The yellow warning for rain is in place from 00:00 on Friday until 09:00 and covers Blaenau Gwent, Bridgend, Caerphilly, Cardiff, Carmarthenshire, Ceredigion, Conwy, Gwynedd, Merthyr Tydfil, Monmouthshire, Neath Port Talbot, Newport, Pembrokeshire, Powys, Rhondda Cynon Taf, Swansea, Torfaen and Vale of Glamorgan.
Storm Éowyn is the fifth named storm of the 2024-25 season, which began in October last year.
“There is a chance that damage to buildings and homes could occur, with roofs blown off and power lines brought down,” a Met Office spokesman said.
“Road, rail, air and ferry services are likely to be affected, with longer journey times and cancellations possible.”
