After a balmy beginning to the month forecasters predict it could be the warmest November day on record.
West Wales, where temperatures reached a record-breaking 22.4C (72.32F) on the first day of the month, could see an even warmer day.
The unseasonably mild weather means parts of the UK are hotter than Rome, where it is 20C (68F) and Lisbon which is a cool 16C (60.8F).
Simon Partridge of the Met Office said there is a 30% chance of Sunday's high in Trawsgoed, Wales being beaten.
The UK's previous record November temperature was in Prestatyn, Wales, in 1946 when the mercury reached 21.7C (71.1F).
The highs in some parts will be contrasted in others where thick fog will keep temperatures 10 degrees C lower.
Mr Partridge said: "There will be quite a big difference between the east and west of the UK.
"If you are in the south-east especially it will be quite a chilly day, but you'll have more luck in the west where it looks set to be a very pleasant day for November."
While the fog will be most dense in the south-east of the country, Northern Ireland and parts of central Scotland will wake to foggy conditions.
Dozens of flights were cancelled at London's Heathrow airport on Sunday and another 45 have been "pre-cancelled" this morning.
The warm temperatures are not expected to last much longer, with another foggy start to Tuesday and unsettled, wet weather on its way later in the week.