AccuWeather.com

By Bill Deger, Meteorologist

Sep 29, 2011; 6:50 PM ET

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A strong thunderstorm rolling in to New York City (shared by LaceyxLynch early Thursday afternoon).

The stubborn storm system that has plagued the weather across the Midwest and mid-Atlantic over the past several days has shifted its focus to the Northeast this afternoon, heightening the risk for flash flooding and severe weather.

Bands of showers and thunderstorms, many containing heavy rain, will continue to sweep across the Northeast into this evening, specifically areas along the I-95 corridor from Philadelphia north through New England and west to the Hudson Valley of upstate New York.

While many areas will experience prolonged dry periods in between showers and storms as explained earlier by Expert Senior Meteorologist Alex Sosnowski, rain in heavier storms will quickly cause issues with flooding.

On Thursday, persistent heavy rains led streams and rivers over their banks across northeastern Pennsylvania and the Catskills region of New York.

The mighty Susquehanna River flooded again from near Conklin, N.Y., south to the Scranton-Wilkes Barre, Pa., area, while flooding along smaller creeks forced evacuations.

More than 3 inches of rain are again possible through tonight over a short amount of time in spots across the Northeast.

AccuWeather.com meteorologists are most concerned about the flash flood risk that exists across the New York City metro area north through western Connecticut, the Berkshires of Massachusetts and New York's Hudson Valley as steamy air is funneled in directly from the Atlantic Ocean.

Newark, N.J., Albany, N.Y., Waterbury, Conn., and Pittsfield, Mass., lie in this enhanced threat zone. Any additional rain will only exacerbate preexisting flooding across parts of northwestern New Jersey, northern Pennsylvania and southern New York today as well.

Those with long commutes or the need to drive long distances across the region the rest of the day will quickly notice the localized nature of the storms and flooding, with long stretches of dry weather followed by blinding downpours.

Never, ever drive through flooded roadways. It is often hard to judge the depth of the water, which could put your life in risk if your vehicle is swept away.

River flooding will continue to be a concern, especially in areas that have been soaked by record rains in recent weeks and months.

This graphic created by the National Weather Service Flash Flood Guidance illustrates that an inch of rain or less per hour is all that is needed to cause flash flooding in areas shaded in red, dark red or purple. Across most of the areas threatened today, at least 2 or 3 inches of rain (green or blue) will be needed for flooding.

As NOAA's Northeast River Forecast Center projects, it will likely take more rain to cause significant rises on rivers over this part of the region as compared to water-logged northeastern Pennsylvania and southern New York. Because of this, the flood threat will be more pronounced along smaller creeks and streams.

Another aspect of this storm system will continue to be the threat for isolated severe thunderstorms. The same region highlighted for an elevated flash flooding risk above faces the prospect of a damaging thunderstorm or two, especially this afternoon and evening as the atmosphere warms.

While dousing communities Thursday, strong storms produced isolated wind damage over southeastern Pennsylvania into southern New England. Similar storms into this evening could generate brief wind gusts above 50 mph, enough to bring down trees and power lines.

Another area of low pressure swooping down from Canada with plenty of unseasonably cool air will keep clouds and the threat for showers across the Northeast into the weekend. In fact, parts of the mid-Atlantic could see their first snowflakes of the season.