Weather Outside Your Window: As floodwaters recede in Northeast, weather to aid Ida cleanup

By Alex Sosnowski, AccuWeather senior meteorologist

Updated Sep. 3, 2021 2:40 PM EDT

Deadly and disastrous flooding inundated the mid-Atlantic and Northeast as Ida triggered excessive rainfall and severe weather, including several tornadoes. The extreme rainfall amounts also led to rises along area rivers, with some places recording historic water levels. However, AccuWeather forecasters say that rivers will crest and begin to recede as cooler and less humid conditions prevail for the beginning of clean-up and damage assessments.

"The zone from Philadelphia to New York City has endured focused flooding rainfall as bad as Floyd (1999) or Agnes (1972), or in some cases worse," AccuWeather Director of Forecasting Operations Dan DePodwin said.

"The chance of rainfall occurring on this scope is estimated in any given year as 1 in 200, 1 in 500 or perhaps greater," DePodwin added, noting that these types of historic rainfall events have been all too common in recent years.

Following two month's worth of rain that fell in less than 24 hours on Wednesday into Thursday and destructive tornadoes that touched down as Tropical Rainstorm Ida moved through the region, an extended period of rain-free and quiet weather is in store for most areas from the central Appalachians to New England. The tranquil weather should last into this weekend.

Even though torrential rain avoided the upper watersheds of the Susquehanna, Delaware and Connecticut rivers, communities along small streams and secondary rivers in the region took a knockout punch from Ida.

The general 4-8 inches of rain and locally higher amounts that fell from southern Pennsylvania to northern New Jersey, southeastern New York state and southern New England dumped 70 million to 1.4 billions of gallons of water per square mile, which was too much for rivers such as the Schuylkill, Conestoga, Raritan, Passaic and Yantic to handle.

These secondary rivers surged to major flood stage late Wednesday and Wednesday night with record levels reported on the Schuylkill River at Philadelphia and Norristown, Pennsylvania, and the Raritan River at Raritan, New Jersey. The Brandywine at Chadd's Ford, Pennsylvania, reached 26.85 feet and shattered the old record of 25.1 feet.

Since the rainstorm occurred over a short duration and it was compact, waters on the small streams were already receding as of Thursday. Waters are forecast to do the same along the secondary rivers into Friday. The majority of streams and rivers in the region are forecast to be below flood stage by this weekend. However, it will take the longest for water to recede where the terrain is flat or close to tidal regions.

AccuWeather forecasters urge people traveling through recently flooded areas to check their route ahead of time, as some roads may still be underwater or damaged in the wake of Ida.

"Unfortunately, the cleanup process may take days, especially in terms of flooded homes and a staggering number of basements in the region," AccuWeather Senior Meteorologist Brett Anderson said.

"But, at least the weather will cooperate as most areas are likely to remain free of rain through Saturday. Temperature and humidity levels will be more typical of mid- to late-September, rather than the dog days of the middle of summer," Anderson said.

For millions in the Northeast, the pattern into this weekend will usher in some of the coolest conditions since early July when rain accompanied a cool spell.

"What a difference 24 hours can make,” AccuWeather Chief Meteorologist Bernie Rayno said, explaining that Ida’s rainfall was enhanced by a dip in the jet stream. Drier weather filtered into much of the region on Thursday as that energy moved away from New England.

“It has been steamy in the Northeast for what seems like weeks,” Rayno added, noting that Mother Nature has flipped the switch to more of an early fall-like feel into the start of the weekend.

In the zone from Philadelphia to New York City, which was slammed with some of the heaviest rain and flooding from Ida, high temperatures are forecast to be in the middle to upper 70s F with nighttime lows ranging from the middle 50s in the suburbs to near 60 in the urban areas.

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"Following the steam bath weather conditions from August, it should feel refreshing into this weekend," AccuWeather Senior Meteorologist Adam Douty said.

"People will be able to turn off the air conditioners and open the windows," Douty added.

The weather will offer nearly ideal conditions for the difficult task of drying out household items, pumping out basements and the hard manual labor that accompanies flood clean-up operations. In some cases, damage to roads and infrastructure may take many days to repair. Some low-lying areas that drain poorly may take an extended period of time for water to recede.

After an area of high pressure from Canada promotes dry and cool conditions into Saturday, with the exception of spotty showers in New England on Friday, forecasters expect the next chance of rain to arrive in the region Sunday into Monday.

"Rainfall from an approaching cold front is not expected to be heavy enough to spark new flooding concerns from later this weekend to early next week, and rivers and streams should be back in their banks by that time," Douty said.

AccuWeather forecasters warn that heavier showers and thunderstorms could return by the middle of next week as another cold front presses through the region. The midweek storm system may also stir up some wind. Since the ground is wet, trees may be more prone to toppling over.

It will be that second front and an associated dip in the jet stream that could play a role in keeping Hurricane Larry out to sea. Alternately, the pattern, depending on the exact setup, may potentially draw the system closer to Atlantic Canada and perhaps the northeastern U.S. later next week. Given the uncertainty in the track beyond early next week, all interests in these zones are being urged to track the system in the coming days.

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