The above chart is 500 mb for 00Z 4 August 2009. The chart shows that the center of the anticyclone is well-established over the White Mountains of Arizona. It is an excellent pattern to allow thunderstorms to propagate westward into the Tucson metro area, assuming that low-level moisture and CAPE have also been brought into place over the lower elevations. This pattern is not favorable and will not bring these necessary ingredients into the Tucson area, unless other features are in play in concert or before. Thus, we have thick anvil cloud but no covection this evening in Tucson because at sounding time the low-level atmosphere was hot and dry and ugly and there was not even a sliver of CAPE. The lack of subtropical, low-level moisture locally probably relates to the fact that the 500 mb anticyclone is HUGE with its southern periphery reaching to Yucatan.
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