It’s been a very, very wet week in America.
That’s more rain than Long Island normally sees in an entire summer.
The previous record was set less than three years ago when Tannersville, New York, received 11.6 inches during Hurricane Irene, unleashing catastrophic flooding throughout the Catskills region.
Over the past day or so, a tropical conveyor belt has sent moisture streaming northward and compressed it, with storms riding along wind currents streaming into a slow-moving storm system that’s lingering over the Great Lakes. The effect has been similar to a slow-moving tropical storm, only with less wind at the coast. Although floods were predicted, with this kind of system, it’s kind of impossible to know exactly where they would occur.
Had Wednesday’s rain swath shifted just 50 miles or so to the West, I’d be talking instead about one of the biggest flash floods in New York City history. (The most rain that’s ever fallen in a single day in Central Park was a measly 8.28 inches back on Sept. 23, 1882. The New York Times account of that storm is amazing.)
As it was, the storms carved a path of heavy rain right through the heart of the urban Northeast:
Swath of excessive rain totals from DC area into southern NJ, LI and CT over last 24 hours. pic.twitter.com/tvSR2t0LaA— Anthony Sagliani (@anthonywx) August 13, 2014
The impressive Northeast rainfall totals were produced by an atmospheric process known as “training” (as in, choo-choo). Individual storm cells line up like a row of box cars, hitting the same areas over and over and over again. Training isn’t uncommon during times when slow-moving tropical flow is present, but usually it lasts only a few hours. In the case of these rainstorms, the train kept coming for the better part of a day.
Scenes from the floods have been dramatic:
There was so much water in Long Island this morning, there were white caps on the Sunrise Highway, one of the main commuting routes into New York City. Seattle’s intense rains were caused by sporadic thunderstorms moving across the Northwest, an area of intensifying drought. At one point on Tuesday, remarkably, there was a flash flood watch at the same time and place as a Red Flag warning for wildfires in Washington State.
This week’s floods are a glimpse of a changed climate. As the atmosphere warms due to human greenhouse gas emissions, it can hold more water vapor. That means rainstorms have become more intense in recent years, and are expected to continue getting more extreme in the coming decades. This year’s National Climate Assessment showed that extreme rainfall events are increasing quickest in the Northeast. On Tuesday, NOAA released new data showing that so far in 2014, the weather pattern over America has been one of the most extreme on record.
Earlier this year, Pensacola, Florida, experienced an equally remarkable episode of training—racking up more than two feet of rain in little more than a day—with devastating consequences.
The National Weather Service warns that the flash flooding event isn’t over yet. Extremely heavy rain continues across much of New England, where more flooding is likely during the day on Wednesday.
The Washington Post has a comprehensive breakdown of the meteorology behind the series of flash flood events, focusing on the Baltimore area.