The Met Office has issued a yellow alert, warning of isolated heavy thunderstorms, accompanied by hail.
These it said are very likely to to affect mainly mountainous, inland, southeastern and eastern coastal areas.
The precipitation rate may exceed 35 millimetres per hour, it added.
The alert is in force from 12 noon to 6 pm on Saturday.
Meanwhile, its early morning weather bulletin the Met Office said that low pressure and an unstable air mass are affecting the area.
Today will be partly cloudy and at intervals cloudy with scattered showers or thunderstorms which are expected to accompanied by hail.
Temperatures will rise to 21 C inland and on the coasts and around 12 C in the mountains.
Tonight will become mainly fine although in the west and north there may be local low cloud.
Temperatures will fall to 11 C inland and on the south and east coasts, around 14 C on the west coasts, around 12 C on the north coasts and 4 C in the mountains.
Sunday will be mainly fine but clouds that develop in the afternoon may lead to isolated showers in the mountains. In the evening there will be increased high cloud, and locally there may be patchy fog.
On Monday, there will be increased high cloud but will gradually become mainly fine.
But clouds that develop at midday and in the afternoon inland and in the mountains, may lead to scattered showers or thunderstorms.
Temperatures will edge up and this will be particularly noticeable inland and in the mountains, to close to the seasonal average.
Tuesday will be mainly fine, but clouds at midday and in the afternoon may lead to scattered showers with temperatures expected to edge up further.