The zone of heavy rain affecting England and Wales on Friday is then expected to move slowly northwards on Saturday, when a further yellow weather warning is in place.
Storm Callum is the third named storm of the season.
Storm Callum bringing periods of heavy rain during Friday and continuing into Saturday - particularly over south facing hills.
There is a chance that homes and businesses could be flooded, causing damage to some buildings.
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.
Areas of high ground are expected to be most affected with the potential for a further 50-80 mm of rain quite widely.
A Met Office weather warning remains in place until midnight.
Gusts of 50mph to 60mph could reach some places, with the chance of reaching 70 miles per hour around some coasts and hills.
He also urged extra care along the Pembrokeshire coastline.
There are now no reports of power outages or trees down in Laois.
Train services between Swansea and Carmarthen are being suspended during parts of Friday because of Storm Callum.
Arriva Trains Wales has suspended its Swansea to Camarthen line and amended part of its weekend timetable.
It is because the storm is likely to coincide with high tides around the south Wales coast.
Sioned Warrell, Natural Resources Wales duty tactical manager, warned flooding could occur as a result of the rain.
NRW will issue flood alerts and warnings if rivers reach trigger levels, with teams monitoring river levels 24 hours a day.
"Widespread flooding from surface water and rivers is probable across south Wales on Friday and Saturday and is possible elsewhere in Wales and parts of western England".
The deluge poses a risk of communities being cut off by flooding.
Met Office Shawbury said wind was expected to pick up on Friday along with outbreaks of rain.