The southwestern monsoon officially starts June 15 and ends September 30. The National Weather Service began using these dates in 2008 to identify a discrete monsoon season as opposed to relying on varying assessments using dew point temperature and onset of precipitation events. Prior to 2008, the historical start date of the monsoon was based on observed conditions (primarily elevated dew point), which varied across the region in a way that reflected the generally westward migration of the monsoon onset (Fig. 1). Firm start and end dates may not perfectly align with regional variability (especially if monsoon conditions are not present at the calendar start to the monsoon), but standardized dates do provide a more consistent time frame to compare year over year and emphasize the spatial and temporal variability of the monsoon within and across years.
Monsoon Tracker (Jun 15 - Aug 17)
The Southwest saw the first strong burst of widespread monsoon activity near the end of June. Most of the first half of July was characterized by a distinct break in monsoon activity, as atmospheric circulation patterns and lack of available moisture limited opportunities for widespread storms to develop, especially at lower elevations. As July progressed, there were increasingly favorable conditions for storms to develop and spread, culminating in an extended period of widespread activity during late July and early August. Tropical Storm Javier helped jumpstart activity in mid-August, just as the previously mentioned extended run was winding down, and provided a brief extension to storm activity via a surge of moisture from the Gulf of California. The remainder of the monsoon window will be a waiting game to see if favorable moisture and atmospheric circulation patterns develop, as well as the potential influence of eastern Pacific tropical storm activity that could supplement storm activity and provide additional moisture to fuel storm activity.
Based on cumulative totals since the official start of the monsoon, most of Arizona recorded average to below-average precipitation, even while clusters of the state have recorded above-average precipitation as a percent of average (Fig. 2a) and in terms of outright totals (Fig. 3a). The percent of days with rain highlights the spatial variability of the monsoon and emphasizes the clustering of storms in the southeastern corner of the state (Fig. 4a). Precipitation plots from specific stations further highlight this variability, with Douglas, Flagstaff, and Tucson stations all on pace for average to above-average monsoon totals (Figs. 5a-c) but with the Tacna station in the southwest corner of Arizona yet to record a precipitation event (Fig 5d).
Cumulative totals since the start of the monsoon show that much of New Mexico recorded average to below-average precipitation, but with wider areas of above-average rainfall as well (Figs. 2b – 3b). The percent of days with rain also highlights the more regular coverage seen in New Mexico, with a wider swath of increased activity that covers most of the state (Fig. 4b). Station plots also demonstrate the lagging precipitation in select locations, with Albuquerque and El Paso, Texas, lagging behind seasonal totals (Figs. 5e-f). Stations closer to southern Arizona, such as the Animas 3ESE station, are recording above-average precipitation (Fig. 5g).
The southwestern monsoon is characterized by a high degree of spatial and temporal variability. Over the course of the season, storm events are interspersed with breaks of limited activity as migrating high pressure systems and available moisture dictates where in the Southwest rain might fall. This results in highly variable precipitation totals on a daily or weekly scale. Regional climatology gives some indication as to the expected cumulative total precipitation any location might expect but says less about how those precipitation totals will be achieved. Any given year of monsoon activity is difficult to categorize on a week-to-week basis, and simple score-carding using seasonal precipitation to date will be skewed by recent runs of heavy rain or extended breaks in the monsoon. Looking at daily precipitation event maps, there are very few days in which no rain fell anywhere in Arizona or New Mexico; there are numerous locations that saw little to no rain while nearby regions saw extensive precipitation. These totals should generally start to even out over the course of the season and vary around the long-term average, but outliers and extremes are always possible. As stated above, the remainder of the season is a waiting game to determine if and when a favorable atmospheric circulation brings additional storm activity into the region, or if tropical storm activity can help jumpstart these storms with additional surges of Gulf moisture.