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 marks a dynamic shift from summer, with increased upper-level winds, the fading of the monsoon, and the onset of more energetic frontal systems due to cooling landmasses and warming oceans.
The Great Basin High becomes dominant, generally bringing clear, cold weather but also leading to significant local phenomena like valley inversions, fog, and strong Santa Ana winds which increase turbulence and wildfire risk in California.
The season sees an increased risk from tropical storm remnants, bringing heavy precipitation and low ceilings to the Southwest, especially in September, and the establishment of the Pacific storm track later in autumn, introducing widespread rain and snow.
Pilots should be prepared for specific regional hazards including persistent "tule fog" in California's San Joaquin Valley (reducing visibility to near zero) and strong windstorms with clear air turbulence east of the Rockies.

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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?