USA spots on 10m WSPR today

Several USA stations spotted my 500mW 10m WSPR beacon today, despite the low sunspot number and only "fair" 10m conditions.

The first to spot me today was KB4SC (6560km) at 1424z and the last was W8QYT (5798km) at 1918z. It was better than I was expecting on 10m this afternoon.

The question is how low can the sunspot number fall and still have decent 10m conditions? I guess there are very many factors to consider. As I've said before, I have worked 11000km+ with QRP SSB at solar minimum on N-S paths. I wonder what WSPR will be capable of?  Remember you only need an opening of 120 seconds for WSPR to be successful and WSPR is 12-14dB better than CW and CW is better than SSB.   If people stick around on 10m we may be in for some real surprises in the coming "quiet years".

UPDATE 2016z:  Well, the opening lasted even longer as W8QYT was still spotting me strongly (-9dB S/N) at 2006z. Often the strongest signals are just before the band dies on a particular path - something to do with the MUF being optimum and lowest attenuation? By the same logic you'd expect optimum propagation just as the MUF rises through 28.124MHz as a path opens. Can't say I've noticed this, but it could account for the early morning spots by 4X1RF?