Mid‑September Monsoon Surge Set to Hit Phoenix

All the right ingredients are finally lining up for a classic Arizona monsoon week. Starting Monday, Sept. 14, hot surface temperatures will mix with a deep pool of tropical moisture sliding northward from the Gulf of California. It’s the kind of set‑up that makes forecasters sit up and take notice.

On a larger scale the subtropical ridge is parked over the Mississippi Valley while a trough is digging into the West Coast. Arizona sits right in the sweet spot between those two features, so the moisture gets funneled straight into the state’s lower levels. The result? A spell of active weather that should stretch all the way to Thursday, Sept. 17, with the biggest bang on Tuesday and Wednesday.

Monday’s start will be a little quiet in the Valley—rain chances are spotty and mostly isolated. By the afternoon, though, the eastern Mogollon Rim and southeast Arizona will feel the first showers. Metro Phoenix can expect the first real drizzle early Tuesday morning, right as you’re heading out the door, and then again in the late afternoon into the night.

Storm probabilities climb to about 60 % in the East Valley, with the highest odds lingering over the desert‑rim communities. Expect heavy rain to be the headline threat, but don’t rule out the usual monsoon companions: lightning, gusty winds that can top 35 mph, and occasional dust whirlwinds when the sun briefly breaks through.

Because the rain is likely to be so intense, a flood watch is already in place for parts of southeastern Pinal County, plus bits of Pima, Santa Cruz, Graham, Greenlee and Cochise counties. The forecast calls for 1‑2 inches of rain in those areas, enough to cause flash‑flooding on low‑lying washes. If Tuesday‑Wednesday storms expand into the Valley as expected, a second flood watch could be issued for the metro area.

Temperatures will feel a little cooler than usual, thanks to the rain‑cooling effect and cloud cover. Highs should hover between 96 °F and 101 °F—just a few degrees below the seasonal average—while overnight lows stay in the low‑80s, a touch warmer than normal. By Friday, the upper‑level trough will have nudged into the Great Basin, flipping the wind direction from the south to the west. That shift brings drier air, and the monsoon will start to wane, with highs rebounding to the low‑102 °F‑104 °F range for the weekend.

All of this is coming from Tevian Whitehurst, the meteorologist who covers extreme weather for The Arizona Republic and azcentral.com. You can follow his updates on Instagram and Facebook at @TevianWX.

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