Why is it raining in Arizona today?

Cool and very wet weather have overspread the mountains and lower deserts of Arizona today (12/3/2022). As the cover image shows, estimated rainfall in the last 24 hours is adding up. The large-scale weather pattern has provided parts of Arizona a great opportunity to receive beneficial late autumn precipitation. In fact, the total water content of the atmospheric column (termed “precipitable water”) over Arizona is unusually moist given the calendar month of December. For context, a nose of subtropical caliber precipitable water values exceeding 1.2 inches have pushed northward into Arizona from Mexico and the East Pacific. Those values are comparable to summer monsoon moisture level expectations. Unusual, indeed.

Excessive moisture is one thing, but you still need a source of lift to force water vapor up to cooler higher altitudes for condensation. Heating the surface from the Sun to spur convective lift can be ruled out due to an impressive cloud shield streaming across Arizona. Also, it has become a cool and wet landscape, yet the rain does continue to fall from above. The atmosphere is not unstable enough to generate thunderstorms, so localized outflow wind boundary collisions for concentrated lift are not present either. A sweeping cold front to provide strong rising motion is not an instigator either. It seems calm enough at the surface, but what we do have is a powerful fetch of winds higher up in the troposphere that are spreading outward over Arizona!

At the moment, the Desert Southwest is geographically sandwiched between an upper level cyclonic circulation near northern California and an upper level anticyclonic circulation offshore of Texas. The tight pressure gradient between the two (strong change in pressure over a shorter distance) have resulted in a band of faster winds with a southwest to northeast orientation that tapped into the subtropical moisture. These strong winds aloft entering the region and then diverging are the main catalysts for widespread upwelling (rising motion) in the atmosphere and beneficial rains!

Data Source: NOAA NWS Weather & Hazards Data Viewer