Cardiff, like the rest of Wales, has unpredictable weather that can be wet, windy and unforgiving. Rail is possible any time, but peaks seriously in November and December. But you don't go to Wales for the weather.
Rain starts in earnest in October and continues through the winter.
Cardiff is most popular in summer, when days are long and warm and the streets are clogged with British visitors. Cardiff's driest months are from April to July.
To avoid crowds, go in Spring, remembering that many B&B's close between mid-October and Easter.
The climate charts below show the historic, month to month high and low temperatures and average monthly rainfall for Cardiff, Wales.
Sleeperz Hotel Cardiff (value)