UPDATE 2 1146PM ET: The line had 78 MPH winds as it came through northern Maryland and did quite a bit of tree/power line damage, but it quickly lost its punch and is no longer severe.
UPDATE 1 1000PM ET: The line is coming into Maryland and Amish country in Pennsylvania with 60 MPH winds.
A line of severe thunderstorms (not a derecho) is moving through Pennsylvania right now with winds of 60-70 MPH possible along its leading edge, prompting severe thunderstorm watches and warnings to be issued for the I-95 corridor between Washington DC and Philadelphia.
Here's the new watch, in effect until the storms pass through later this evening:
Here's the line of storms as of around 915PM EDT as it moves into southeastern Pennsylvania. Provided the line is able to sustain itself on what's left of the instability in the I-95 corridor, it should stay severe as it impacts the metro areas.
The storms have a pretty good chance of staying together, I think, so long as the cold pool doesn't jog too far ahead of the storms. If that happens, it's game over and the line dies. That's always a possibility. However, there's plenty of moisture in the area, as shown by the dewpoint map as of 900PM EDT:
...and the temperatures are still pretty warm...
...and the most unstable convective available potential energy (CAPE; essentially, the fuel storms use to generate updrafts and get strong) is between 2000 and 3000 joules per kilogram, which is more than enough to support thunderstorm development...
...so it's very possible that the line of storms will maintain itself or even strengthen as it moves into this soupy tinder box that is the I-95 corridor.
If you live in the area or know anyone who does, please let them know to prepare for a potent line of storms.
National Weather Service Main Page
National Weather Service -- Washington DC/Baltimore
National Weather Service -- Philadelphia/New Jersey
National Weather Service -- Central Pennsylvania
National Weather Service -- New York City area
Storm Prediction Center Main Page
Storm Prediction Center -- Current Severe Weather Watches
Storm Prediction Center -- Convective (Severe Weather) Outlooks
Storm Prediction Center -- Mesoscale Discussions
Storm Prediction Center -- Storm Reports
Storm Prediction Center -- Mesoscale Analysis Pages
Wunderground's Detailed Radar (click the + nearest to you to see your local radar)
TwisterData.com's excellent GFS/NAM/RAP model website.