The Southwestern Monsoon
The monsoon in the Southwest U.S. officially starts June 15 and ends September 30. The National Weather Service began using these dates in 2008 to identify a discrete monsoon period 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 to year and emphasize the spatial and temporal variability of the monsoon within and across years.
The southwestern monsoon is characterized by spatial and temporal variability. 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 timescale. 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. Totals should even out over the course of the season and vary around the long-term average, but outliers and extremes are always possible.
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. By mid-to-late July, 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 again in mid-August, providing a brief extension to storm activity via a surge of moisture from the Gulf of California. The remainder of August and early September saw a decline in widespread monsoon activity, even while numerous areas did see intermittent precipitation, particularly at higher elevations. On September 7, Hurricane Newton generated significant precipitation activity in a swath across southwestern Arizona and extending into central New Mexico, although observed values were less intense and widespread than initially forecast. Recently, the monsoon has been in active decline, with activity shifting to the northwest and regional activity looking more and more like fall across much of the region, although eastern New Mexico has seen an uptick in activity based primarily on surges of moisture from the Gulf of Mexico.
In what remains of the monsoon, limited opportunity exists for moisture and atmospheric circulation patterns to develop into widespread events, although intermittent convective activity could always develop across the region. Eastern Pacific tropical storm activity has the potential to contribute directly via rainfall events or at least to supply additional moisture to the region that could fuel storm activity, but this period of activity extends well beyond the bounds of the official monsoon (and also highlights the ongoing debate as to whether tropical storm events should “count” when discussing the performance of the monsoon).
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 rainfall (Figs. 2a-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 showing that their seasonal average for monsoon precipitation had been surpassed by early September (Figs. 5a-c), while other stations such as Tacna 3 NE in the southwest corner of Arizona have only recorded a few events and are well below their seasonal total (Fig. 5d).
Cumulative precipitation totals since the start of the monsoon show New Mexico has caught up in the last month. Much of the state has now recorded average to above-average precipitation, although some areas still are below average (Figs. 3a-3b). Relatively widespread, uniform precipitation is indicated by the observation that most of the stations across the state recorded rain on at least 30 percent of days in which rain was recorded anywhere, and a slightly smaller yet still significant region recorded rain on 50 percent of such days (Fig. 4b). Station plots also demonstrate the lagging precipitation in select locations such as Albuquerque (Fig. 5e), while others such as El Paso, Texas, and the Animas 3ESE station recorded above-average precipitation (Figs. 5f-5g).