The rain is accompanied by strong winds, which when combined with high tides may lead to some coastal impacts due to large waves.
Met Éireann forecaster Deirdre Lowe said strong winds would be followed by a period of heavy rain sweeping up from the south east, with the possibility of flooding.
Not yet formed, but later Thursday into early Friday, a rapidly deepening Atlantic storm depression (936hPa), will advance towards the west of Ireland and track northwards, eventually pulling away towards Iceland daytime Friday.
"On Friday, the winds will ease somewhat in many areas during the morning, becoming fresh or strong southerly".
The main impacts for the United Kingdom will be from heavy rain across some western areas on Friday and Saturday.
Before all that happens Thursday will be a day of heavy rain in many places.
South Wales will get up to a month's worth of rain on Friday and Saturday, and double that in high and exposed locations.
Met Éireann has issued Orange and Yellow warnings for Storm Callum.
The Met Office said that "leaves and twigs, brought down by the wind, could block drains and culverts, increasing the likelihood of flooding".
"It's vital that we all take a look at our own road behaviour and adapt it accordingly to minimise incidents during this weather", says Conor Faughnan, AA Director of Consumer Affairs.
James Nicholas of Arriva Trains Wales said: "There is significant rain expected this weekend across Wales coupled with high winds and very high tides, particularly up on the Cambrian Coast - linking in to Shrewsbury. You can check our website or call Floodline on 0345 988 1188 for any flood warnings that may be in place and keep an eye on weather reports and local news for details of any disruption in your area". Localised coastal/tidal flooding is probable on Friday along the south Wales coast and is also possible for west Wales, the Wye estuary, and some north-west and south-west coasts of England.
Mean (sustained) wind speeds - Gales 39-46mph.