The high pressure system that has been in firm control of our weather this week loosened its grip slightly on Thursday allowing for some scattered afternoon storms to develop. For July, sea level pressure readings this week have been near the top of the observed range thanks to the strong upper level ridge. The featured chart takes a look at the maximum observed hourly precipitation by sea level pressure for Des Moines during July. A general and intuitive relationship is shown with maximum precipitation values generally decreasing as pressure readings get to either extreme of the observed range. While the reason for the decrease in precipitation for the highest pressure seems obvious as higher pressure implies less upward vertical motion in the atmosphere, why do values decrease at the lowest pressures as well? The speculative reason likely has to with the most intense storms being displaced from the surface low pressure center and perhaps also due to stronger winds associated with lower pressures and leading to faster storm motions with less training of storms over the same location.

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