A Very Rainy September in DC

by robekulick

Between the High Holidays, getting sick, and midterms coming up I haven’t had time to blog very much, and I’ll probably be pretty sporadic in my blogging until after midterms. But since coming to the DC metro area in late July, a few of my fellow first year PhD students have commented that it seems to rain here non-stop. I’ve assured them that the amount of rain we had, particularly in September was not the norm, but while taking a break from studying I did the following calculation.

According to NOAA data in rained 8.84 inches in DC in September. While the NOAA didn’t have a data series for historical rainfall amounts in September that I could find quickly, it did have a series for Maryland going back to 1895. Using the mean for that series as an estimate of the population mean and the variance as an estimate of the population variance, the z-score associated with 8.84 inches of rain in September is approximately 2.63. If this methodology were completely correct, this would suggest that in a given September there is only about a 0.5% chance of getting more rain, or in other words the amount of rain we got was in the 99.5 percentile of the September rain distribution.

Of course, since these sample estimates aren’t the true population numbers and a number of other simplifications involved in the calculation, these probabilities aren’t exactly right. But it does give a pretty good indication of how much damn rain we got this September and suggests that the situation should improve from here.