Strong Cutoff Spinning West Of Washington State

The system that will affect Arizona at the start of next week is currently spinning away over the Pacific to the west of Washington state (see WV image at 4 pm MST top). The NAM 300 mb 12-hour forecast valid at 5 pm MST this afternoon (middle) has forecast the system to be further north and east of where it appears to be, at least in the WV imagery. A strong 300 mb jetstream is forecast to be approaching the cutoff from the northwest, i.e., the feature that will help the system dig down the coast. The current water vapor loop shows the cutoff to be moving slowly southeastward at this time. The NAM 60-hour forecast for 500 mb (bottom) indicates that the system will dig nearly due south and be off the northern California coast at 5 pm MST on Sunday (March 20th) afternoon with central heights lower than 5220 m. Thus, a very impressive system. However, after 60 hours the models forecast the system to weaken, become an open trough and move rapidly eastward. The GFS ensemble forecast from this morning shows that most of the members forecast a frontal band of precipitation to move eastward across southern Arizona Monday night, quite like what happened with the last system in February. However, most significant precipitation remains to the north. There is another large plume of subtropical moisture over the eastern Pacific that may be pulled into the digging cutoff. So it is yet another sittion that we'll have to watch evolve as it approches the Southwest.