Mid‑September Monsoon Storms Return to Phoenix

Starting Monday, Sept. 14, the desert will feel a little different – the heat is still on, but a thick layer of low‑level moisture is already hanging around, especially over the eastern half of Arizona. That combination is the classic recipe for monsoon activity, and this week the ingredients are all lining up.

On a larger scale, a subtropical ridge is parked far to the east over the Mississippi Valley, while a trough is beginning to dig down the West Coast. Arizona sits right in the sweet spot between those two features, which means tropical moisture is being funneled northward from Mexico and the Gulf of California. The result? A week‑long stretch of showers, thunderstorms and, most importantly, heavy rain.

The busiest days are shaping up to be Tuesday and Wednesday, Sept. 15‑16. During those two days the storm threat spectrum is broad – from lightning‑filled storms and gusty winds over 35 mph to the dreaded flash‑flood potential. In the Valley, storm probabilities climb to about 60 % on Tuesday, with the East Valley seeing the highest odds.

Monday’s start will be a bit sluggish for Phoenix. Rain chances are isolated at first, mainly over the eastern Mogollon Rim and the southeastern corner of the state. By the afternoon, though, the clouds push eastward, and by Tuesday morning the metro will feel the shift. Expect showers on your commute, a brief lull, then another round in the late afternoon that can linger into the night.

Heavy rainfall is already forecast for southeastern Arizona. In fact, a flood watch is in effect until 11 p.m. Monday for the southeastern half of Pinal County and parts of Pima, Santa Cruz, Graham, Greenlee and Cochise counties. Those storms could drop one to two inches of rain. If the same moisture expands into the Valley on Tuesday‑Wednesday, officials may issue a broader flood watch for the Phoenix area.

Temperatures will stay a few degrees below normal while the clouds linger. Expect highs ranging from the mid‑90s up to about 101 °F, with overnight lows holding steady in the low 80s – a touch warmer than usual for this time of year. By Thursday, the trough that’s been nudging the West Coast will have moved into the Great Basin, turning the wind direction from the south to the west. That shift will dry out the lower atmosphere, and rain chances will taper off heading into Friday and the weekend.

When the drier air finally settles in on Friday, Sept. 18, the heat will bounce back. Temperatures are projected to climb back into the 102‑104 °F range, bringing the typical late‑summer feel back to the Valley.

All of this is being tracked by Arizona Republic meteorologist Tevian Whitehurst, who covers extreme weather and climate for azcentral.com. You can follow his updates on Instagram and Facebook at @TevianWX.

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