This is a movie loop of the past 6 hours.
All times are Eastern.
This image is an infared picture, unlike visible imagery which uses reflected sunlight. The satellites measure the temperature of the clouds and then generates the imagery. The brighter the white on the imagery, the colder the clouds tops and higher the the thunderstorms are. Because the satellite is measuring the temperature of clouds, you can get a picture 24 hours a day. The satellite gives us a picture every 30 minutes.