Homes and businesses are likely to face flooding and some communities could be “cut off” as heavy rain is expected to hit Wales.
The Met Office issued an amber rain warning across most of Wales from midday on Saturday to 06:00 BST on Sunday.
It said fast-flowing or deep flood-water could cause “danger to life”.
Many places will see 1-2in (25-50mm) of rainfall, with totals of 2.5-3.5in (70-90mm) expected on higher ground.
More than 4.5in (120mm) is expected in some of the most exposed high ground of Snowdonia.
The warning, which also covers large parts of south-west England, comes after a day of downpours on Friday, as Storm Alex moved in from France.
The warning covers Blaenau Gwent, Caerphilly, Carmarthenshire, Ceredigion, Conwy, Denbighshire, Flintshire, Gwynedd, Merthyr Tydfil, Monmouthshire, Neath Port Talbot, Powys, Rhondda Cynon Taf, Torfaen and Wrexham.
The Met Office warned that delays and cancellations to train and bus services were likely and conditions would make driving difficult.
“The unusual wind direction associated with the rainfall will mean that the peak rainfall totals are likely to occur in some areas that are usually well sheltered and drier during unsettled spells of weather,” its forecast warned.