Strong Pacific Front Across Arizona Later This Week

First low temperatures - Yesterday was the first morning of the fall with a low temperature in the 30s here, with a min at house of 39F vs 50F at TUS. This morning, in contrast, lows held in the lower 40s at the house. But at the airport, TUS appears to have had a min temperature only in the low 60s - east-southeast winds prevailed at TUS during the night while things were calm here near the Rillito.

At 500 mb a closed low develops over southern California by 5 MST by Wednesday afternoon - above is 500 mb forecast from this morning's new NAM forecast valid at that time. By 5 pm on Thursday afternoon (below) the forecast low has opened up and moved rapidly east-northeastward. The associated vorticity maximum moves across northern Arizona, with strong winds likely ahead and behind the front. The current NAM forecast is quite dry for the southern half of Arizona.

However, several of the GFS ensemble members forecast subtropical moisture intruding into Arizona as the front moves through. The 5 am MST morphed PW product (below), from CIMSS at the University of Wisconsin, indicates the presence of two atmospheric rivers along and off the west coast, as well higher moisture over the southern third of the GoC. The evolution of these moisture sources will be important for the precipitation possibilities with this next weather system.