Rainlog Climate Summary – August 2017
After an exceptionally wet July for many areas across Arizona, monsoon storm activity slowed down to a crawl leading to unusually dry conditions over the past month. August is typically the wettest month during the summer monsoon season in Arizona and the lack of thunderstorm activity stands out against the frequent and heavy activity that characterized much of the month of July. There were three days in August where no rainfall fell anywhere in Arizona (August 8th, 16th, and 18th), a strange and unusual occurrence in the heart of the monsoon season. The long-lived surge of moisture from the Gulf of California and reasonably favorable position of the upper level high pressure system that supported the frequent thunderstorm activity in July broke down in early August leading to much less favorable conditions for monsoon activity. The upper level high retreated south into Mexico ushering in drying upper level winds from the southwest and suppressing much of the thunderstorm activity across the state. Some higher elevation areas have been able to fire off afternoon thunderstorms on a handful of days in August, but most lower elevation areas have been left out of the action.
The overnight period of August 12th into the morning on the 13th was one of the heavier, widespread rain events that occurred during the month. Tropical Storm Jova passed west of Baja California around this time helping to push a slug of moisture up into southern Arizona. This provided the fuel for an outbreak of heavy thunderstorm activity in the overnight hours on the August 12th. Rainloggers in the Tucson metro area reported widespread amounts in excess of 1” with a handful of reports above 2.5”. Reports of daily totals in excess of 1” extended from southeast Arizona (Bisbee, Douglas, Sierra Vista) up through Phoenix and as far north as Prescott with this event.
Overall, August precipitation totals look bleak across much of Arizona with respect to average levels. Some isolated areas have received average to above-average August totals, but most of Arizona has observed less than 75% of average with some locations (far northeast and parts of central AZ) seeing less 25% of their typical average total August precipitation. The monsoon season typically winds down through the first couple weeks of September, but the threat of tropical storms and associated moisture will keep the prospect of additional precipitation alive for this season.