If you thought yesterday's heavy rain was bad, wait until you see this: Islip, New York recorded more than 13 inches of rain this morning, 11 inches of which fell in just three hours. That breaks the all-time New York State record for the most rain recorded in a 24-hour period.
The unprecedented rainfall event occurred as a result of the confluence of two extreme weather events: thunderstorm training and extremely high precipitable water values. As I discussed last night after areas near Baltimore, Maryland received a foot of rain, thunderstorm training occurs when thunderstorms keep redeveloping and moving over the same areas over and over again, much like train cars move over train tracks, hence the name.
Precipitable water (or PWAT) values help show the moisture content in the atmosphere. A PWAT of 1.50" means that if you condensed all of the water vapor out of that column of the atmosphere, from top to bottom, and it all fell as rain, it would produce 1.50" of rain. Higher PWAT values indicate high levels of moisture in the atmosphere.
PWAT values over Long Island were pushing 2.00" or more this morning when the record-breaking heavy rainfall event occurred. According to model data compiled on WeatherBELL, PWAT values that high over the area were more than two standard deviations higher than normal.
When a constant stream of heavy thunderstorms developed over Long Island this morning and tapped into the ocean of moisture in the atmosphere, it created a perfect storm.
For good measure and to keep it in perspective, the world record for the most rainfall ever recorded in one day was on La Réunion, a small volcanic island east of Madagascar in the Indian Ocean. The island recorded 71.9" of rain in 24 hours back in January 1966 as a result of Tropical Cyclone Denise.
[Images via AP / Gibson Ridge / WeatherBELL]