(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 35 million people are under flood watches across the region, from Baltimore and Philadelphia to the New York metropolitan area and southwest New England.
Widespread rainfall totals of 3 to 5 inches are expected, and isolated spots could see up to 7 inches, especially from northern New Jersey and northeastern Pennsylvania to the Lower Hudson Valley and southwestern Connecticut. The higher end of that range is more than New York City’s average July rainfall of 4.60 inches.
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 major state highway, and asked residents to “avoid all unnecessary travel.”
Storms were ongoing over parts of New York and Pennsylvania as of early afternoon. The worst of the flood threat closer to the Interstate 95 corridor is expected to ramp up by mid-to-late afternoon. The most dangerous threat will continue to unfold in the evening and overnight into Wednesday morning.
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.
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.
The forecast
The most drenching storms for the New York metro area to Philadelphia will likely get underway late Tuesday afternoon and 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 delays are likely at airports and on roads throughout the region. Hundreds of flights have been cancelled Tuesday at major airports in the region, according to FlightAware.
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 a bit farther north on Wednesday into southern and central New England, including Boston and Hartford, but New York City will also remain at risk.
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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