Met Eireann has added Leitrim, Roscommon and Sligo to today's weather warning as stormy conditions are expected to reach further west than expected.
The forecaster had already issued a status yellow thunderstorm and rain warning for Leinster, Cavan, Donegal and Monaghan.
But now 18 counties are under the new warning which begins at midday and will last until 8pm tonight.
The forecaster said: "Scattered heavy showers with localised thundery downpours will give a risk of spot flooding."
It is expected to be generally miserable countrywide today before it begins to brighten up in the coming days.
A Met EIreann forecaster said: "Rain in the east and northeast today with sunshine and showers elsewhere. There'll be heavier showers with a chance of thunderstorms and localised flooding from around lunchtime until early evening; areas at risk the counties north and east of a line from Sligo to Wexford . Highest temperatures 17 to 20 degrees, with mostly light southerly winds veering westerly later.
"Tonight, showers will largely die out this evening with just isolated showers remaining. Cloud will increase from the west later, with patchy light rain or drizzle developing in the southwest by morning. Mist thickening to fog in places with lowest temperatures 10 to 13 degrees with light westerly or variable breezes.
"Sunday will be dry in most areas, with sunny spells developing, the best in the east and northeast. Overall, rather cloudy, with patchy drizzle, in parts of Munster and along the west coast. Highest temperatures 17 to 20 or 21 degrees, in light westerly or variable breezes.
"Mainly dry, settled weather for the rest of the week, with some sunshine and above normal temperatures."
