Conditions continue quite dry over southern Arizona and northwest Mexico with PW values ranging from the upper 30 to lower 40s of mm. The TWC sounding (top image from the SPC) now indicates some decent southerly wind speeds below 500 mb, as the anticyclone strengthens over the southern US (see middle and bottom analyses from NCAR RAP). So, even though we remain in the flow regime of the US lobe of the Atlantic subtropical high (at least in lower-half of troposphere), the atmosphere is relatively dry. At 500 mb, the weak deformation zone remains over the lower Colorado River Basin, separating the flow around the Atlantic and Pacific subtropical anticyclones. The Pacific trough is forecast to take over this zone, a bit off the coast, as the 500 mb high strengthens and shifts westward. Note that in the upper-half of the troposphere the flow (see 250 mb analysis) has a fetch through the Pacific trough (at least over most of Arizona) and is not part of the subtropical flow regime - in the past, I've referred to this type monsoon flow setting as being mixed mode, i.e., about half subtropical and about half westerlies.----------------------------------------------------------
As the lower troposphere heats up today and tomorrow, I think we can expect upticks in thunderstorm activity each day, especially over the mountains. Although the flow is becoming more southerly, deep moisture remains quite far away - so initially, the development of increased CAPE will have to be due primarily to heating and related moisture recycling from the recent heavy rains.
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