The chance of an isolated tornado forming is below 5%, but forecasters said some storms could become severe. Those storms may produce wind gusts exceeding 60 mph and large hail, Lojero said. Hail could occur with any strong or severe thunderstorms, with no specific area identified as more at risk.

A flood watch is in effect for much of central and northern Arizona from 8 p.m. Wednesday until 5 p.m. Thursday. The weather service said storms could drop 1 to 2 inches of rain per hour in some areas. If storms repeatedly pass over the same locations, a phenomenon known as training, the risk of flash flooding could increase.

The system is drawing together a strong low-pressure area moving in from the California coast and moisture from the south. The Valley has a 50% chance of rain late Tuesday and early Wednesday, before the most significant impacts are expected Wednesday afternoon into Thursday morning. Drier air should bring gradual improvement Thursday afternoon and evening, Lojero said.

Arizona averages four to six tornadoes a year, according to the Arizona State Climate Office. They are typically rated EF0 to EF2 on the Enhanced Fujita Scale.