Right, the GFS model runs 4 times a day. They are the 00z, 06z, 12z and 18z which start to be released at 3.30am, 9.30am, 3.30pm and 9.30pm respectively. The 18z is known as the "pub run" because of the time it is released and it's tendancy to show some insanely extreme charts on occasion. Hope that helps