Similar scenes unfolded in July when back-to-back deluges flooded the New York subway. A National Weather Service balloon launch found record atmospheric moisture during that event.
Now residents in the Big Apple are preparing for yet another potential flood event, with flash flood watches up for the city of 8 million. The heaviest rains will accompany a band of thunderstorms along a cold front that will sweep through the region Thursday night into Friday morning. The downpours will not be as extreme as those three weeks ago but could still drop one to two inches of rain in a short time.
“The rainfall combined with wet antecedent conditions in a short period of time will lead to increased potential for flash flooding,” the National Weather Service wrote.
A ribbon of high PWATs, or precipitable water values, is aiming toward New York like a fire hose. PWATs describe how much moisture is present in a column of atmosphere and is available for clouds to tap into to produce rainfall. PWATs exceeding two inches are moving into the region. The atmosphere is like a soggy sponge, and it won’t take much to squeeze that moisture out of the sky.
The bulk of the rain is expected through early Friday. Some severe weather is possible, too, with sporadic bouts of damaging wind and a low risk of an isolated tornado.
The stretch of repeated drenchings began in July when the remnants of Tropical Storm Elsa worked up the East Coast. Moisture streaming north ahead of the storm interacted with a cold front on July 8 to drop 2.27 inches of rain, with an additional 2.06 inches falling the next day. Areas northwest of the city picked up significantly more, the high rainfall rates overwhelming the drainage capacity of New York’s aging infrastructure. Paved surfaces became waterways, with floodwaters gushing into the subway.
The week ended with 1.42 additional inches of rain July 12, contributing to a monthly total topping 11 inches. The average is just above four.
August was equally wet, with 10.32 inches coming down and once again bringing widespread flooding. Tropical Storm Henri dropped 7.12 inches in two days — Aug. 21 and 22 — 4.45 of which fell on just the 21st. Parts of Monmouth and Middlesex counties in New Jersey picked up more than eight inches of rain from stalled downpours. Flash flood warnings were hoisted far and wide during that event.
As if that wasn’t enough, September kicked off with the extreme flooding from Ida’s remnants. New York received 7.13 inches Sept. 1, while Newark Liberty International Airport got 8.41 inches. It proved Newark’s wettest calendar day on record.
Both locations also set records for hourly rainfall, with 3.15 inches collected in an hour in Central Park and 3.24 inches at Newark. That obliterated records.
“The previous [New York City] record we have in the data set was 1.76 inches on Sept. 8, 2004, associated with [Hurricane] Ivan,” said Ross Dickman, meteorologist in charge at the National Weather Service in New York. He explained that it was from a PRE, or predecessor rain event, which occurs when tropical moisture streaming north ahead of a named storm is focused along a cold front.
“Obviously [this summer] has been a challenging time period,” he said. “We’ve had our share of high-impact events with these systems coming across and the impacts that they’ve been having.”
Four of New York’s 20 most extreme one-hour rainfalls have come this year. On Aug. 21, it received 1.69 and 1.84 inches in back-to-back hours as Henri came through. Another top-20 one-hour rainfall occurred July 8, when 1.54 inches fell in a single hour amid the remnants of Elsa.
During the height of Ida’s rains, which the Weather Service referred to as “catastrophic,” it opted to issue the first-ever flash flood emergency for the New York metro area. That’s the highest-tier flash flood warning that can be issued, indicating a likelihood for rare life-threatening flooding.
“People’s phones went off even before the flash flood emergency was issued,” Dickman recalled. “When they were, there were five consecutive emergencies that started at 8:41 p.m., then 9:28 p.m., which pretty much was the whole New York City metropolitan area. These are supposed to be once-in-a-career type issuances, but to issue five in one night for any forecaster is certainly an eye-opening experience.”
Now, with flash flood watches up again and another shot of heavy rainfall incoming, the concern is that saturated soils won’t be able to absorb much additional runoff. In fact, the ground is already holding about four extra inches of water as it is.
“We had Elsa, we had Henri, and then we had the remnants of Ida,” Dickman said. “That was sort of a trifecta of high-end rainfall. With wet antecedent conditions, now every time it rains, everyone’s on edge, since it doesn’t take much.”
Much of what has transpired this summer can be attributed to atmospheric steering patterns, which have prevented storms from recurving out to sea and instead allowed them to scrape along the East Coast. That, along with nearby warm ocean waters driven by human-caused climate change, has helped put more moisture into the air.
“In the past five years, I’ve heard a lot of comments about, ‘Wow, these dew-point temperatures,’ ” Dickman recalled. “If that pattern were to continue, I would think our summer severe-weather season would extend longer, too.”