In the world of weather, some things don’t mix, such as major cities and heavy rainfall. New York City has been slammed by multiple flooding rain events in the past week, and a third could target the Big Apple and bring additional problems on Monday night.
“Conditions may develop that lead to flash flooding,” wrote the Weather Service in New York. “Flash flooding is a very dangerous situation.”
Up to four inches of additional rain may fall during the evening. The Big Apple has already seen over eight inches of rainjust before the halfway point of July; the monthly average is 4.7 inches.
The rains first made headlines last week, when tropical moisture streaming north ahead of Tropical Storm Elsa interacted with a cold front Thursday. At least 2.27 inches came down during the afternoon hours as severe thunderstorms festered across the Tri-State area, also producing tennis ball-size hail in Bergen County, N.J., and snapping trees with 50-60 mph winds. Areas farther south in the Garden State saw gusts approaching 80 mph.
At its peak, the deluge dropped 1.56 inches on Central Park in a single hour, snagging eighth place on the list of heaviest one-hour rain tallies recorded in the city since 1943. The rapidity with which the water came down spurred significant flash flooding, causing underpasses and highways to become rivers as traffic ground to a standstill. Floodwaters gushed into subway platforms, leading some commuters to wade through waist-deep toxic water.
The heaviest rainfall occurred between 5 and 6 p.m., coinciding with the peak of the evening commute. Flash flood warnings blanketed the city, with numerous water rescues resulting.
Areas farther south in New Jersey saw up to 4.92 inches of rainfall, much of which saturated the ground and set the stage for additional heavy rains areawide.
Additional significant rains came with Elsa itself early Friday. The city tallied 2.06 inches during the calendar day, with another quarter-inch coming down Saturday. Flooding once again resulted, the sodden soils offering little opportunity to drainage of renewed floodwaters. In Connecticut, amounts approached a half foot.
Now, New Yorkers are bracing for another serious rainfall event during the first half of July to kick off the workweek. Monday began with a morning batch of storms that released 1.35 inches in a single hour, rivaling what had occurred just four days prior and adding to the list of only a dozen times so much rain has fallen in such an interval since 1943.
The instigating weather map feature is a west-to-east stationary front, which is draped just north of the city. That stalled front allows waves of rain and thunderstorms to ride along it, a phenomenon called the “training,” or repeating storms drenching the same areas time and time again. About 16 inches of water are trapped in the soil, about two or three inches more than average and in the top 2 percent of soil moisture values measured by NASA. That makes it easier for new rains to trigger flooding, while also replenishing moisture in the atmosphere and enhancing future rains.
On Monday morning, the National Weather Service released a weather balloon that found record “precipitable water” values, or total atmospheric moisture, over Long Island. More than two inches of liquid-equivalent water is present in columns of atmosphere overhead, illustrating the availability of moisture to juice-up storms. Moisture continues to pool along the boundary.
Thunderstorms were firing during the midafternoon hours south and west of New York along the stalled front. One slow-moving storm complex had spurred the issuance of flash flood warnings in central New Jersey, including Trenton and a stretch of Interstate 95. Rainfall from those storms totaled to two to 3.5 inches. The “nearly stationary” storms were also producing wind and hail, and showed signs of potentially back-building toward Philadelphia.
High-resolution weather models indicate storms will increase in aerial coverage and intensity during the evening hours, possibly lingering well into the night in the vicinity of northern New Jersey and New York City. There is even some data to hint that storms could redevelop overnight along the front, riding into the Big Apple and making for a messy commute Tuesday morning.
The National Weather Service Weather Prediction Center has drawn a level 3 out of 4 “moderate risk” for flash flooding around the city. Mid-level flow, which steers thunderstorms, is parallel to the boundary triggering the storms to begin with. That is a perfect recipe for having two, three or four rounds of storms impact the same areas.
“Twelve-hour [weather model] probabilities suggest low-end potential for spots of 5 inches,” warned the Weather Service. “Recent heavy rainfall … is 300 percent plus of normal the last 7 days.”
Rainfall rates could flirt with two inches per hour.
The flood risk will continue through the overnight hours, making for a very dangerous situation if roadways become inundated. Conditions should improve during the day Tuesday, though dangerous floodwaters could linger.
The Northeast has seen a significant increase in the intensity of the most extreme rain events in recent decades, coinciding with rising temperatures from human-caused climate change. Higher temperatures increase the speed of evaporation and make more water available in the atmosphere for storms to draw from.