Along with a spell of heavy rain and high tides there is a risk of coastal flooding and damage.
South Wales will get up to a month's worth of rain on Friday and Saturday, and double that in high and exposed locations.
The forecast deluge poses a risk of communities being cut off by flooding, while widespread gales are expected to cause disruption across western areas of the UK.
Train services between Swansea and Carmarthen are being suspended during parts of Friday because of Storm Callum.
The storm is expected to bring heavy rain between 6am on Friday and 6pm on Saturday, particularly over south-facing hills.
She said the west coast was "not out of the woods" in terms of strong winds and there was still a danger to people on the roads.
"Now we've already got the rain and the gusty winds coming in from the west, gusts currently about 50-60 miles per hour".
At this time of year the wind is also likely to cause more leaves to fall on the roads, which can block drains and increase localised surface flooding.
The main impacts for the United Kingdom will be from heavy rain across some western areas on Friday and Saturday, with yellow Met Office weather warnings for wind and rain now in place for Friday and Saturday.
The other 13 counties have been issued with a yellow warning which will also end at 9am on Friday.
NRW said its emergency response teams were checking flood defences, clearing gullies and ditches and installing temporary defences.
"If you are travelling you should ensure you and your vehicle are adequately prepared for the conditions making sure you have sufficient fuel and supplies such as warm clothing, food and water in the event you are delayed for several hours".
"The rain is expected to be accompanied by strong winds, which when combined with high tides may lead to some coastal impacts due to large waves", Met office forecasters said last night.