The top temperature across the UK on New Year's Eve is expected to be 11C.
The south will see brighter skies than the north on Christmas Eve.
The showers will be heavy at times, with the risk of thunder.
Storm Atiyah approaches parts of Wales and south-west England later on Sunday.
Sunday will see top temperatures of 8 Celsius (46 F).
Southwestern parts are likely to turn wet and windy later.
Temperatures will be close to the seasonal average but it will feel chilly in the rain, and with a brisk breeze in the northeast.
Heavy rain will bring the risk of localised flooding in places.
Travellers have been warned to brace themselves for disruption, particularly on Tuesday.
After a chilly but sunny start for most, much of the UK will stay dry on Sunday with bright or sunny spells continuing throughout.