Pattern Is A Confusing Mess This Morning

Just going to show a couple of graphics this morning, since the upper-air pattern is really a mess. The NAM analyses for 12 UTC at 500 mb (above) and 250 mb (below), serve to illustrate this. At 500 mb the anticyclone is centered over Nevada and the wave feature that has moved across northern Mexico the last two days is (at 500 mb) is now over the central GoC, with ongoing convection near its center. An inverted trough extends north-northeastward from this feature and weakens along the northern border between New Mexico and Arizona. The height gradient between this feature and the anticyclone results in easterly to northeasterly winds at 500 mb over Arizona. This feature (inverted trough) appears to have maximum cyclonic vorticity at about 400 mb just east of Tucson.

At 250 mb this morning (NAM analysis for 12 UTC below) the closed cyclone has weakened dramatically, with only a weak, inverted trough present over northwestern Mexico that extends northward to the southern portion of the New Mexico - Arizona border. The 250 mb anticyclone is also over Nevada and nearly vertical with the 500 mb anticyclone - this pattern gives northeasterly winds in the upper-troposphere.

The middle-level, inverted trough moves westward across southern Arizona today. Exactly what convective convective storms will do as this happens is very difficult to forecast. 

The TUS morning 12 UTC skew-T plot (above from SPC) indicates a wind profile that is nearly uni-directional, i.e., not much vertical wind shear. It is not clear how far westward low-level easterlies will intrude into Arizona, but the possibility exists for deep, downslope flow this afternoon - certainly not a good situation for storms in the low deserts near Tucson. CAPE is minimal at low elevations. So the situation does not appear much improved for storms at low elevations. As I said, a really messy pattern with best CAPE present over higher elevations. The 12 UTC WRF-NAM forecasts (total rainfall through midnight, below) another day with best storms and rain staying south of the Tucson metro area, i.e., more of the same.

I will just keep an eye on things and hope that some however the afternoon situation down here in Tucson evolves more favorably than the models indicate.