A weather warning for thunderstorms is in place for parts of north, south and west Wales overnight.
The yellow warning came into force at 20:00 BST on Sunday and is set to last until 04:00 on Monday.
The Met Office warned up to 30mm (1.2in) of rain could fall in less than an hour.
It warned some areas could also see hail, gusty winds and "frequent lightning" which could cause damage to buildings.
It also said the heavy rain could lead to flooding and cause power cuts, as well as delays to train services.
The 11 local authority areas covered by the warning are Bridgend, Carmarthenshire, Ceredigion, Conwy, Gwynedd, Anglesey, Neath Port Talbot, Pembrokeshire, Powys, Swansea and Vale of Glamorgan.