Firefighters had to rescue four people from flood water when the two cars they were in got stuck.
Two passengers were given precautionary checks for hypothermia.
Part of the A470 at Dolgellau, Gwynedd, has been shut since the two cars got stuck at about 04:05 GMT. Several roads remain closed due to flooding, according to Traffic Wales.
There is a Met Office yellow warning in place for rain in mid and south Wales on Saturday from 22:00.
It said the rain could be “especially persistent” in areas “currently saturated and highly sensitive to further rainfall”.
Prince Charles met flood victims and their rescuers during a visit to Pontypridd on Friday.
Storm Dennis hit more than 1,000 homes and businesses in Rhondda Cynon Taff and damage to council infrastructure could cost up to £30m, according to council leader Andrew Morgan.
The weather warning is place from Saturday evening until 11:00 on Sunday.
It covers 15 of Wales’ 22 local authority areas at Blaenau Gwent, Bridgend, Caerphilly, Cardiff, Carmarthenshire, Ceredigion, Merthyr Tydfil, Monmouthshire, Neath Port Talbot, Newport, Powys, Rhondda Cynon Taff, Swansea, Torfaen and Vale of Glamorgan.
The Met Office said between 20 to 40mm was likely to fall, with 50 to 60mm in mountain areas.
There are currently six flood warnings in place covering the River Vyrnwy, Severn and the Lower Dee Valley between Llangollen and Wrexham.