Motorists on Britain's roads were being urged to take care today following overnight downpours.
A Met Office spokesman said a spell of heavy and persistent rain is likely to cross parts of southern, southwest and central England along with southeast Wales during the first half of Wednesday.
The rain should gradually ease from the west this morning, but the Met Office said the public should be be aware of the possibility of some issues with surface water and localised flooding on Thursday and Friday after parts of South Wales saw 20.2 mm of rainfall between midnight and 8am this morning.
A second band of persistent rainfall is forecast to affect most parts of Wales from the early hours of Thursday morning into the evening.
Environment Agency Wales is urging people to remain vigilant as localised intense downpours could lead to surface water flooding, which could be made worse by leaves and other debris blocking drains.
Strong onshore winds are also likely to cause rough conditions along the south and west Wales coastline, which could also lead to some coastal flood alerts being issued.
People are advised to take extra care on the roads tomorrow as driving conditions are likely to be particularly hazardous.
Agency officers are working today and tonight to clear any blockages in watercourses and check flood defences.
They will be monitoring conditions and river levels closely over the next 48 hours.
The Met Office is advising the public to be prepared for disruption to transport today.
There are ongoing delays to Gower bus services 115, 117 and 118 at Llanddewi due to flooding and congested lanes.
The AA said the M5 corridor – from Gloucester to Plymouth – is bearing brunt of flood-related incidents.
Darron Burness, the AA’s Head of Special Operations, says: “It’s been very bad today in the Midlands and South-west with the weather causing extremely challenging driving conditions.
"Unfortunately, the forecast for tomorrow is even worse with heavy rain forecast across a wider area – particularly South West, South Wales and Scotland – so drivers need to plan accordingly and keep tuned to local traffic and weather reports.
“It’s important to keep your speed down and maintain a bigger gap between you and the vehicle in front. If you hit water and the steering becomes unresponsive, don’t hit the brakes – just ease off the accelerator and slow down gradually till the tyres regain their grip.
“Also, stay out of flood water as it’s very difficult to gauge how deep it is and it often contains unseen hazards like dislodged manhole covers. If conditions are bad, heed any police advice about whether it’s safe to travel and accept that you may need to make alternative arrangements.”
The Environment Agency this morning issued 50 flood alerts for the South West of England, and the warning has been upgraded to Amber for parts of Devon and the West Country where the greatest impacts are expected from the heavy rain.