Heavy rainfall could present a risk to life, the Met Office said, as it issued a 36-hour amber warning for Wales where almost a month's rain could fall over 48 hours.
Storm Callum has been upgraded from a yellow to an amber weather warning, which covers a large part of Pembrokeshire on Friday, October 12 and Saturday, October 13.
Storm Callum is now hitting the United Kingdom with wet and windy weather conditions, bringing heavy downpours and winds of up to 76mph to some areas.
"Areas of high ground exposed to the south and south west will be most affected, with the potential for a further 40-60 mm over hills.
In addition, leaves and twigs, brought down by the wind, could block drains and culverts, increasing the likelihood of flooding".
Friday's weather could impact travel plans during the morning commute, with Met office warning "road, rail, air and ferry services may be affected, with longer journey times and cancellations possible".
Flood warnings were put in place in Scotland and around 2,000 homes and businesses in England were left without power as some flood defences were breached.
A man who died after a landslide in south Wales during Storm Callum has been described as a "witty, charming, loyal and lovely son".
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The highest winds of the morning were 77mph recorded at 7am at Capel Curig in Gwynedd, north Wales, with 76mph gusts in the Scilly Isles and 64mph at RNAS Culdrose, in Cornwall.
Storm Callum is the third named storm of the season to hit the United Kingdom - with the worst of the weather being felt in Scotland, Wales and South West England.
The Met Office had issued a yellow weather warning for Northern Ireland.
Driving conditions will be hard, there is the risk of flooding.
Despite there being no weather warning now in place for Wrexham on Saturday, further heavy rainfall is expected to hit the area throughout most the day.
The Scottish Environment Protection Agency issued five flood alerts for Friday in Argyll and Bute, Ayrshire and Arran, Dumfries and Galloway, Scottish Borders, Skye and Lochaber and Orkney.
Train services between Preston and Carlisle have resumed after being cancelled due to "severe weather" on Saturday - but passengers should still expect delays. There is the continued risk of coastal flooding due to high seas.