The morning 500 mb plot and analysis from the University of Wyoming UA site is shown above - I didn't have time to annotate or enhance any charts this morning. The main anticyclone center is now out over the Southern Plains and there is band of southerly flow over Arizona. The dry and warmer, middle-level air has also shifted eastward. There is also a pronounced upper-tropospheric cyclone spinning aloft between Brownsville and Del Rio, Texas. The models forecast this feature to move westward, extend downward into the lower troposphere, and brush across southern Arizona over the weekend. The forecast evolution appears to result in some strange hybrid feature that is not clearly either an upper-level inverted trough nor a distinct, lower-tropospheric tropical wave. This will be an interesting feature for us to watch over the weekend.
This morning's Tucson sounding plot (above, also from Wyoming) and the 12 UTC WRF-NAM forecast sounding for 4 pm MST is below. Note that the deep moisture has returned overhead. The forecast CAPE for this afternoon appears realistic to me. Steering flow for afternoon storms will be stronger than the past several days, but will also have a strong southerly component. So the main forecast dilemma is whether storms that develop over higher terrain to the south-southeast will be able to propagate into lower elevations of eastern Pima County. The WRF vertical shear profile forecast for this afternoon indicates that there could be some chance for storm cores to move northward and leave their anvils trailing behind. This scenario would favor low-elevation storms, but the usual case with deep southerly flow is that storm anvils spread ahead of storm cores and shut down lower elevation development. Close watch of the storm structures will be needed this afternoon and evening.
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