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.
A status orange warning for winds in place for Dublin, Louth, Wexford, Meath and Waterford has been extended from midnight on Thursday until Friday morning. Temperatures in the capital could quite easily get to 78F (26C) over the course of the day.
Natural Resources Wales said that over the last few days, rain gauges have recorded totals of more than 160mm (five inches) of rain in the Brecon Beacons.
According to Met Éireann, there's going to be further rain through Saturday.
Coasts on the South and Western parts of the country are set to be most badly affected.
Some areas could see more than 200mm (8in) of rain by the end of today, and forecasters say there could be a danger to life from fast-flowing or deep floodwater.
Making sure you know what the situation is like where you live is really important.
Met Office meteorologist Simon Partridge said: "We could see up to 100mm of rain for some places (western parts) and the main area of concern is down across southern Wales".
A yellow weather warning is in place tomorrow across large parts of north west and west Wales, with further heavy and persistent rainfall forecast.
Those planning to travel ahead of the weekend should note there may be significant disruption following the storm.
"Storm Callum" is the third storm of the 2018/19 season, with the strongest winds likely to hit western Ireland, although North Western parts of the United Kingdom can expect severe gales.