Arizona will get a taste of tropical moisture as soon as Thursday evening, Sept. 3, but not because a hurricane is hitting Southern California.
Tropical Storm Marie, soon to be Hurricane Marie, formed Tuesday morning, Sept. 1, in the eastern Pacific Ocean off the coast of Mexico. Before its official formation as a tropical storm, social media was ablaze with unreliable model outlooks and bad forecasts that showed a powerful storm slamming into SoCal. Tropical forecasts are like local forecasts: the farther out one looks and the fewer models used, the more unfounded the information.
But now, consensus is growing on the track of the storm and the impacts on the U.S. Southwest, which should lead to an increase in storm-enabling tropical moisture
heading into the end of the week.
Marie is now forecast to curve to the west/northwest, pushed that way in part by a bulging high pressure that’s currently situated in the deep south and moving back westward. Marie is not expected to get close in latitude to the international border.
Marie is still developing as it sits on the edge of equatorial waters over 85 degrees and is expected to reach peak strength on Friday, Sept. 4. Current forecasts show peak winds of 105 mph, which is high-level Category 2, near Category 3 strength. To reach Category 3, winds over 110 mph would need to be detected.
It’s during this strengthening that moisture will move toward the Southwest, including Arizona. Scattered storms will be possible in the high elevations to the east and southeast, with good chances for those storms to survive in the Valley. It won’t be a washout by any means for Phoenix, but strong storms could lead to flash flooding. Parts of the eastern Mogollon Rim and areas south of Tucson could see over an inch of rain.
Excessive rainfall outlooks place southeast and south-central Arizona under a slight risk of flooding on Thursday and Friday due to tropical moisture. Other storm risks include frequent lightning, outflow winds over 35 mph and blowing dust that could lower visibility.
Meanwhile, drier air will be pulling into western Arizona and California through the end of this week, while south-central and southeast Arizona get the higher rainfall chances. As high pressure draws a bit closer from the east, some of those better moisture chances finally make it to these regions late Saturday, Sept. 5, into the start of next week.
All of these features at play make the Labor Day weekend forecast a bit uncertain for the Valley, as cloud cover and lower chances for rain could keep Saturday cooler, while high pressure could start to raise temperatures next week.
Is it common for tropical systems to impact Arizona?
During the month of September, Pacific tropical storms have a higher chance of curving northward so that moisture can be advected into the Southwest. Ocean waters get much cooler the further north they are, so tropical storms rarely survive making it as far north as the U.S. to make direct impact. If the actual system makes it far north enough, it usually weakens into a remnant system that drops storms and heavy rainfall. For Arizona, advected tropical moisture and remnant rainfall are well-documented as sources for late monsoon activity.
Does El Niño play a part?
El Niño does not have much of a direct effect on the Southwest monsoon, but it does have an effect on the Pacific hurricane season. Warmer ocean waters and lower wind shear across the equator lead to conducive conditions for tropical storms to form. The record strength of this El Niño and the amount of heat detected in the ocean has led to an impressive 15 named storms in the Pacific Ocean so far in 2026. The more tropical systems that can form, the higher the opportunity for some of the moisture to pull Arizona’s way.
Meteorologist Tevian Whitehurst covers extreme weather and climate for The Arizona Republic and azcentral.com. Follow him at @TevianWX on Instagram and Facebook (Meteorologist Tevian Whitehurst).
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This article originally appeared on Arizona Republic: Tropical weather raises chance of heavy rain, flood risk for Arizona
