The springtime storm season is in full swing and we are beginning to see the catastrophic effects of heavy precipitation throughout the country. Not only is the US getting its first taste of the hurricane season, but thunderstorms were apparent throughout the northern, central, and eastern US yesterday. The most extreme flooding from yesterday occurred just outside of Ellicott City, MD, where there was just under 9.5″ of precipitation that fell as reported by a CoCoRaHS observer:
Below is a MetStormLive 24-hour precipitation map, showing the maximum observed precipitation colocated with where the CoCoRaHS observation was taken.
The following image is for the same 24-hour period of the MetStormLive image, but is generated from the NWS Stage IV product.
To better quantify this storm, the 6-hour period where the majority of the precipitation occurred has been plotted and translated into an Average Recurrence Interval. In just 6-hours roughly 9.5″ of rain fell which is equivalent to a greater than 1 in 1000-year precipitation event. In other words, this storm had a less than a 0.1% of occurring in any given year.
Unfortunately, Ellicott City has experienced events like this several times before, as the characteristics of the topography in this area makes the City prone to flooding. The most recent event occurred in July 2016, where just over 7″ of rain fell in 6-hours. This amount of precipitation over this window of time translates to a 450-year ARI, or a 0.22% chance of occurring in any given year.
With this storm dropping significant precipitation in the region so close to when the last major event occurred, a common remark is that these events should literally only happen once in x-number of years. However, when dealing with probability and statistics it might be better to consider it as a storm with a 0.1% of occurring in any given year, meaning there’s a small chance of it occurring in any year. Regardless we hope those affected by this flood have a speedy recovery.
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