The swollen Susquehanna River is slowly retreating to its banks in both Pennsylvania and New York after recording some of the highest floodwaters ever seen in the northeast.

Tens of thousands of people have been displaced within the affected areas because the river forced officials to evacuate them from their homes.

Tropical Storm Lee dumped heavy rain on an already soggy U.S. northeast on Thursday leading the river to break high-water records set nearly 40 years ago in the aftermath of Hurricane Agnes.

The river crested at nearly 42.7 feet on Thursday night in Wilkes-Barre, Pa., which officials said is beyond the design capacity of the region's levee system. That was also higher than the record set during Agnes in 1972.

More than a dozen death have been blamed on Lee's and its remnants: six in Pennsylvania, three in Virginia, one in Maryland, and four killed when it came ashore on the Gulf Coast last week.

U.S. President Barack Obama has declared states of emergency in Pennsylvania and New York. This will ensure federal aid is directed to the areas affected.

The flooding came about a week after Hurricane Irene drenched the East Coast.

Forecasters with The Weather Channel have said that there is only one word to describe Tropical Storm Lee and its moisture - stubborn.

From the very beginning of its development as a depression, its stubborness was on display, Tim Ballisty, editorial meteorologist, at The Weather Channel explained in a post. Forming in the central Gulf of Mexico on September 1 and intensifying into a tropical storm the following day, Lee was content on staying put.

Lee swirled in the north-central Gulf for days and was stationary, then it meandered, then stalled, then made a slight movement, then stalled again, meandered some more, and finally made a push onshore over Louisiana on Sept. 4, Ballisty said.

All the while, the cylcone was dumping rain over Louisiana, Mississipi, Alabama and Florida, he noted. But the story only begins there. It's what happened after its push onshore over Louisiana that has made Lee notorious.

He said the same meteorological factor that helped in finally pulling Lee onshore and into the interior southeast U.S. was the same mechanism that made Lee a historic rain maker over the Mid-Atlantic and Northeast.

Here's his explanation:

An upper level trough resided over the eastern third of the United States on Monday, September 5. Just ahead of the trough at the surface, a slow-moving cold front was making its way across the East producing areas of heavy rain and the occasional thunderstorm. Even without Lee, the East Coast was on its way to getting wet during the first week of September.

We know about Lee and its stubborn tendencies but this upper trough was just plain ol' stuck. The forward movement of upper troughs are different from one trough to the next. Is the weather pattern progressive or is the atmosphere blocked like a car in a traffic jam?  This upper trough was in a nasty Los Angeles traffic jam.

Enter Lee.

Lee and its moisture was pulled northward by the trough and without getting into the complexities, the upper trough along with Lee's energy and moisture became a unified rainmaker over the Mid-Atlantic and Northeast - a rainmaker with a rich tropical connection.

To make a comparison, the upper trough capturing Lee and its moisture is like a stock car receiving four new tires and a full tank of high-ocatane gas - rejuvenated, renewed stamina and efficient.

So now we are left with a prolific rainmaker that isn't moving anywhere. On top of that, soil across the two regions were already or nearly saturated. All in all - a bad combination that leads to extensive flooding.