Flash flood warnings were issued by the National Weather Service across portions of Arizona, Utah, and California due to the rainfall associated with a low pressure system moving across the region. This can be seen in Figure 1. The system formed originally as a tropical invest region off the west coast of Mexico, then moved northeastward over the Gulf of California. Over the Gulf of California, the system was able to continue to grow as the warm, moist air above the water contributed to convection. This can be seen in Figure 2. This figure depicts the moisture transport associated with the system. As shown, the moisture is being advected from over the Gulf of California northward to the Mexico and Arizona border.
Figure 1. National Weather Service issued map of warnings and advisories. Issued 19 November 2019 at 2006 UTC. Depicts majority of Arizona under a flash flood warning. Over 13 million people were under a flash flood warning at the time this was taken.
Figure 2. SPC Mesoanalysis Page produced moisture transport plot. Taken 2000 UTC 19 November 2019. Depicts the transport of precipitable water (fill) northward. Moisture transport vectors are plotted. https://www.spc.noaa.gov/exper/mesoanalysis/new/viewsector.php?sector=12#
Figure 3. Visible Satellite Image taken 1956 UTC 19 November 2019. Depicts Southwestern United States, including the system over Arizona bringing rainfall to the region. http://www.aos.wisc.edu/weather/wx_obs/GOES17_sw.html
The system is expected to propagate northeastward, bringing rain to more of the southcentral United States, and even snowfall at higher elevations. The current cloud structure can be seen in Figure 3. The system is currently located over the border of Mexico and Arizona. The flash flood warning will remain in place through Wednesday evening as the system moves northward, bringing continuous rainfall to the rest of Arizona. In regions that are as dry as the southwest, it does not take a lot of rainfall to induce flooding. Though there is some potential for hail formation and strong winds, the main threat will be the flooding.