Thousands of homes have been without power, roads shut and trains cancelled, as the UK continues to be battered by heavy downpours.
The River Usk in south Wales burst its banks to block off one road and the main A40 was also cut off by storms.
Footage on the BBC showed strong winds reversing a waterfall on the Isle of Skye.
Rail services were plagued by delays, with severe weather affecting services between Preston and Scotland, across Wales, and in South West England.
Strong winds have seen speed restrictions imposed on many routes - with reduced Virgin services between London Euston, Birmingham New Street, and Edinburgh and Glasgow, Network Rail said.
Flooding and falling trees have seen major disruption across most rail services in Wales, while trains in Devon and Cornwall are also likely to be delayed. Motorists were also warned over tricky driving conditions brought by Callum.
An amber warning for rain remains in place for parts of south Wales, with further flooding and power cuts likely.
A yellow warning covers much of the rest of Wales, as well as parts of Scotland and the north west and south east of England.
Thousands of households were left without power and dozens of flights grounded yesterday as torrential rain and winds of more than 112km/h buffeted the Republic of Ireland, Northern Ireland and the west coast of Britain.
One part of south Wales, the Brecon Beacons village of Libanus, recorded 182mm of rain in just 48 hours - way above the monthly total for the region of 169mm.
Forecasters are warning of more of the same, although some parts of England will escape the worst.
Met Office meteorologist Helen Roberts said: "There will be more wet and windy weather for similar parts of the country - predominantly western areas - which could cause more flooding.
"The east and south east of England will see the best of the drier and brighter weather, where it will be even warmer than yesterday and could reach 25C - way above average for the time of the year."
Several police forces reported accidents on the roads due to surface water, while Network Rail said it was expecting disruption to train services throughout today and advised passengers to plan ahead.
Volunteers have been helping the owners of one of Wales' finest Tudor homes to build a 300m sandbag defence to help prevent further flood damage.
Peter Welford and his wife Judy Corbett have spent nearly 25 years restoring the Grade I listed 16th Century Gwydir Castle, near Llanrwst in the Conwy Valley.
The Environment Agency has issued flood warnings for parts of the Dorset coast, as well as 58 flood alerts across England, while Natural Resources Wales has 28 flood warnings and 39 flood alerts in place.
Storm Callum was named by Met Eireann and is the third named storm of the season to hit the UK.
US strikes Houthi targets in Yemen after pilots rescued from Red Sea.