Strange Brew of Ingredients

Yesterday - several small, high-based Cbs developed southwest of Tucson late - probably in response to the very hot temperatures at surface and the realatively cool middle-level air. These were quite nice visually at sunset but I didn't get a photo. No stations in the Pima County ALERT network measured rainfall in past 24-hours.

A very complicated situation has developed overnight.

First, strong storms occurred much further north over Mexico (see 06Z IR image above) pushing some outflows into the borderlands. Additionally, at low-levels a nice anticyclone set up over northwestern Sonora and there has been a very pronounced surge of moist air flowing around this high. The net result is that stations in south-central and southwest Arizona are all reporting dewpoints 10 to 20F greater than at this time yesterday and Yuma is actually up 34F.

At 500 mb there is 1) an inverted trough extending from south end of Gulf of California northward to Arizona - New Mexico border; 2) there is a very distinct circulation and vorticity maximum in this feature along 110W just south of 30N; 3) a shortwave that has been moving slowly southward from the northwest U.S. extends from north of San Diego to the nebraska Panhandle; 4) a cutoff low is at the end of trough 3 over the Pacific west of San Diego; 5) a large anticyclone is located over Oregon to the north of trough 3 with very warm and dry air in its core; and 6) a second anticyclone is centered near the northern coast of Baja.

Throw in a very strong deformation zone in the upper-troposphere and it's clear that summertime synoptics are not aways dull over the Southwest. The way that all these features interact during the next 48 hours will determine the degree of storm activity across the southern quarter of Arizona. It appears that the northern anticyclone will dominate and push southeastward. This forces the Mexican trough/vorticity maximum off to the northwest toward San Diego. If southern Arizona stays in the flow around the vort max, activity will increase this late afternoon through tomorrow. But if the fetch around the northern anticyclone dominates, then thunderstorm activity will be pushed down to the border and south. The NAM forecasts are right on the fence this morning.

Looks like some isolated storms this afternoon, even though we're in a typically unfavorable west to northwest flow regime. The NAM for tomorrow evening indicates that all of Arizona is in the dry circulation around the northern anticyclone. But, it appears that tomorrow will have a distinct binary character to it depending on which flow regime dominates - either a day with significant strong storms or another mostly suppressed day. Very hard call - just have to watch to see how things evolve.

The NAM indicates the the strong southerly flow that develops between the cutoff and the dominant anticyclone interacts to pull the moist, subtropical air north again Friday and Saturday with the strongest push into the lower Colorado Basin and southern California - looks like a chance for severe storms in the Mojave on Friday and Saturday.