Environment Agency Wales has issued flood alerts for Bridgend, Vale of Glamorgan, Rhondda Cynon Taf, Carmarthenshire and Pembrokeshire.
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Showers in the north and east will merge into longer spells of rain for a time.
The Environment Agency have issued two Flood Warnings and eight Flood Alerts across Wales.
The four-day event in Portmeirion has become one of the UK's best regarded small festivals.