Monsoon Recap (Cont.)
The variability and intensity of these storms is only hinted at in seasonal totals however, and the monsoon is notoriously spatially variable and inconsistent. It is not uncommon for 1–2 inches of rain to fall in midtown Tucson and be dry in the foothills, or vice versa, for example, and many metrics are based on the placement of a limited number of rain gauges. This is an important point because during numerous precipitation events this season, a single storm dropped a season’s worth of rain in a day (and in a few cases, in a few hours), but the coverage was not always consistent or uniform.
These storms also drive above-average seasonal totals, which has a limited effect on mitigating drought compared to steady and consistent rains, increase disaster potential due to intense precipitation (e.g., Tropical Storm Norbert in Phoenix and Tucson), and underscore the complexity of planning a large urban area for a possible storm event (e.g., Tropical Storm Odile in Tucson). Citizen science enterprises such as rainlog.org provide a more detailed and nuanced picture of monsoon variability through crowdsourcing of precipitation measurement, but the standard measure is still a comparison of the seasonal totals measured at stations in the Southwest (Fig. 4). The intensity and percent of days with rain (Figs. 2-3, above), help illustrate different patterns of steady vs. intense precipitation, but cumulative monsoon precipitation plots also help clarify variation in precipitation intensity.
Three plots (Figs. 5-7) help explain these different types and intensities of rain events. In September, Phoenix (Fig. 5) received almost all of its monsoon precipitation in two single days, and a majority of the rain fell in a single day when Norbert pushed in from the Gulf of California on Sept. 7. The precipitation plot for the Coronado National Monument (Fig. 6) shows a much more even precipitation pattern, with numerous smaller storms spread out over the season; the largest single-day rain event was associated with the incursion of moisture from Odile). The precipitation plot for Tucson (Fig. 7) falls in the middle, with more frequent and smaller storms compared to Phoenix, but longer dry spells and gaps in monsoon precipitation compared to Coronado NM HQ. Norbert is also clearly identifiable in this plot.
This comparison helps illustrate that while extreme precipitation events may provide short-term drought relief and precipitation totals may indicate water deficit improvements, long-term drought conditions will persist and are best mitigated by similarly long-term patterns of above-average precipitation, especially as we look forward to the winter precipitation season.