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.
There are some really poor driving conditions in parts of South Wales this morning. ☔️— South Wales Police (@swpolice) September 24, 2019
↔️ Leave a safe and increased distance to the vehicle in front
⏰ Give yourself extra time for your journey
💡Use dipped headlights
🚲🚘🚶♂️ Be considerate of all other road users
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.
⚠️Due to heavy rain flooding the railway between Mountain Ash and Aberdare all lines are blocked.— Trafnidiaeth Cymru Trenau Transport for Wales Rail (@tfwrail) September 24, 2019
Train services running to and from these stations may be cancelled or delayed.
Disruption is expected until 11:00 24/09.
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.
🚌 We're experiencing some delays due to traffic congestion in the city centre and slower traffic due to the weather. Please allow extra time for your journeys and accept our apologies in advance. ☔️— Cardiff bus (@Cardiffbus) September 24, 2019
Find out more about our city centre diversions here - https://t.co/j5q9SZQjpA pic.twitter.com/YEJTR3SlX6
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.
*Road closure*— Neath Port Talbot Council (@NPTCouncil) September 24, 2019
The A4107 between Abergwynfi and Treorchy has been closed. A landslip has blocked the road approximately 1 mile outside Abergwynfi going towards Treorchy.
The road will remain closed for most of the day while Council workers clear the highway. Updates to follow. pic.twitter.com/CnjeppIFGM
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".