With high pressure aloft over Arizona & NM and troughing to our northwest, the flow aloft has been west-southwesterly. This pattern has brought drier and warmer-than-normal conditions to Arizona recently. Expect a change Tuesday and Wednesday as the high pressure builds to our east and a trough approaches from the west. A south-southwesterly upper-level jet stream will strengthen over northern Arizona, with Arizona being on the equatorward side of the jet entrance. This will help create lifting and vertical wind shear. Meanwhile, breezy southerly low-level wind will stream northward, bringing a deep layer of moist, unstable air from the Gulf of California into northern Arizona. The net result will be a chance for thunderstorms mainly Wednesday – Wednesday night. Some storms could become severe, with large hail approaching 1” diameter and possible rotation. Storms that form will move northeast at about 30 mph, and will also be capable of frequent lightning and heavy rain. This could be the end to the summer thunderstorm season.
From Thursday – Saturday, the air over Arizona will remain 5-10 degrees cooler than today, with possibly a slight chance for isolated showers or thunderstorms. The cooling will be just in time to usher in the first day of fall (Sept. 22).
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