The average high temperature for Tucson, Arizona on this date is about 97 F, but the high hadn't been that low since July 18, until just this week (Wednesday's high was 91). The model forecasts for Friday are well below that, and the NWS has predicted 91. Why? Because of expected thunderstorms that are fairly common in July and August, the two wettest months of the year for Tucson. The dew points are expected to continue in the 60's which is very high for Arizona. Let's predict Friday's Tmax and see if the thunderstorms come and how much they can "cool" it down.
Question: How do they get such humid air (high dew points) in the middle of the desert?
Tucson did have monsoon thunderstorms, but not until after 4 pm, and by then the high had already risen to 92 -- moderate for them.
Of course the desert is usually dry but flow from the direction of the nearby Gulf of California (also called the Sea of Cortez) can really increase the dew points and increase the chance of rain, just as the Gulf of Mexico so often does for the Southeast and other parts of the U.S.
Think about how strong seasonal effects can be -- for the two months of summer monsoon in Arizona, the weather can be dramatically different from the rest of the year.
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