DISCUSSION: Widespread flash flood watches were issued Sunday across portions of the Mid-Atlantic from Maryland to New York. There was plenty of available moisture in the atmosphere, as precipitable water values (or PWATs, as they are often referred to) reached upwards of two inches across the Mid-Atlantic coastline, seen below. Remember that precipitable water indicates the amount of rain that would fall if all of the moisture were to be squeezed out of a column of air.
At the 300mb pressure level, a potent trough was driving through the Great Lakes region. Take a look at the purple contours below, which indicate divergence. If air diverges at upper levels, then that means air from lower in the atmosphere must rise to take its place. This is important to the flash flooding setup, as moisture-laden air that is forced to rise can create thunderstorms with torrential rains.
A bit closer to the surface, at 500mb, we can still see the trough anchored in the Ohio Valley, but we can also see vorticity as shown by the green and yellow shadings. Vorticity is a measure of fluid rotation, and an area of vorticity moving towards a particular location is another sign of rising air. For this situation, note the vorticity maximum near Atlantic City (marked as an "X"). Over the previous four hours, that "X" had migrated northeastward from the Washington DC area, meaning that air would be forced upwards above Delaware and southern New Jersey.
Combine the high PWATs, upper-level trough, and mid-level vorticity movement, and you've got the perfect recipe for flash flooding. Check out the loop below, courtesy of COD Meteorology, as storms quickly form in Delaware and southern New Jersey - the exact areas where upper-level divergence and rising air were at a maximum. It's a satisfying feeling to see the principles you learn about play out in real time.
While these were the conditions between roughly 11PM and midnight Eastern time, dangerous flooding had already occurred earlier on Sunday. A slow-moving storm in northern Delaware and extreme southeastern PA dumped over four inches of rain in spots, with two inches falling in a single hour in the town of Chadds Ford, PA. Not to be outdone, the airport in Harrisburg, PA measured an amazing 4.27 inches in one hour, in which 1.83 inches fell in just 21 minutes! These totals were extremely localized, however, as rainfall amounts greatly diminished only five miles away from each of these locations. Even high-resolution models can't predict the weather on that small of a scale, so it's very important to keep an eye on the radar whenever a flash flood situation is possible.
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© 2017 Meteorologist Jake Spivey