Autumn in the Southwest

Heading to California? Arizona? Nevada? Even Utah, Colorado, or New Mexico? We’ve got the perfect article for you.

Tuesday, September 24, 2024 at 04:11 AM ET

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This early fall pattern in September shows the Southwest heat low over western Arizona and a very strong North Pacific High dominating weather along the coast. (Tim Vasquez/AWIPS plot)

Key Takeaways:

Autumn in the Southwest brings a transition from summer's complacency to dynamic weather, driven by decreasing solar radiation, colder air masses from Canada, and strengthening upper-level winds.
The regional weather shifts from the "Southwest heat low" to the dominant "Great Basin High," which often provides clear, cold conditions but also fuels hazards like Santa Ana winds (with associated turbulence and wildfire risk) and persistent valley fogs (e.g., California's tule fog).
Early autumn carries a risk of heavy precipitation and low ceilings from remnants of tropical storms originating in Mexico, while later autumn sees the Pacific storm track shift south, bringing frontal systems and widespread precipitation to the West Coast.

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Complacency can easily creep in during the summer. It seems like all we have to worry about is the air mass and orographic thunderstorms that pop up late in the day. Once autumn arrives, we end up with a new set of ingredients on our plate. The weather becomes more dynamic, upper-level winds increase, we start dealing with frontal systems, and the tropics explode with hurricanes. How can we make sense of all these changes?

Here, our attention is on the Southwest, including not only the Rockies but the Great Basin and California. What are good strategies for planning your flights in September, October, and November? What can you expect?