The Dewpoint Rule—A Historical Look at the Evolution of Defining the Onset and Demise of the North American Monsoon in Arizona

Human nature drives us to label and classify the world around us. For many years, forecasters and climatologists over the southwest United States recognized summer thunderstorms were more frequent in July and August, but frequency of occurrence varied noticeably from year to year. In the mid-1960s, meteorologists with the NOAA/National Weather Service (NWS) in Phoenix, AZ, devised an “Arizona Monsoon” onset rule predicated primarily on atmospheric moisture content over Phoenix. For many years, the monsoon was defined as having begun on the first of three consecutive days when Phoenix, AZ, recorded an average surface dew point of 55 °F or greater; this was referred to as the “55 degree dew point rule”. Typically, this moisture increase was associated with a transition from deep-layered westerly flow to an easterly or southeasterly flow, and was deemed sufficient to increase the threat for thunderstorms over Arizona. However, the seasonal wind shift, a principal aspect of the monsoon, was not explicitly included in the monsoon onset definition. The end of the monsoon was more subjectively determined, and was declared when westerly flow aloft returned while average dew point fell below 55 °F. Scientific advances allowed meteorologists to learn that the so-called Arizona Monsoon was in actuality just part of the much broader North American Monsoon (NAM). The Phoenix-centric focus of the start/end dates meant many locations typically influenced by the NAM often experienced periodic thunderstorms with intense lightning, heavy rain and localized flooding, and outflow-generated dust storms prior to official monsoon onset. Over time, this became increasingly problematic for other southwest United States NOAA/NWS Weather Forecast Offices (WFOs), including Tucson, Flagstaff, El Paso, Albuquerque, San Diego, and Las Vegas. In addition, the underlying research supporting the 55 °F dew point rule became more and more outdated as environmental sensing tools improved and knowledge of the NAM mechanics evolved. Finally, it was believed a hyper-fixation on start/end dates driven primarily by the 55 °F dew point rule detracted from the real mission of the NWS: to provide partners and constituents with useful guidance regarding the protection of life and property. Subsequently, in 2008, the southwest United States WFOs agreed to define the NAM with fixed start and end dates, 15 June to 30 September, an approach analogous to the “hurricane season” as well as “climatological summer”. This talk will detail the history of the 55 °F dew point rule and the move away from it through scientific analysis along with interviews from current and former NWS employees and various collaborators. Improvements in messaging and customer interaction will also be provided since the change occurred ten years ago.