Monsoon Classroom: The Transition

What drives the Arizona Monsoon? The blog’s “Monsoon Classroom” breaks down the 5 main patterns that help fire up our Summer storm season.

Type IV
Courtesy: NWS Tucson

If the Four Corners High marks the start of the Monsoon, the Transitional Pattern serves as the final chapter. During the month of September, high pressure is forced toward the Central Plains as a Pacific trough or cold front moves into Arizona. This develops a boundary between Monsoon moisture and dry air.

Along this boundary, thunderstorms develop and move from west to east. Some of these storms can become severe, capable of damaging winds, large hail and even an isolated tornado. Once the boundary sweeps across the state, winds aloft switch from southeast to northwest. This northwesterly flow draws in dry Pacific air, essentially shutting down our Summer storm season.

Like the Four Corners High, the Transitional Pattern occurs each and every Monsoon. While it does not guarantee severe weather, it does mark our shift toward Fall weather in the Sonoran Desert.


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