Met Eireann issued the Status Orange warning for Galway on Wednesday evening ahead of Storm Callum which will hit Ireland on Thursday night.
The storm is expected to bring heavy rain between 6am on Friday and 6pm on Saturday, particularly over south-facing hills.
Strong winds at this time of year can increase the rate of leaf fall which can potentially block drains or culverts and, with the heavy rainfall expected over Friday and Saturday, could well heighten the potential for flooding.
Met Office meteorologist Greg Dewhurst said: "Storm Callum is coming from the middle of the Atlantic".
"Some exposed locations could see as much as four inches by the end of Friday".
Power cuts, travel disruption and hard driving conditions are also predicted, with yellow weather warnings for wind covering Northern Ireland and western parts of Wales, England and Scotland throughout Friday. There is the risk of localised flooding, tricky conditions on the roads with surface water and spray. Winds will be strongest overnight and early on Friday, gusting between 90 and 110 km/h at coasts.
"Storm Callum is the 3rd named storm of the 2018/19 season".
It is because the storm is likely to coincide with high tides around the south Wales coast.
The national emergency group includes Met Éireann, local authorities, the ESB, OPW and government.
There is also a warning for rain in place for Cumbria from 12am to 23.59pm on Saturday (October 13).
"Our teams are doing all they can to reduce the risk for communities, but if there is flooding we want to make sure people are doing all they can to keep themselves safe".
Shropshire is on the edge of the yellow warnings we have issued, with the most persistent and heaviest rain expected across Wales, north west England and south west England.