The launch today was scrubbed...but the graphics below should be useful whenever it goes up!
NASA Wallops is estimating the probability of a 5PM launch this evening to be a mere 45% due to the cloud cover. Looking out my back window, it appears that the estimate is optimistic. However, it is always possible that the weather at Wallops could clear while it is still overcast here locally. The Washington Post published a couple of graphics that should help you spot it. Here, it should be visible in the southeast at about 10 degrees elevation starting about 3 minutes after launch. I don't know how visible it will be in daylight. The night launch of the Minotaur-1/ORS-1 in June 2011 was visible here but wasn't easy to see. Luckily, the Antares is a much bigger rocket so it should be brighter.
Launch coverage begins at 4PM on NASA TV.