An extensive period of torrential rain triggered widespread flooding across New York today. Three to nine inches of rain fell, leading to one of the most severe flooding events in the history of the city. The deluge has brought the city to a standstill, submerging streets, inundating subway stations, shutting down airport runways, and prompting emergency rescues.

Storm Impacts

The torrential rains and rising floodwaters produced a wide range of significant impacts. Many areas faced power outages due to the widespread flooding as electrical substations were compromised by rising waters. Property damage from rising flood waters in buildings, leaking roofs, and uprooted trees was extensive. 

The most severe impacts were the countless disruptions to the transportation system. The city's subway system bore the brunt of the flooding, with numerous stations and tunnels submerged under cascading waterfalls and fast-flowing currents. The Metropolitan Transportation Authority (MTA) was forced to suspend multiple subway lines, leaving commuters stranded and seeking alternative routes. The storm caused flight delays and cancellations at major airports, school cancellations, and major disruptions to the bus system as well. 

The JFK airport received over 8.5” of rain yesterday, while parts of Brooklyn received over 7”. The hourly rainfall was just as important as the total rainfall yesterday. Below you can see the maximum hourly rainfall estimates from radar. Much of the flooding occurred where the hourly rainfall rates were highest, rather than the most rain fall in total.

Training rain resulted in a prolonged period of intense rainfall rates in several areas.

Storm total rainfall, from radar and gauges, is shown below. You can see a large area above five inches with a few small areas of more than eight inches of rain. 

The Cause

Four key elements came together to create the record setting event yesterday. First, an inverted trough present over the Northeast acted as an initiation point for showers and thunderstorms. Troughs are areas of lower atmospheric pressure that can lead to the development of low-pressure systems, which often produce unsettled weather, including precipitation and storms.

Inverted troughs are a type of low pressure system distinguished by an elongation of the low pressure area. Most troughs of low pressure in the mid-latitudes are characterized by decreasing atmospheric pressure from south to north, while inverted troughs are characterized by decreasing pressure from north to south.

Image: Traditional trough (left) versus an inverted trough (right).

Inverted troughs can lead to significant precipitation events in the Northeast due to several factors. More pronounced convergence of air at the surface leads to enhanced lift, which can produce more intense and organized convective activity. This mechanism yielded extremely heavy rainfall rates over New York yesterday.

Inverted troughs in the Northeast often interact with other weather systems, such as coastal lows or frontal boundaries. These interactions can enhance lift and moisture convergence, leading to more widespread and heavier precipitation. Inverted troughs can also tap into and transport abundant moisture available in the Atlantic Ocean, providing the necessary fuel for heavy rainfall. A coastal storm situated off the Atlantic Coast supplied extra moisture and lift yesterday, leading to prolonged periods of heavier rainfall.

This type of storm can also interact with remnants of tropical systems or tropical moisture plumes. These tropical connections can introduce additional moisture into the system, further increasing the potential for heavy rainfall. Remnants of Tropical Storm Ophelia, which dissipated earlier in the week, fed additional tropical moisture into the storm system, enhancing the rainfall due to the additional moisture source. Below you can see tropical moisture being pulled into the Northeast.

Finally, regions of high pressure to the north and west allowed the storm to remain mostly stationary for much of the day, causing a small region to be inundated for hours.  

Check out some more shots of the flooding below.

This storm produced some fascinating satellite imagery.

Emergency services worked throughout the day, rescuing stranded residents and ensuring the safety of those in vulnerable areas. Communities have come together to take on the task of cleanup and recovery, but it will take time. Although it's not rare to see inverted troughs combined with other weather systems and tropical moisture, when they occur over the most densely populated part of the country, the consequences can be significant.

Luke Stone
Forecaster, OpenSnow