The remnants of Hurricane Ida unloaded a historic and catastrophic deluge on New York City and the surrounding region Wednesday and has been linked to at least 41 deaths in New York, New Jersey, Pennsylvania and Connecticut.

The death toll from the storm in the Northeast, more than a thousand miles from where it made landfall in southeastern Louisiana, may seem surprising. Four main factors contributed to this disaster:

  1. The exceptional rate at which the rain fell, up to 3 to 4 inches per hour, spurred by a complex set of meteorological factors.
  2. The region’s vulnerability to flooding, due to urban and suburban sprawl that creates areas where water can’t run off, and inadequate infrastructure to manage the stormwater: New York City’s sewer system was only designed to handle 1.75 inches of rain in an hour.
  3. Very heavy rain before the event, which left waterways already near capacity. For example, New York City received over 10 inches of rain in August, its fourth-highest amount on record during the month.
  4. Climate change intensifying excessive rainfall.

Before we visit the meteorology of this event, it’s worth summarizing the amount of rain it generated, which was exceptional and, in several instances, record-breaking:

  • New York City’s Central Park received 7.13 inches of rain Wednesday, its fifth-wettest day on record. Between 9 and 10 p.m., 3.15 inches fell, the greatest amount in a single hour ever observed in the city.
  • Newark had 8.41 inches of rain, its wettest day on record; a record 3.24 inches fell in a single hour.

Rainfall this extreme is expected to occur only once over a 200-to-500-year period. So how did it happen?

The meteorology behind the deluge

The remnants of tropical cyclones that travel inland over the central and eastern United States are sometimes marked by deadly “second acts” that can unfold days and hundreds of miles beyond coastal landfall. Often this is because extreme tropical moisture interacts with preexisting weather systems in the mid-latitudes. These elements include fronts, while jet stream disturbances add additional energy and provide a source of invigorated uplift to tropical remnants.

These interactions have led to several historic flood disasters in the Mid-Atlantic and southern New England (both flash floods and river floods), including from storms named Camille (1969), Agnes (1972), Eloise (1975), Gaston (2004) and Lee (2011).

Extremely heavy rains cascaded down on New York City and surrounding regions for an extended period as the remnant circulation of Ida approached from the southwest. Meteorologists knew several days in advance of an increased threat for heavy rain and life-threatening flooding as Ida’s remnants began to interact with a potent set of atmospheric conditions in the Mid-Atlantic.

The National Weather Service issued flash-flood watches for the region 48 hours in advance and predicted general, widespread rains of three to five inches, with locally higher amounts. A rare “high risk” warning, for the most serious flood potential, was declared by the Weather Service a day in advance, as shown below.

The Weather Service strongly messaged the flooding potential, calling it “life-threatening.”

Flash floods are inherently multi-factor natural disasters. It’s not just about the amount of water falling from the sky but also the rate at which the rain falls and the ground’s ability to infiltrate and process that water. Naturally vegetated surfaces do a far better job of acting as a giant “sponge” than do the extensive, impervious surfaces common to sprawling suburban and urban regions.

The graphic below zooms in on the New York City area, showing total rain accumulation across the New York boroughs, courtesy of a dense network of meteorological stations that are part of the New York State Mesonet run by SUNY Albany. Peak amounts were in the 7.5-to-8.5-inch range — half of the amount delivered over southeastern Louisiana by Ida’s core.

Despite the lesser rainfall in New York City relative to southeastern Louisiana, the flooding was arguably worse. This is because the extensive paved surfaces and rooftops generated a tremendous volume of runoff with nowhere to go.

As mentioned, the peak rainfall rates in New York City were record-breaking and exceeded those in Louisiana even though less rain fell overall. The efficiency of such rain generation is indicative of the exceptional meteorological circumstances present Wednesday.

Many of the key processes are synthesized on the graphic below, from the National Weather Service. The core of the approaching Ida remnants cannot be seen in the diagram (they’re off to the left, over West Virginia), but their large wind circulation began pushing a surge of mild, moisture-laden and unstable air northward along the coastline of New Jersey, New York and southern New England.

Along this push, a boundary called a “warm front” became established (red, scalloped lines). A conduit of moisture-laden, low-level wind — called a “low-level jet” — accelerated up and over the warm frontal boundary, the air rising along an invisible ramp (blue arrow). The focus of the jet was squarely on southeastern New York, New York City and Long Island. The buoyancy of an unstable air mass (red contours) ensured that the air would rise in narrow, intense updrafts inside thunderstorm cells.

This setup, once established, persisted for many hours, with cloudbursts of rain periodically arriving from thunderstorms transiting the same regions.

The enormity of the rain swath generated along and ahead of the warm front is shown in the next graphic, again from a Weather Service bulletin during the late afternoon. In this figure, the core of Ida (red “L”) can now be seen. The red and orange splotches are the small, convective cores of embedded thunderstorms.

The next graphic shows the setup many hours later, during the evening. The warm front has shifted to directly over New York City as the core of Ida draws closer. The record-setting cloudburst of 3.15 inches in a single hour occurred over Central Park at this time, between 9 and 10 p.m.

Heavy, persistent rain generation over the large area was being aided by other processes. First, a wavelike disturbance in the jet-stream flow was approaching from the west, merging with the high-altitude low-pressure zone associated with the remnants of Ida. Second, a fast-moving pocket of air moving through the jet stream at high altitudes, called a “jet streak,” was encouraging air to rise vigorously across the region.

And third, the airflow in mid-levels was being deformed into a narrow, concentrated front along the northern edge of the storm center, focusing strong uplift of moist air through a deeper layer.

These processes are unique to the mid-latitudes, acting in this case onto a vortex of tropical origin and a soppingly humid, tropical air mass imported with and by that vortex. The storm that dumped so much rain on New York City was a hybrid system of sorts — part tropical, part extra-tropical. The history of the Mid-Atlantic and New England is replete with instances of these devastating, hybrid weather systems in late summer and fall.

Climate change and urbanization increase flooding risk

The setting in which these storms now occur has also shifted, over many decades, in that climate change has led to a greater frequency of heavy rain events in the eastern United States. The latest Intergovernmental Panel on Climate Change assessment demonstrates a 7 percent increase in heavy rain for every 1.8 degrees of rise in atmospheric temperature. Years of weather-balloon launches have shown a steady increase in low-level atmospheric humidity levels. That’s more “raw material” available to generate rain.

In the past two weeks, New York City has had three of its top 20 heaviest one-hour downpours on record; four of the top 20 have come this year. On Aug. 21, it received 1.69 and 1.84 inches in back-to-back hours. Another top-20 one-hour rainfall occurred on July 8, when 1.54 inches fell in a single hour.

And it’s more than just climate change that makes populated zones increasingly prone to heavy-rain disasters. Humans have altered the landscape as more and more of the United States becomes urbanized and covered with suburban sprawl. The vast increases in impervious surface — and denser concentrations of at-risk citizens in these locations — has heightened our vulnerability to fatal flash floods. The horrific, $125 billon flood delivered by Hurricane Harvey over the Houston region in 2017 underscores the contribution of land-surface change to flood vulnerability.