Actually, the closure at about 0630 on Sat was caused by a fresh snowfall starting to accumulate on the runway. Although it had been treated, the surface contamination started to increase to the point that braking action was reported as medium/poor. With the snow still falling the airport authority had absolutely no option at this point but to close and sweep; whilst completing this they also re-treated with antiice. Before the runway reopened, the cloud dropped to 200' hence low visibility procedures commenced, which dropped the movement rate further once the runway became available, hence the large backlog of delays for departures (getting the inbounds out of the holds being the priority). Once all the runway lead-offs and all the taxiways were clear and treated, the other runway and its taxiways could be attended to; once they opened then the departure rate upped nicely.