I was going off the quote in the OP which comes from the Guardian article which is credited to the Met Office. It dates from 2011 and says:

> A warmer atmosphere can hold more moisture

Nothing in that specific causal claim is dependent on season. If anything the obvious inference is that rains should be harder in summer and less so in winter. If there's a seasonal dependency there'd need to be further elaboration as to why. There might be one but the article doesn't supply it, actually it goes on to say that:

> So far, any impact that climate change may have had generally on regional rainfall cannot be distinguished from natural variations

So they seem to be agreeing with what I wrote above.