The large-scale setting today is somewhat unusual and is similar to transition situations often seen at the end of the summer. Graphics above are 12 UTC analyses from NCAR RAP of 300 mb (top) and 700 mb (just above). At 300 mb a fairly strong trough is located out west near the Colorado River. However, the 700 mb trough has lagged behind, or been overrun, by the features in the upper-half of the troposphere. The 700 mb flow is much weaker and with a fetch from the subtropics and high moisture content.
Below are morning sounding plots (skewTs from SPC) for Tucson (top) and Yuma (bottom). The Tucson sounding indicates a two layer troposphere - moist and tropical airmass below 500 mb and a dry, westerly trough above. The Phoenix sounding is similar. However, the Yuma sounding is more typical of a Plains severe thunderstorm environment, as is the Flagstaff sounding.
This meteorological split personality results in a situation today where the main threat for the southeastern third of the state remains heavy rain storms, while the northwest third could experience severe thunderstorms. The middle third has a poissiblity for a bit of both.
A very interesting day on tap.
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