WRAL WeatherCenter Blog

I'm curious to know the storm level that would categorize the rainfall on Sunday, April 15. Was it a 5-year or a 10-year or a 100-year storm?

Posted April 16, 2007

MIKE MOSS SAYS:      Peggy,    Keeping in mind that rainfall in convective storms like those can be highly variable over a short distance, we did end up with anywhere from about 1 to 3 inches of rain around the region for the day, with reporting sites in Wake County ranging from around 1.75 to 2.75 inches and averaging about 2 inches.

How you slice and dice information to translate it into return intervals as you asked about also involves some interpretation. If we go by rainfall total over a 24-hour period, or even assume that most of the average 2 inches fell in about 12-18 hours, for our area this is not an especially unusual occurrence and has a historical recurrence interval of about one year or even a little less (i.e. that much rain in over half a day sometimes occurs more than once a year). For locations that received closer to 3 inches, the recurrence interval is about once in 2 years.

We can also look at short-term rainfall intensity and find a little different result for a given point location. A couple of the storm cells that developed last evening gave radar signatures indicating instantaneous rainfall rates as high as 2 to 3 inches per hour. A rate of 2 inches per hour would be expected to happen about every 5-10 years, while a 3 inch per hour rate has roughly a 75 year recurrence interval.

You can read more about a tool used to estimate these kinds of precipitation statistics in a WeatherCenter Blog entry from Jul 2006. It is located at

http://www.wral.com/weather/blogpost/1102164/

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