It is warning of the risk, nothing else, to be fair. They are a governmental organisation with a duty to portray the risks.
Kent's post above is the best analysis of the situation so far, but suffice to say, things are ever changing and as I type, it is currently snowing in quite a bit of our region, albeit patchy and light in nature.
Please refrain from the MetO slagging off type posts because one or two of the netweather members on here are affiliated with them.
Plymouth remains to be in area of risk and I suspect that you will see a little snow over the coming hours.