Flooding and tornado threat: Debby threatens Northeast US including I-95 zone from DC to NYC, Boston

Dangers from flash urban flooding, severe thunderstorms packing strong wind gusts and fast-moving tornadoes are possible from Interstate 95 to coastal communities into the end of the week.

While torrential rain will lead to small stream and river flooding in the Appalachians of the Northeast, Debby's eastern flank will bring dangers from flash urban flooding to severe thunderstorms, a few tornadoes and storm surge in the zone from Interstate 95 to the coastal communities into the end of the week, AccuWeather meteorologists warn.

A tropical system that tracks well inland undergoes a transition where the steadiest rain tends to fall near and just north and west of the storm track, AccuWeather Senior Meteorologist John Feerick explained.

"Places such as Philadelphia, New York City and Trenton, New Jersey, may have actually received more rain Tuesday night than they will from Debby into Friday night," Feerick said, "So with Debby taking a path generally up the spine of the central Appalachians then toward the St. Lawrence Valley later on, that steady rain corridor will avoid much of the I-95 zone."

However, although Debby will likely to become a tropical rainstorm before reaching the interior Northeast, it will continue to cause major problems. AccuWeather refers to former tropical systems that continue to cause risk to lives and property as a tropical rainstorm to continue to raise public awareness of a potentially dangerous situation.

The AccuWeather RealImpact Scale™ for Hurricanes in the U.S. is a 3, and for Canada is a 1.

I-95 zone still at risk for flooding problems from Debby

Despite not being in the steady rain zone, bands of showers and thunderstorms farther to the east, originating from the Atlantic Ocean will continue to feed Debby and interact in coastal areas.

These bands will be troublesome for the I-95 metro areas, including Washington, D.C.; Baltimore; Philadelphia; New York City; Boston; Hartford, Connecticut; Providence, Rhode Island; Portland, Maine; Atlantic City, New Jersey; and Richmond and Norfolk, Virginia. Intense downpours lasting minutes to hours will occur within the bands. The nearby northern and western suburbs of the major cities will experience similar problems.

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"Where the downpours persist, the risk of flash flooding in urban areas and along small streams will increase substantially," Feerick said.

Because torrential downpours earlier this week dumped debris on some areas, rivers may also rise quickly and significantly from southeastern Pennsylvania to New Jersey, southeastern New York and southeastern New England.

Motorists should be prepared to seek an alternate route, as underpasses and other streets and highways that drain poorly can fill with water quickly. Even though the water may appear to be shallow, it may still be rising rapidly or may have washed away the road beneath. This is why experts urge motorists never to attempt to drive through flooded areas.

Debby poses a tornado threat from I-81 corridor to Atlantic coast in Northeast

Along with torrential downpours, severe thunderstorms packing strong wind gusts are likely. Because Debby has shown a history of triggering tornadoes on its eastern flank, AccuWeather meteorologists believe that trend will continue throughout its duration in the United States.

In many cases, the tornadoes will be fast-moving, perhaps exceeding 40 mph, so there will be little time for people to react.

Another concern is that some of the tornadoes will occur during heavy rain or at night, when they may be hard to spot in advance.

People are urged to closely monitor severe weather bulletins and take immediate action if a tornado warning is issued for their location. In some cases with Debby, a tornado may occur without advanced warning, so people should remain aware of their surroundings.

Outside of tornadoes, wind gusts in some of the bands of showers and thunderstorms can be strong enough to knock down trees, cause minor property damage and trigger sporadic power outages. Wind gusts of tropical-storm-force can occur from I-81 to the Atlantic coast.

The increasingly wet soil in the region will have the effect of making trees top-heavy, so even moderate gusty winds could bring some trees down.

Storm surge, coastal flooding are threats from Debby in Northeast

Another serious concern from Debby will be the coastal flood threat.

"A southerly wind initially will cause water to pile through the Chesapeake Bay with no way to escape, meaning water rises of over 3 feet above the routine tides around Baltimore and Annapolis, Maryland," Feerick said, "There will also be coastal flooding during times of high tide along the Delaware Bay and the tidal portion of the Delaware River into the Philadelphia area. New York City Harbor, the East River and the tidal portion of the Hudson River will also to have some of the same problems, as well."

Some minor coastal flooding can be expected in southeastern Virginia, eastern Maryland, Delaware, New Jersey, Long Island, Connecticut, Rhode Island, Massachusetts and New Hampshire. The worst conditions will occur around high tide.

Some access roads and streets in low-lying coastal communities may be submerged.

Weather to improve this weekend

Conditions will improve as the weekend progresses, as Debby speeds away across Atlantic Canada.

A sweep of dry and cool air is forecast to filter in from the Midwest.

However, some of the water that was pushed into the back bays may flood the western side of the bays for a time. River flooding set into motion by torrential rain will continue. Some rivers may not crest until next week.

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