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East Coast storm bringing worst coastal flooding since 2003 to parts of Mid-Atlantic
Floodwaters are expected to flood roads, enter homes and close businesses
Coastal flood warnings span from the Virginia Tidewater region north through central New Jersey, including the entirety of the Delmarva Peninsula and the shores of the Chesapeake Bay. The flooding will be notable both for its unusual intensity and particularly long duration, with concern lasting through at least Sunday morning before the elevated waters recede.
The setup
The instigating storm system was wrapping an arc of intense downpours and embedded thunderstorms up the Eastern Seaboard as the low-pressure center remains to the west. That places the entire East Coast on the warm side of the system, which will bring southerly or southeasterly winds, an onshore flow and gusts topping 50 mph near the shoreline, and 30 to 40 mph inland.
A general one to 1.5 inches of rainfall is possible in most spots too, with a few localized totals closing in on two inches.
The storm system will slowly fade this weekend as it treks north over the northern Appalachians, but continued easterly winds will maintain coastal flooding issues through early Sunday. The storm could even bring the highest water levels observed in Delaware Bay, beating out any flood events tied to past hurricanes or winter storms. It’s an important reminder that storms can be problematic even if they don’t have a name or fit neatly into a categorical box.
Dangerous flooding likely
The National Weather Service is forecasting water levels on Delaware Bay at Reedy Point to crest at 9.5 feet Friday evening, corresponding to “major” flood stage. The value corresponds to anticipated water levels above the “mean lower low water” datum, or the average lowest low tide mark over a 19-year window.
That would be nearly three inches above its previous crest at 9.24 feet on April 16, 2011. That occurred during a strong springtime storm system that whipped a barrage of tornadoes across the Carolinas which also caused severe weather and coastal flooding in the Washington region.
“Flooding may become severe enough to cause some structural damage along with widespread roadway flooding near tidal waterways,” wrote the National Weather Service in Mount Holly, N.J. “Some locations may become isolated by the floodwaters.”
The weather officials note that the Friday evening high tide and that occurs Saturday morning will be the most affected by flood concerns. This is especially true in the Chesapeake Bay, where officials are already distributing sandbags to help coastal residents protect vulnerable property and infrastructure.
The forecast water level “would be five feet for Annapolis,” said Connor Belak, a meteorologist at the National Weather Service office in Sterling, Va., which serves the greater Baltimore-Washington region. “The only two higher instances were 7.16 from Isabel on Sept. 19, 2003, and 6.17 feet on August 23, 1933.”
The tidal gauge is located in Santee Basin off Holloway Road. While Belak notes that it would take another foot of surge for Annapolis to experience major coastal flooding, he emphasizes that the duration will be a complicating factor.
“Most will be in moderate flood stage for the better part of 36 hours, which is pretty long duration,” he said. “In Annapolis, [the threat will persist] through Sunday morning at 8 a.m..”
For the D.C. waterfront, Belak says that most historic crests were around seven feet; his office is forecasting 6.92 feet, which could bring splashover around the Jefferson Memorial and Tidal Basin.
Growing impacts
The effects are already manifesting. Belak states that St. Mary’s and Calvert counties in Maryland have “dozens of roads closed, and lots of instances of houses and basements taking on water, along with garages.”
Those same areas, along with Atlantic coastal Cape May in New Jersey and the Delaware beaches, are also under high wind warnings because of near-shore easterly gusts that could approach or exceed 50 mph.
Some of the tributaries that flow into the ocean and into various bays will also experience flooding as onshore winds back water up and prevent drainage. The Northwest branch of the Patapsco River in Baltimore near Fort McHenry should see moderate flooding, possibly tying for its fourth-greatest water level recorded.
The Delaware River in Philadelphia is possibly going to hit major flood stage for a time during the Friday night high tide.
The city of Alexandria in Virginia, located just southwest of Washington, is handing out sandbags to residents on a first-come first-served basis; there’s a limit of five per address.
A disturbing climate trend
Alexandria suffers regular coastal flooding within its main business district where King Street meets the tidal Potomac River. That’s thanks to King tide flooding. King tide flooding occurs when the moon is closest to the Earth in its orbit. It is often enhanced in the fall and winter because of prevailing winds pushing water onshore because of the positioning of key larger-scale weather features. King tide flooding affects most of the East Coast and is even threatening several cities.
Nowadays, it’s not uncommon for coastal cities to see two or three of these larger events per year, and sea level rise because of human-caused climate change is making it even worse. Charleston, for instance, sees regular king tide flooding, and Miami has seen a 12-fold increase in action-tier flooding since 1996.
Between 2000 and 2015, the incidence of high-tide flooding in the Mid-Atlantic doubled from an average of three days per year to six, according to a 2018 NOAA report.
The fact that the Mid-Atlantic is bracing for serious flooding from a meager storm system is yet another anecdote illustrative of an irreversible trend that will only grow more disruptive — and costly — with time.
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