New York City, Philadelphia are at the center of a serious Northeast flash flood threat

By Meteorologists Chris Dolce, Dakota Smith and Briana Waxman

(CNN) — The Northeast — including New York City and Philadelphia — is at risk of significant and life-threatening flash flooding through Tuesday night from an unusually strong storm. It could drop more than a month’s worth of rain in parts of the region.

More than 36 million people are under flood watches across the region, from Philadelphia to the New York metropolitan area and southern and western New England.

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Widespread rainfall totals of 2 to 5 inches are expected, and isolated spots could see higher amounts, especially from northern New Jersey and northeastern Pennsylvania to the Lower Hudson Valley to western New England. The higher end of that range is more than New York City’s average July rainfall of 4.60 inches.

Storms have been active over parts of New York, New Jersey and Pennsylvania since Tuesday afternoon. The worst of the flood threat closer to the Interstate 95 corridor started to ramp up Tuesday evening. The most dangerous threat will continue to unfold overnight into Wednesday morning.

A considerable flash flood warning, the second highest level of warning, was issued for Baltimore late Tuesday afternoon, where 2 to 3 inches of rain fell in a short amount of time.

Flooding was already being reported in northern Pennsylvania late Tuesday morning. In Bradford Township, just a few miles south of the Pennsylvania-New York border, police said “multiple roadways” were flooded, including parts of a major state highway, and asked residents to “avoid all unnecessary travel.”

The Rescue Hose & Ladder Co. fire department said it responded to Coalport in central Pennsylvania, where crews rescued three people and two dogs from a residence.

In Cumberland County, Pennsylvania, video shows floodwater slowly rising through city streets on Tuesday as heavy rain continues to pour down.

Also on Tuesday, in Westover, Pennsylvania, swiftly moving floodwater swept through a resident’s backyard, inching closer to the home’s raised balcony.

The big concern is that rain could fall at torrential rates of 1 to 3 inches per hour over urban areas and quickly overwhelm stormwater infrastructure. New York City can handle rates up to 1.75 inches per hour, but that threshold is getting surpassed more often. Less than two weeks ago, water rushed into subway stations and stalled cars on city roads when several inches fell in a short period.

Extreme rainfall rates are becoming more common as planet-warming pollution pushes temperatures higher because warmer air holds more moisture. Weather systems can then wring that moisture out like a water-laden sponge, yielding heavy and often highly localized downpours.

Monday brought an ominous preview in northern New Jersey when isolated storms fired up ahead of Tuesday’s system. A slow-moving thunderstorm dumped over 3 inches of rain near Glen Rock and Ridgewood, according to radar estimates, flooding highways and streets across parts of Bergen and Passaic counties.

In Orange, New Jersey, video shows several stranded cars mostly submerged in muddy floodwater.

Video from Glen Rock showed a BMW nearly submerged. Two people trapped inside escaped before police arrived, but the water was initially too deep for a tow truck to reach the vehicle. Numerous water rescues were reported around Glen Rock, Hawthorne and Fair Lawn, New Jersey.

The storm’s strength on Wednesday, when it slows down and intensifies, will be more on par with a system in colder months of the year — not late July. Storms like this are usually weaker in summer because the jet stream doesn’t add as much punch to intensify them.

At the same time, the system is tapping into souped-up, moisture-rich pipelines of Atlantic and Gulf air. The amount of water vapor in this air mass will reach levels well above average for even midsummer’s humid reputation.

Those extreme opposites will clash and cause multiple rounds of heavy rain and thunderstorms to flourish over the Northeast.

Overnight threat

The most drenching storms for the New York metro area to Philadelphia will continue overnight. Some cities will see multiple rounds, but it’s difficult to pinpoint the timing for each bout.

Any places where bands of rain slow down or stall out for multiple hours will see the most serious flooding impacts. Travel impacts have been piling at airports and on roads throughout the region.

More than 2,000 flights have been canceled at airports across the US, according to FlightAware. New York City’s airports are the most impacted, along with Newark Liberty International Airport in New Jersey. Thunderstorms forced ground stops at airports across major cities, including New York, Newark, Washington, DC, and Baltimore.

All American Airlines flights, which were temporarily grounded for less than an hour Tuesday evening due to an IT problem affecting the carrier, have resumed, the airline said.

It was not immediately clear what caused the outage, but the airline said it issued the groundstop while teams worked to resolve the issue.

Flash flooding is one of the deadliest weather hazards and is particularly dangerous in urban settings. Dozens were killed nearly five years ago in the Northeast when Hurricane Ida dumped 7 to 10 inches across a similar corridor as this week’s threat.

Severe thunderstorms are also a threat on Tuesday from around Philadelphia and Washington, DC, to the Carolinas. Wind damage is the primary concern from any storms that intensify, but isolated tornadoes are still possible.

The most expansive flash flood threat will shift north on Wednesday into southern and central New England, including Boston and Hartford, but New York City will remain at risk too.

Northern and central Connecticut to the western half of Massachusetts and southern Vermont are most at risk for serious flooding Wednesday.

Downpours will likely be ongoing across this area as the day begins. Flash flooding could unfold as one or more narrow bands of heavy rain spread northeastward through the afternoon. New storms could form behind this area as well.

Pockets of rainfall could persist in parts of the Northeast on Thursday, but the concern for additional flooding is low. The storm will weaken and push out of the Northeast on Friday, but a few scattered showers and storms could still billow up over the region.

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CNN’s Amanda Jackson, Alea Motwane and Aaron Cooper contributed to this report.