The following link gives a good (though by no means infallible) weather forecast for Wales:http://www.bbc.co.uk/weather/ukweather/wales/
If you click on the individual places on the map (for example Swansea or Merthyr) you can then look at a five-day or 24 hour forecast for that particular place. Personally, I have found the 24 hour forecast to be surprisingly accurate.
It is also worth bearing in mind that, generally speaking, more rain tends to fall in the west than the east. Hence it may well be a good deal wetter in the Swansea Valley than over at Llangattock even though rain is forecast for both areas, although this may not always be the case.
As with all forecasts bear in mind that it is precisely that, a forecast. Other factors to take into account when planning a trip into a flood liable system include the amount of rain which may already have fallen locally in the preceeding few days and how saturated the ground may or may not be to begin with. Always seek local advice if it is available. If in doubt I would suggest erring on the side of caution - the cave will still be there another day.
Hoping this is of some assistance to anyone thinking of caving in South Wales.