Monsoon gets a tropical boost

Tucson residents should prepare for a shifting weekend weather pattern that could bring anything from ordinary afternoon thunderstorms to very heavy rain and localized flash flooding. Showers and thunderstorms are expected to become more active in the afternoons and evenings as tropical moisture moves north into Southern Arizona. The storms could affect people traveling across town, working outdoors, hiking, commuting through washes or low lying areas, and spending time outdoors over the holiday weekend. Temperatures are expected to remain close to normal or a few degrees below normal, even as rainfall becomes the primary concern.

A cluster of morning rain and showers was moving north just west of Tucson, giving the area a temporary break before daytime heating helped fuel more storms. Forecasters expected a 50 percent chance of showers and thunderstorms during the day, mainly after late morning, with a high near 96 degrees. The chance of storms continued into the night, when skies were expected to become mostly cloudy and temperatures fall to about 71 degrees. Winds were forecast to remain light, shifting from southeast or calm conditions to a west southwest breeze during the afternoon.

Friday was also designated a First Alert Weather Day because another round of unsettled weather was possible. The forecast called for a 30 percent chance of showers, with thunderstorms possible after the morning, and a high near 96 degrees. Rain chances were expected to decrease for the rest of the holiday weekend, at least according to the latest forecast, but they would not disappear. Saturday brought a 20 percent chance of afternoon showers and thunderstorms, with a high near 97. Sunday was expected to be mostly sunny with a high near 100 and only a 10 percent afternoon storm chance, followed by a slight chance of showers overnight.

The forecast remained unsettled into next week. Labor Day was expected to reach about 98 degrees with a 20 percent chance of afternoon showers and thunderstorms. Storm chances then increased to 30 percent Tuesday, 40 percent Tuesday night, and 40 percent Wednesday, with daytime highs remaining in the upper 90s. Another forecast described the broader pattern as scattered to numerous thunderstorms each afternoon and evening through the weekend, reflecting the uncertainty in how much moisture will reach Tucson and where storms will develop.

The biggest question is not whether moisture is present, but how the atmosphere will arrange it over Southern Arizona. A trough of low pressure moving down the Pacific coast from near Alaska is expected to interact with moisture from tropical systems off Mexico. If the trough moves slowly, more moisture could be drawn into the Tucson area and produce periods of very heavy rain. If it moves quickly, rainfall totals could be considerably lower.

That uncertainty has produced a wide range of possible outcomes. Gary Zell, a meteorologist with the National Weather Service in Tucson, said higher end forecast models showed the possibility of three to five inches of rain in the Tucson area over the weekend. Lower end models showed less than half an inch. Zell said the outcome depends on where the moisture settles and how much is pulled northward ahead of the trough as it enters the western United States. The difference between those scenarios could determine whether Tucson sees only scattered storms or a weekend of significant flooding concerns.

Forecasters have linked the incoming moisture to tropical activity, although the systems identified in the weather discussions differed. One forecast said moisture would drift north from the circulation of Hurricane Marie, which the National Hurricane Center expected to move northwest to west northwest over the weekend. That outlook did not indicate that a major surge of tropical moisture would reach the Southwest, though later forecast runs could change. Another report said Hurricane Priscilla, located off the tip of Baja California, was expected to cross the peninsula, break apart and send remnants toward Southern Arizona. In either case, the central forecast message was that tropical moisture would be available for Tucson, particularly Saturday through Monday.

The setup has a notable local precedent. Zell pointed to the October 1983 storm, when tropical storm remnants interacted with a Pacific trough and produced torrential rain and widespread flooding in the Tucson area. He emphasized that it is too early to know whether this weekend’s pattern will develop in the same way. Forecast models are still varying widely, and officials expect to gain more clarity as the event approaches. For now, Tucsonans can expect a daily cycle of possible storms, with the greatest activity generally during the afternoon and evening, and should keep in mind that even a forecast with modest overall rainfall can still produce dangerous flooding in isolated locations.