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It's been a while since this happened, so I don't have many details. About 3 years ago, we had a popup storm here in Louisville. It was completely unexpected and unpredicted. That's not the strange part though. I took at look at the radar (it was about 5:00 PM) and noticed that on a 2 hour loop, the storm appeared as if it was brewing right over the city landfill and drifting towards the north-east. That was strange. They were essentially just thunderstorms with a lot of lightning, no tornado threat.

But then I zoomed out of the radar, and noticed that the same thing appeared to be happening over other nearby major cities (Cincinnati, Nashville, Indianapolis, St. Louis...). I'm not sure where the landfills are in those cities, but I'm guessing that may have been a pattern. That night was unusually muggy, like 100% humidity. The date was 7/20/2011.

(Click here for almanac data for area on that day)

Luckily I found this snapshot I saved

What could explain this phenomenon?

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  • $\begingroup$ Hmm, this is interesting. I'd have to look into this specific case, but it might turn out that it wasn't really a storm in all locations, and it was just air pollution/stuff in the air. I know that happened in Austin, TX on one of their hot days. I'm not sure what happened here, though. $\endgroup$
    – hichris123 ♦
    Aug 9, 2014 at 19:50
  • $\begingroup$ @hichris123 Pollutants are much smaller particles compared to cloud droplets and rain drops, and would appear more scattered and of much lower intensity on the radar than it is in this case. See my answer below. $\endgroup$
    – milancurcic
    Aug 9, 2014 at 20:23
  • $\begingroup$ I was beginning to think this was shortly after Taco Bell released some new hot meal, thus causing a reaction at the city landfill :P $\endgroup$
    – Jerry Dodge
    Aug 9, 2014 at 20:34

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This case is an excellent example of urban-induced convection.

Urban areas have very different surface and soil properties, leading to a different heat balance relative to more natural environments. At times when solar shortwave radiation forcing is high - summer time, early afternoons - urban areas act as heat islands. Sensible heat flux and outgoing longwave radiation are then increased relative to the environment. While latent heat fluxes are decreased because of lack of soil moisture and vegetation in the cities, there is enough moisture coming from the environment to trigger convective cloud formation and rainfall downwind from the urban area.

As to the question why did this occur in four urban areas at the seemingly similar time, it just happened so.

See this review article on urban-induced convection and associated physical processes for more details.

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  • $\begingroup$ I am not an expert in monsoon meteorology, but as far as I understand, monsoons operate on a larger scale. A large urban area, like Los Angeles valley, may affect the seasonal patterns, however, I believe that the desert is the main driver for the circulation change. $\endgroup$
    – milancurcic
    Jan 21, 2015 at 15:19

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