Tuesday's rainfall was a 500 to 1,000-year event, meteorologists say

Nearly four inches of rain fell in the span of one hour at BWI on Tuesday, an event that can be expected to occur only once every 500 to 1,000 years, according to the National Weather Service.

What weather service meteorologists called an "incredible" 3.91 inches fell between 12:29 p.m. and 1:32 p.m. at Baltimore-Washington International Thurgood Marshall Airport, they wrote in a forecaster's discussion Wednesday.

There is a 0.1 percent chance of that much rain falling in an hour at BWI during any given year, weather service hydrologist Jason Elliott wrote.

The 2.7-inch rainfall from 1 p.m. to 2 p.m. that was reported in the weather service's regular hourly updates could be considered a 50-year rainfall event, Elliott wrote. And the 4.19 inches that fell between noon and 2 p.m. is a 200-year event for rainfall in a two-hour period.

But that window from around 12:30 p.m. to 1 p.m. included the heaviest rain, and Elliott noted that BWI wasn't even at the center of the most intense precipitation. Radar indicates as much as 11 inches of rain fell throughout the day in the Nabbs Creek and Marley Neck areas east of the airport, he wrote.

The heaviest reported rainfall in the region was 10.32 inches in the Green Haven area of Pasadena. The 6.3 inches measured at BWI was the second-highest single-day rainfall total for Baltimore on record, dating back to 1871.

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