The Washington Post

Weather Service: BWI witnessed “between a 500 year and 1,000 year rainfall”

Doppler estimated rainfall totals in northern Anne Arundel County, including BWI Airport (National Weather Service)

The extreme rain that inundated northern Anne Arundel County was truly exceptional; a rare event that comes around every so many hundred years, according to analysis from the National Weather Service. The record-setting rain partially submerged dozens of cars in two long-term parking lots at BWI Airport, and led to more than two dozen high-water rescues in Anne Arundel County.

Here is what the National Weather Service found, taken verbatim from its report on the event; note I’ve bold-faced the key points:

As we already know…6.30 inches of rain was recorded at Thurgood Marshall BWI Airport. This is the second highest daily rainfall total ever reported for any day in Baltimore. The highest is 7.62 inches set on August 23rd 1933 from the great Chesapeake Potomac hurricane of 1933. Daily precipitation records in Baltimore go back to 1871.

Based on the BWI observations…2.70 inches of rain fell between 12:54 p.m. and 1:54 p.m. Based on precipitation frequency studies…this occurs on average once every 50 years in a one-hour time period. 4.19 inches of rain fell between 11:54 a.m. and 1:54 p.m….Which is approximately a 200 year rainfall event for a two-hour time period.

During the heaviest rain…between 12:29 p.m. and 1:32 p.m….an incredible 3.91 inches of rain fell. Although this time period is not exactly one hour…that amount falls between a 500 year and 1000 year rainfall for a one-hour time frame. This means there is around one-tenth of one percent chance of this amount of rain falling in an hour in any given year at BWI.

Amazingly…BWI was only on the edge of the heavy rain axis. Dual-pol radar estimates – which were extremely close to ground truth in the areas where we have ground truth – indicate over 11 inches of rain in the Nabbs Creek/Marley Neck areas east of BWI. Our highest ground truth report was from Green Haven…just south of Nabbs Creek…where 10.32 inches was recorded.

This remarkable rainfall event occurred from the same weather system that caused severe flooding in Detroit Monday, Long Island Wednesday morning, and Portland (ME) Wednesday night.

* In Detroit, 4.57″ was measured at the airport – the second wettest day on record. Over 6 inches fell in some of its suburbs (more information).
* In Islip, Long Island, 13.26″ was recorded – the most on record in a 24-hour period in New York state history (more information).
* In Portland, Maine, 6.43″ fell (4.21 inches between 9 and 11 p.m.), its second wettest August day on record.

Rainfall totals around Portland, Maine (National Weather Service)
Jason is currently the Washington Post’s weather editor. A native Washingtonian, Jason has been a weather enthusiast since age 10.
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