Roads have been closed due to flooding as most of Wales is told to expect heavy rain until Sunday.
The A4086 at Nant Peris and the A498 between Beddgelert and Pen y Gwryd, both in Gwynedd, have been flooded.
Flooding has also closed the B5106 between Llanrwst and Gwydir Castle. A 30mph speed limit is in place on the A55 Britannia Bridge due to high winds.
The mountains of north-west Wales could see 3.9in (100mm) to 5.9in (150mm) of rain fall, according to the Met Office.
Natural Resources Wales (NRW) warned coastal areas, Snowdonia and northern parts of Ceredigion could be hard hit, with "potential for large waves".
The yellow "be prepared" warning started at 09:00 GMT.
On Thursday night 27 flood alerts - meaning there was a risk of flooding - were in place for communities living close to rivers in north, mid and south Wales.
With big swell forecast today and over the next few days we urge to #RespectTheWater if you live within walking distance of the coast. Avoid exposed coastal areas like cliffs, seawalls and piers.— Porthcawl RNLI Lifeboat (@PorthcawlRNLI) October 28, 2020
Watch the conditions LIVE safely from home: https://t.co/yEbomj4Y6z pic.twitter.com/yJ3hZ4Ug44
"The predicted heavy rain is likely to cause flooding up and down the country over the next few days," said Gary White, NRW duty tactical manager.
Of Wales' 22 local authority areas, 20 are covered by the weather warning: Anglesey, Blaenau Gwent, Bridgend, Caerphilly, Cardiff, Carmarthenshire, Ceredigion, Conwy, Denbighshire, Gwynedd, Merthyr Tydfil, Monmouthshire, Neath Port Talbot, Newport, Pembrokeshire, Powys, Rhondda Cynon Taf, Swansea, Torfaen and Vale of Glamorgan.