I was thinking that this seems reasonable enough:
but then Mat Collins tweeted that it will "will also be used for making more reliable climate projections" which is a bit of a hostage to fortune. (Though technically, it may take so long to check this out that we never know if he is right or not).
Way back in the mists of time senior scientists at FRSGC (as it then was) imagined that their new superduper Earth Simulator would solve all the problems of climate change...
As for weather prediction, it's easy enough to demonstrate experimentally that higher resolution has the potential to improve performance (even though computer power is far from the only influential factor). So I expect they can claim quite confidently that the bigger computer will improve this.