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Why so Much Rain? Recapping a Wild Week of Record-Setting Floods

15 Aug 2014, 10:53 am

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First, it was Detroit on Monday. On Tuesday, Baltimore – particularly the south side of the city – got blasted with historic floods. Wednesday, it was Long Island’s turn and finally on Wednesday night and into Thursday morning Portland, Maine took the final hit from a torrential downpours.

Historic flooding covered highly-populated areas of the eastern third of the country, the result of a slow-moving center of low pressure that barely nudged along before finally departing into the Canadian Maritimes this afternoon.

So what caused a record-setting 13.57″ of rain to inundate Islip, NY (southern shore of Long Island)? What were the ingredients that led to epic rains (6.30″) at BWI Airport just south of Baltimore, MD on Tuesday? And how did Detroit wind up with over four-and-a-half inches of rain in the heart of Monday’s afternoon commute, forcing the closure of several major interstates there?

Let’s take a look.

Firstly, a big mid-latitude low pressure system — far more typical in the spring and fall months rather than the middle of summer — moved slowly across the eastern third of the country, with little in the way of steering mechanisms to push the low along. Next, the low ran into warmer air than it might normally find were it taking place in, say, April or October, as these types of lows typically do. Warmer air at the surface forces the cooler air aloft to rise, leading to increased lift and diffluence aloft, often the recipe for big rains. With warmer temperatures and very cool air behind the system in the Great Lakes, a vast temperature contrast helped strengthen the low even further. Another important piece was the deep plume of tropical moisture this low was able to tap into, particularly the ‘warm sector’ (the area between the cold and warm front) that fueled the majority of the huge downpours that led to much of the flooding. Finally, a strong piece of upper level energy brought added lift to the atmosphere as well.

In short: a stronger low than normal for this time of year plus infusions of tropical moisture and strong energy in the upper atmosphere made this an epic soaker for many along the east coast.

The good news? Nothing anything close to this type of rain appears to be on the horizon, at least not anytime soon. A more typical pattern for mid-to-late summer resumes this week, with scattered thunderstorms returning to the northeast for Saturday and chances for storms every 2-3 days likely for next week. Hopefully, we’ll avoid any record rains such as the ones we saw this week. But of course if does happen, we’ll keep you posted with all the latest right here on WeatherNation and weathernationtv.com.

Meteorologist Chris Bianchi

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