The Met Office has issued a yellow weather warning for thunderstorms across the area.
The warning, which covers Pembrokeshire, Carmarthenshire, and Ceredigion, is in place from 5am until 10pm on Thursday, August 27.
Forecasters predict a broad band of thunderstorms and heavy rain moving north across Wales throughout the day.
Rainfall totals of 30mm to 40mm are expected in many places, with some areas potentially recording more than 50mm in a relatively short period.
Some storms may become slow-moving during the afternoon and evening.
Heavy rain is expected in Fishguard until at least 4pm; and Cardigan until 5pm; while it's set to brighten up slightly earlier in Tenby and Carmarthen.
The Met Office has warned of potential flooding, which could lead to difficult travel conditions, road closures, and disruption to public transport, as a yellow weather warning for thunderstorms comes into effect.
Delays and cancellations to bus and train services are possible where flooding or lightning strikes occur.
There is also a chance that some communities could become isolated if roads flood.
Homes and businesses may be affected by rapidly developing flash flooding, while power cuts and disruption to other services are also possible.
Lightning strikes and hail measuring between 1cm and 2cm are also listed among the potential hazards.
Residents are being urged to consider whether their property is vulnerable to flash flooding and to prepare accordingly.
The Met Office advises people to check travel conditions before setting out and to make sure they have essential items available in case of any power outages.
The Met Office is urging people to keep up to date with the latest forecast throughout the day as conditions develop.
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