 DISCUSSION: Over the past 24 hours, a storm similar to a Nor’easter dropped an immense amount of rain in the Mid-Atlantic region. Highly unusual for this time of year, this coastal storm (with a track that is typical of a winter Nor’easter) carried lots of tropical moisture which allowed it to drop nearly 6-7 inches of rain near Washington, DC. Rain gauges in parts of southwestern Delaware measured more than 10 inches of precipitation! While the rain is still continuing, a map detailing 24-hour precipitation totals for the region showcases the effects of this coastal storm (see above, courtesy of NWS WPC). Early this morning the NWS Weather Prediction Center released a Mesoscale Precipitation Discussion (MPD #:0592) warning the Delmarva area of the threat of flash flooding from this coastal storm. The graphic they released for the MPD (below, courtesy of NWS WPC) shows the conditions that were set up for the storm. A low pressure center with counterclockwise winds around it was riding along the coast. This allowed for warm, moist air from the south to mix into the stationary front, providing it a source to draw more energy from, and cooler air from the northeast to drive strong winds into the area. The storm had a history of heavy rains, and with convection that continued to pop up along and in front of the stationary front, the storm had another source to draw energy and moisture from that permitted it to slow down and downpour.  In addition, the coastal storm had the help of the unusually sharp contrast of sea surface temperatures in the Atlantic for this time of year, (see below, courtesy of NWS Eastern Region HQ) which is typically a driving force for Nor’easters. This contrast also shows the Gulf Stream pushing further north than we usually see it. With the combination of all of these conditions, rain poured at large rates over a long duration, allowing for incredible amounts of precipitation for the area.  To learn more about other flooding events from around the world, be sure to click here!
©2017 Meteorologist Katie McCracken
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 Torrential rain has hit parts of Kansas and Missouri prompting water rescues. Radar indicates 3 to 7 inches of rain has fallen during this event with flash flooding imminent. Indian Creek in Kansas City has risen over 10 feet in 4 hours. Tomahawk Creek is also a concern for the people living along the banks. Water rescues are ongoing at the time of this article and will continue as water levels are expected to continue to rise even though the rain has ended. Buildings have been closed due to flooding as well as road closures including major highways in the area. Officials are asking people to not drive through flooded roadways for safety. The complete Flash Flood Warning from the National Service in Pleasant Hill/Kansas City, Missouri is as follows:
“* Flash Flood Warning for... Wyandotte County in northeastern Kansas... Southeastern Leavenworth County in northeastern Kansas... Johnson County in east central Kansas... Clay County in west central Missouri... Southern Ray County in west central Missouri... Lafayette County in west central Missouri... Eastern Platte County in west central Missouri... Jackson County in west central Missouri... Northern Johnson County in west central Missouri... * Until 1100 AM CDT * At 807 AM CDT, Doppler radar indicated heavy rain across the warned area. 3 to 7 inches of rain have fallen. Flash flooding is already occurring. * Some locations that will experience flooding include... Kansas City, Overland Park, Olathe, Independence, Lee`s Summit, Shawnee, Blue Springs, Lenexa, Leawood, Raytown, Liberty, Gladstone, Grandview, Prairie Village, Grain Valley, Excelsior Springs, Merriam, Mission, Smithville and Kearney. * Extensive flooding has occurred near Indian Creek with water over the road at 103rd and Wornell Rd and multiple water rescues. Indian Creek has crested and is starting to fall. The Blue River has also experienced Minor to Moderate flooding and is starting to crest currently. Tomahawk Creek reached record flooding overnight but has crested and is starting to fall. * This Flash Flood Warning replaces previous Flash Flood Warnings that will expire at 830 AM CDT. This includes the following highways... Interstate 70 in Missouri between mile markers 0 and 62. Interstate 70 in Kansas between mile markers 410 and 423. Interstate 35 in Missouri between mile markers 0 and 27. Interstate 35 in Kansas between mile markers 212 and 235. Interstate 29 between mile markers 0 and 19. Interstate 635 between mile markers 0 and 12. Interstate 470 between mile markers 0 and 16. Interstate 435 between mile markers 0 and 83. Kansas Turnpike between mile markers 222 and 226.” Remember, “Turn around. Don’t drown!” Stay up-to-date on flooding events around the world here! ⓒ 2017 Meteorologist Brandie Cantrell 
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.
To learn more about other flooding events from around the world, be sure to click here! © 2017 Meteorologist Jake Spivey |
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