Update from Orbital Sciences:
We have a new T-0 time for the #Antares launch of 10:58 a.m. EDT #COTSDemo #Cygnus
— Orbital Sciences (@OrbitalSciences) September 18, 2013

Tomorrow, September 18 2013, Orbital Sciences Corp are launching their Cygnus cargo spacecraft atop an Antares launch vehicle to resupply the International Space Station from Wallops Island, VA. The launch window is from 11:16 to 11:31 a.m. 10:50 to 11:05 am Eastern Daylight Time. If the weather is clear, that should mean a good sighting opportunity for those of us in the mid-Atlantic region.
Unlike the LADEE launch earlier this month, however, Antares/Cygnus is going to be harder to spot for a few reasons.
For one, it’s a daytime launch. Nighttime launches are always going to be easier to spot from a distance.
Second, it’s launching to the South East, “away” from those of us in Virginia, Maryland, & Philly. If you’re in NJ, NY, etc., it will seem to go toward the East more. Here’s an expected view from my home in the Westminster, MD area:

My friends & family in the Philly area will have a somewhat better view as they’re further east and will see the trajectory arc a little more eastward.

Third, the first stage is a liquid-fueled rocket. Liquid-fueled rockets burn “clean” in that they do not have a long fiery and smokey exhaust like solid-fueled rockets do. If the vehicle can be seen at all during the first stage, it will look like a small bright dot with a small puff of smoke behind it. Depending on the atmospheric conditions, it may leave a contrail. (Antares’ second stage is solid-fueled so it will be much more visible when it lights but the rocket will be further away by that point.)
So, it won’t be quite as easy to spot as LADEE’s launch earlier this month, but it’s certainly worth a shot. Good luck!
Update
When it comes to viewing rocket launches from a distance, timing is everything. You can watch the countdown on your smart phone which will let you know if there are any delays. But bear in mind that the feed to your phone is about a minute (or more) in the past, so be sure to start looking toward the horizon around 30-50 seconds before “launch.”