The Met Office has issued a yellow 'be aware' warning of fog for:
Powys, Monmouthshire, Blaenau Gwent, Newport, Torfaen, Bridgend, Caerphilly, Cardiff, Carmarthenshire, Merthyr Tydfil, Neath Port Talbot, Rhonnda Cynon Taff, Vale of Glamorgan, Ceredigion & Swansea.
The Met Office has issued a warning about fog forming overnight in the east.
Patchy fog will form through Monday evening, perhaps becoming more widespread overnight and into Tuesday morning.
The public should be aware of the potential for difficult driving conditions and some travel disruption.
In some places visibility could fall below 200m.
The fog should quickly clear as cloud, wind and rain spreads eastwards. Some heavy spells of rain are expected in the afternoon. Maximum temperature 8 °C.
Fog is reducing visibility on the A449 between the M4 J24 / A48 (Coldra, Newport) and the A40 (Raglan Interchange, Raglan).
Fog is reducing visibility on the A465 Heads Of The Valleys Road between the A40 Monmouth Road / A4042 (Hardwick Roundabout) and the B4560 Llangynidr Road (Garnlydan / Llangynidr).
Patches of mist and fog are reducing visibility on most of the M4, especially between Cardiff and Bridgend.
Mist and fog patches are reducing visibility on the M4 especially between Cardiff and Bridgend.
There's fog and reduced visibility on the M4 between J36 A4061 / A4063 (Bridgend / Sarn Service Area) and J38 A48 (Margam Off Slip).