The National Weather Service Office serving New York City said Wednesday evening this was the first time this office has ever had to issue a Flash Flood Emergency, on a night in which Manhattan's Central Park saw the most amount of rain ever to fall in a single hour.
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The first Flash Flood Emergency ever issued by the New York office came Wednesday evening for Northeast New Jersey, which was followed by the second one ever put out by the New York City office, covering New York City itself.
Tracking Ida: Storm remnants spawn tornadoes, historic, deadly flooding across NYC, Tri-State
"To be clear... this particular warning for NYC is the second time we've ever issued a Flash Flood Emergency (It's the first one for NYC). The first time we've issued a Flash Flood Emergency was for Northeast New Jersey an hour ago, the weather service, based in Upton, said in a tweet.
Massive flooding across the area, State of Emergency in NJ
A Flash Flood Emergency advises people to move immediately to higher ground, and avoid walking or driving through flood waters.
It is the highest level of flood alert, above warning and watch.
The National Weather Service implemented the Flash Flood Emergency headline to its Flash Flood Warnings on February 4, 2014, the agency told ABC News.
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The terminology should only be used on rare occasions when all of the following criteria are met:
a. Severe threat to human life is imminent or ongoing
b. Catastrophic damage is imminent or ongoing
c. Reliable sources confirm a tornado or flash flood
"These headlines will effectively highlight the potential for large-scale loss of life and catastrophic property damage in these rare tornado and flash flood events," the agency said.
Indeed, the flooding killed at least 16 people, nine in New York City and another seven in New Jersey as flash flood waters overwhelmed homes and cars, and caused severe and widespread damage to property.
Unprecedented amounts of rain fell in a short time, overwhelming a region with already saturated ground.
Central Park observed 3.15 inches of rain in one hour, from 8:51 pm to 9:51 pm. That would make it the wettest hour in New York City record-keeping, dating back to 1870. It smashed a record set just last month, on the night of Aug. 21, when between 10 and 11 p.m., Central Park saw 1.94 inches.
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Between Philadelphia and the Tri-State area, the remnants of Hurricane Ida unleashed flash flooding, tornado watches and warnings and historic amounts of rain in an unprecedented evening of deadly severe weather.
The term "500 year flood" is widely used to describe events like this, but as ABC News' Ginger Zee explains, that 1 in 500 doesn't mean once in 500 years.
It means a particular area has 0.2% chance of flooding in any given year.
Daniel Swain, a UCLA climate scientist, emphasized to ABC that 1 in 100 or 1 in 500 doesn't mean the same in our warming climate thanks to human-induced climate change.
The following is Swain's observation on what's happening:
"But in reality, what was once the 100-year flood, the flood that had about a 1% chance of happening any given year, isn't the 100-year flood anymore. The extreme precipitation that we saw in New York City -- and it really in numerous locations throughout the Northern Hemisphere this summer, there have been a number of really severe and extreme precipitation and associated flood events that have caused quite a lot of damage and taken quite a lot of lives really on a bunch of different continents, North America, Western Europe, Southeast Asia, etc.
And what's really becoming clear is that these very extreme precipitation events are occurring more frequently than they used to and the main reason for that is human-caused climate change, which is increasing the amount of water vapor in the atmosphere and raising the ceiling on how intense the precipitation is under a variety of meteorological conditions."