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Astounding, record-smashing rainfall swamps Long Island; 11 inches in 3 hours

Doppler estimated rainfall shows extreme totals over Long Island (National Weather Service)

In  just a few hours this morning, Long Island witnessed a jaw-dropping downpour unprecedented in New York state history.  Islip logged 13.26 inches of rain from the event, the greatest amount of rain to fall within 24 hours in a single storm in New York weather records.

Incredibly, over 11 inches (11.19″) of rain fell in three hours, between 5 and 8 a.m.  About 10 inches (9.81″) of rain fell in 2 hours,  a phenomenal quantity of water in such a short time.  No hurricane or tropical storm to affect New York State has produced such an output.

As Mashable’s Andrew Freedman put it: An entire summer’s worth of rain fell on New York in just a few hours

Consider, too, Islip almost received about as much rain in three hours as downtown Los Angeles did in all of 2012 and 2013 combined (11.75″).

Cliff Mass, a meteorology professor at the University of Washington, who happened to be on Long Island this morning, shared this eyewitness account:

Moses would have been impressed…it was pouring outside…in a way we rarely see in Seattle.  No lightning.  Water was jetting off the roofs of nearby buildings.  I even heard a frog in the distance…perhaps the next plague was in preparation.  I put out my walking staff to stop it, but to no avail.

Flash Floods overwhelm parts of Sunrise Highway in and around the Islip area, via WeatherGone wild on YouTube

This exceptional rainfall originated from the same weather system that flooded parts of Detroit and Baltimore.  The event unfolded as a plume of deep tropical moisture surged ahead of a slow moving-cold front in the eastern U.S.

Related: Water, water everywhere: The anatomy of Tuesday’s flash flooding event (Baltimore) | Record-breaking rain floods Detroit metro, shuts down interstates

After this moisture-enriched plume doused the Washington and Baltimore area, it surged north.  As the tropical air streaming in from the southeast clashed with chillier, marine air feeding in from the northeast, a coastal front formed – where moisture converged.  Heavy rain fell not only in Long Island, but also in coastal New Jersey and Connecticut.

“The radar evolution of this storm is nothing short of amazing,” says Jeff Halverson, Capital Weather Gang’s severe weather expert. “The heavy rain band was extremely narrow, and its entire length passed over this single location, Islip, Long Island, over several hours.  When we think of echo training, this literally WAS the proverbial “rain train” perfectly rendered by Nature!”

Radar loop showing extreme rainfall moving over Long Island between 2:30 and 8:30 a.m. August 13 (Weather Underground)

Mashable’s Andrew Freedman wrote Wednesday how these type of exceptional rainfall events fit a global warming pattern.  Historical weather records show such heavy rain events already increasing.

Greg Carbin, the warning coordination meteorologist at the National Weather Service Storm Prediction Center, discovered the amount of moisture measured in the atmosphere at Wallops Island, Virginia Tuesday evening – which streamed towards New York overnight – ranked in the 99th percentile.  He examined the metric known as “precipitable water” which “was within the top 20 values on record.”  Of the precipitable water extremes since records began in 1963, “almost all are in the 1990s and 2000s”, Carbin said.

Climate change is not the only risk multiplier when it comes to flash flood situations. Marshall Shepherd, chair of atmospheric sciences at the University of Georgia, notes urbanization plays a major role in the impact of recent flooding events.

As I watch breathtaking flooding in Boulder, Pensacola, Detroit, Baltimore, and Long Island, a rather simple equation comes to mind.

Urban Flooding = Increase in intensity of top 1% rain events + expanding urban impervious land cover + storm water management engineered for rainstorms of “last century”

A take home message here is that extreme urban flooding is increasing – so it should not come as surprise and we need to adapt urban infrastructure to be better prepared and more resilient.

Forecasts for flash flooding must also improve. Whereas meteorologists can generally identify pattern setups conducive to flash flooding, we face difficulties identifying the specific location of the actual events until they are already underway in many instances.

Photos from Long Island, via Twitter

Jason is the Washington Post’s weather editor and Capital Weather Gang's chief meteorologist. He earned a master's degree in atmospheric science, and spent 10 years as a climate change science analyst for the U.S. government. He holds the Digital Seal of Approval from the National Weather Association.
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