Met Eireann has issued a rarely seen Status Orange fog warning for 18 counties, including Waterford.
The counties covered are Dublin, Carlow, Kildare, Kilkenny, Laois, Wexford, Wicklow, Offaly, Westmeath, Meath, Galway, Mayo, Roscommon, Clare, Cork, Limerick, Tipperary,m and Waterford.
"Mist will thicken into fog overnight and will be dense at times," Met Eireann warned.
The warning is valid from midnight until 8am on Tuesday morning, April 9.
Extreme caution is advised on all roads in the affected counties.