6pm Sunday – 11am Monday
The Met Office warns that visibility could fall to around 50 metres or less in some places, potentially disrupting road, bus, rail and air travel.
The warning covers Cardiff, Newport, Rhondda Cynon Taf, Caerphilly, Merthyr Tydfil, Blaenau Gwent, Torfaen, Monmouthshire, the Vale of Glamorgan, Powys, Conwy, Denbighshire, Flintshire, Gwynedd and Wrexham.
Fog is expected to lift gradually during the morning, although some inland areas may remain affected for longer.
Why it matters: Reduced visibility could affect school transport, commuters and public transport reliability. The warning indicates a risk of disruption, not confirmed widespread cancellations.
