Wales weather: Flooding blocks roads and railway lines
Trains and buses have experienced delays and drivers warned to be careful after heavy rain.
The Met Office issued a warning for heavy rain on Tuesday across south and north east Wales, Powys and Wrexham.
It said up to 30mm (1.2in) of rain was expected to fall quite widely, with the potential for up to 70mm (2.8in) in a few locations.
Flood alerts are in place and a landslip has blocked a road in Neath Port Talbot.
South Wales Police said conditions on south Wales' roads were "really poor" on Tuesday morning.
In a tweet, the force urged people to leave a safe and increased distance to the vehicle in front, allow extra time, use dipped headlights and be considerate of other road users.
Traffic Wales said the railway between Mountain Ash and Aberdare had been blocked due to flooding, with trains cancelled or delayed and disruption for most of Tuesday morning.
A car became stuck in flooding under Aberdare railway bridge, with the road blocked.
Cardiff Bus apologised for delays to its services and urged people to allow extra time for their journeys, and there were queues on parts of the M4.
In Neath Port Talbot, the A4107 was shut in both directions after becoming blocked by a landslip between Commercial Street in Abergwynfi and A4061 Bwlch-Y-Clawdd Road in Cwmparc.
Neath Port Talbot council tweeted that the road would remain shut for most of the day while it was cleared by council workers.
There were flood alerts in place in Wales, including at River Neath and River Ely.
Forecasters said the downpour was brought by a burst of low pressure travelling across the UK, along with warm and humid air linked to the remnants of Hurricane Humberto which hit the Bermuda coastline last week.
Forecaster Mark Wilson warned of "thunder, lightning and gusty winds" through the affected areas.
"It's very likely there will be issues with surface water causing flooding," he said.
A "brief respite" in some parts was expected in the early afternoon, Mr Wilson said, adding "more rain is predicted for the evening".