I have used Northwood quite a bit lately. The available schedule is not 100% ,but close enough to use (depending on your equipment). I was mostly interested in the srfc analysis and fcsts.
The transmissions typically started a few minutes early, and I never found the 120 h fcst.
I never got around to it, but I suspect that the answer to your question is to (try to) have look at the 0100/1300 transmission