Satellite images of the monsoon storm system that moved through North Phoenix and the rest of the Valley Sunday night look strikingly similar to space-based images of hurricanes, with bands of thunderstorms loosely organized around a central core, all rotating counterclockwise.
The system was “similar in structure” to a hurricane, the National Weather Service told North Phoenix News, “but much smaller and weaker in intensity.” It was caused by the same underlying mechanism that creates hurricanes: heat released by strong thunderstorms, forming a low pressure system.
Roger Pielke Sr., senior research scientist at University of Colorado-Boulder, went further. Pielke said the storm was a “tropical depression that formed over land,” which is unusual.
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Tropical depressions on land are typically remnants of tropical storms or hurricanes that started out as tropical depressions, their birth and growth all happening over water. They weaken as they move inland because they are robbed of the warm water that is their primary fuel. (The ocean surface temperature must be 80 degrees for a hurricane to form.)
In the Arizona storm system, several factors may have allowed the “well-defined vortex (tropical depression) to form,” Pielke said in an email interview:
- an atmosphere primed for thunderstorms (loaded with heat and humidity)
- flatter desert terrain the storm encountered as it moved southwest
- large-scale wind patterns
- heat stored and released from the desert floor
Similar systems have occurred in Australia, where tropical cyclones move inland and occasionally intensify when hot, sandy soil provides energy, scientists hypothesize. Some call them “landcanes.”
Meanwhile, Pielke has a scientific paper under review that describes a similar tropical disturbance that formed over Louisiana last year and had “all the characteristics of a tropical depression,” ultimately dumping as much as 30 inches of rain in southern parts of the state.
The Arizona storm system didn’t generate such copious rainfall Sunday. But it did pour 2 inches in parts of the Valley and spawn winds in excess of 60 mph. It’s not known how common land-forming tropical depressions might be, Pielke said, adding that a new high-definition NOAA geostationary satellite, launched in November, could help researchers better understand them.