of 54 degrees. The old system was based upon urban weather stations, which are mostly useless for residents of rural communities a hundred or more miles away.
Long time local residents resist this change, sometimes vocally. While it may sound silly to ague the definition of “Monsoon Season”, the attitudes revealed coincide with attitudes toward other Arizona issues. Several reasons exist for the backlash towards the new “Monsoon Season”:
1) The federal government (National Weather Service) is telling the local community what to do. After all, how does Washington DC know more about local weather than people who have lived here for decades?
2) It is a conspiracy. To an Arizonan, anything that seems fishy or goes against long-held understanding is a conspiracy. Conspiracy theories about the new Monsoon definition are likely due to some kind of Global Warming/Al Gore hoax, or as a way of hiding a perceived diminishing amount of storms over metro areas. Perhaps the developers forced the government to change the definition so the effects of urbanization are hidden?
3) The new definition invades a local tradition of speculation about the beginning and intensity of the upcoming Monsoon.
While reasons #1 and #2 are important, I suggest the angst primarily results from reason #3. A cultural “season” begins in Arizona about the time the temperatures in the deserts first climb above 100 in mid-May. Desert dwellers begin opining about when the monsoon will come, how intense it will be, and how long it will last. The conversation becomes almost daily in public places and at dinner tables as sweaty residents become anxious for relief from the monotonous hot sunny days. After all, summer thunderstorms are exciting. Mountain residents also want relief from the windy dry air that brings explosive fire danger. People don’t care about when the Monsoon season officially starts, they care about when the first rain comes as that signifies the end to the “Monsoon Anticipation Season”. The NWS creating a Monsoon season based upon calendar dates destroys that cultural tradition.
So when does Monsoon Season begin? According to the National Weather Service, it begins today, June 15th. The weather today is sunny and dry, with an expected high temperature of about 100.
The NWS also has some pretty good web pages that describe what causes the monsoon and why it is so hard to predict. Click here for the info.
The weather changes that signal the Monsoon begin during the middle of June. Rain in the desert is the last thing to change. Establishing a beginning date of June 15 recognizes the beginnings of the seasonal weather change rather than the completion. This makes sense for those who live in mountainous areas, who are likely to see thunderstorms before the desert valleys.
For those that still prefer a dew point system, weather station data is readily available online. Have at it.