The Valid Time Event Code page is a great resource to look back at past warnings issued by each National Weather Service office. The warning was issued 13 minutes prior to the time the damage was reported by the Storm Prediction Center. According to NOAA this 13 minute warning is directly in-line with the average warning time for a tornado. Here's how to make sure you get a weather warning no matter where you are.
NWS Survey of the Tornado
The official report shows the damage path as well as radar images during the event. The timing of the event is unusual but not impossible. The figure to the right shows the average time of occurrence for tornadoes in the Southeast from 1950-2010. Most tornadoes occur between 3 and 4 PM so this storm was not typical.
Cherrystone campground is located along the Eastern Shore of Virgina in the Delmarva Peninsula. Over 1000 people were camping on the morning of July 24th at the campground when the warning was issued. Many campers reported getting warnings on their cell phones prior to the storm hitting. The emergency alerts can be turned on or off through the notification center in the settings on an iPhone. NOAA Weather Radio is also a way to receive alerts no matter where you are in the United States. As long as you are in range of one of the transmitters you will receive alerts during severe weather.