Forecasters say up to 4cm of rain could fall across Wales in the hours to 4pm.
They say there's a risk of surface water flooding.
The Met Office has issued a heavy rain warning for:
Carmarthenshire, Merthyr Tydfil, Monmouthshire, Neath Port Talbot, Powys, Rhonnda Cynon Taff, Swansea, Blaenau Gwent, Bridgend, Caerphilly, Torfaen, Cardiff, Newport, Vale of Glamorgan, Ceredigion, Pembrokeshire, Wrexham, Denbighshire and Gwynedd
The Met Office says surface water flooding is likely on stretches of the road network, and in places strong winds will lead to considerable spray.
There are hazardous driving conditions because of rain on the M4 in both directions between J22 M49 and J30 A4232 (Cardiff Gate).
The Met Office has issued a yellow "be aware" warning of rain for South and West Wales tonight.
The Environment Agency now has 28 flood alerts (down from 33) in force across Wales.
The Environment Agency has issued 25 flood alerts for Wales from Gwynedd to the Vale of Glamorgan.
It says an alert means flooding is possible. Be prepared.
There are hazardous driving conditions because of rain on the M4 Second Severn Crossing in both directions between M4 J22 / M49 and J30 A4232 (Cardiff Gate).
Environment Agency Wales has issued six flood alerts for Wales. People in areas at risk of flooding are warned to be prepared.
- Rivers Bran and Gwydderig at Llandovery
- Rivers in the Ewenny and Vale of Glamorgan West catchments
- Rivers in the Llynfi and Ogmore catchments
- Rivers in the Neath catchment
- Upper Teme upstream and including Knighton
- Afon Vyrnwy, Afon Tanat and Afon Cain & their tributaries