Weather Gang: Water, water everywhere: The anatomy of Tuesdays flash flooding event

Posted: Published on August 14th, 2014

This post was added by Dr Simmons

On Tuesday morning around 10 a.m.,anareaof very intense rainfall set up over the Washington-Baltimore region, and then parked there for the rest of the afternoon. The precise location of the heaviest rain accumulation was not anticipated in advance, as is often the case in summertime, flash flood-producing stormsan unfortunate shortcoming of even our best forecasting efforts.

By the end of the day, over 10 inches of rain fell south of Baltimore in northern Anne Arundel County, which was the hardest hit area. In D.C., over two inches of rain fell. Dozens of water rescues were made throughout Tuesdays event, and numerous incredible flash flood images were documented on social media.

Todaywe look at the final rainfall totalsand piece together how Tuesdays event took shape, so we can learn some meteorological lessons from this episode.

Impressiverainfall accumulation

Tuesdays advisory map showed what no one likes to see a series of flash flood watches (light green) and warnings (dark red) rushed to cover a rapidly evolving storm system.The Capital Weather Gangand the National Weather Serviceboth forecastthat the day would feature periods of moderate to heavy rain, but not to the tune of five to 10 inches.

Figure 1. Watches, warnings, and advisories from the National Weather Service during Tuesdays heavy rain event. Flash flood warnings are in dark red. (NWS)

The heavy rain was extremely localized. Figure 2 is a map of radar-derived rainfall accumulation. The white blobin Anne Arundel County near Glen Burnieiseight inch rainfall totals. Actual gauge measurements were higher, as radar tends to underestimate heavy rain falling from shallow, tropical-like rain clouds, such as these.

6.30 inches of rain fell at Baltimore-Washington International Airport, which broke the record for the date, previously 4.91 inches set in 1955. It also moved Tuesday into second place for the wettest day overall at the location.The largest amount of rain the airport has seen in a calendar day is 7.62 inches on August 23, 1933.

As if thats not enough, 10.32 inches fell near Green Haven, Md. in Anne Arundel County. Pasadena, Md. was also hard-hit, with 7.55 inches. Dundalk, Md. recorded 8.75 inches, and Pumphrey, Md. saw 7.41.

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Weather Gang: Water, water everywhere: The anatomy of Tuesdays flash flooding event

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