This link will show what is really happening live. Click the arrow and it will play a forward prediction for the next few hours.
This link is also useful:>http://www.weather-forecast.com/maps/United-Kingdom?symbols=cities.forecast.weather
NW Scotland is wetter than Wales but the mountainous parts of Wales can be pretty wet. That said, I have been in north Wales and experienced continuous days of hot sunny weather. So hot, I had to jump into the sea to cool down! Anyway, you can normally expect changeable weather. Yesterday, the whole of Wales had warm temperatures (up to 24C) and plenty of sunshine. Today, it is raining continuously through expected to clear up during the afternoon.
The rain crossing Wales (& the rest of Britain) today is frontal rain - from a low pressure area that has come off the Atlantic. During hot weather, thunderstorms can build up over the inland areas and if you see cumulus clouds forming, head for the coast where such storms are less likely. Having said that, the incidence of thunderstorms in Wales is comparatively rare.
Although not in the immediate Dolgellau area, you might like to consider this place if it is raining:> http://www.llechwedd-slate-caverns.co.uk
Also consider Powis Castle - which is 39 miles (1 hour driving time) east - although you would want it fine to see the gardens.https://www.nationaltrust.org.uk/powis-castle-and-garden