Up to 50 locations in England are forecast to reach 20C or more on Thursday, with the warmest spots in the south, the Midlands and eastern England.
Cambridge, Peterborough and Boston could all see 22C in the afternoon, while London, Oxford, Leicester, Northampton and other areas are expected to reach 21C, according to Met Office maps. Further north, Hull, Scunthorpe and York are among the places forecast to hit 20C.
Unseasonably warm conditions
The Met Office forecast for Thursday says: "Rain, often heavy, edging into northern England and Wales. Brighter and cooler conditions following to Northern Ireland and much of Scotland. Unseasonably warm across England and Wales. Windy in places."
A band of rain is expected to move southwards into Northern Ireland and southern Scotland on Wednesday night before spreading into northern Wales and northern England on Thursday.
Locations forecast to reach 20C or more
The full list of locations forecast to reach 20C or above includes Aylesbury (21C), Banbury (20C), Barnstaple (20C), Birmingham (20C), Boston (22C), Bourton-on-the-Water (20C), Bristol (20C), Cambridge (22C), Chelmsford (21C), Coventry (20C), Corby (21C), Derby (20C), Exeter (20C), Ely (21C), Gloucester (21C), Grantham (21C), Guildford (20C), Halesworth (20C), Hastings (20C), Hereford (20C), Hull (20C), Hunstanton (21C), Ipswich (20C), King's Lynn (21C), Leicester (21C), Lincoln (21C), London (21C), Louth (21C), Milton Keynes (20C), Minehead (20C), Monmouth (20C), Northampton (21C), Norwich (21C), Nottingham (21C), Oxford (21C), Peterborough (22C), Reading (20C), Rochester (20C), Scunthorpe (20C), Sheffield (20C), St Albans (20C), Stratford-upon-Avon (20C), Sudbury (20C), Swindon (20C), Taunton (21C), Tenterden (20C), Thetford (21C), Wells (20C), Worcester (21C) and York (20C).
Cooler conditions to follow
The warmer weather is not expected to last everywhere, with cooler conditions forecast towards the end of the week. The outlook for Friday to Sunday states: "Sunny spells and showers on Friday once early rain clears the far southeast. Often sunny this weekend but showers possible in the east. Feeling quite chilly with rural night frosts."
Looking further ahead, high pressure is expected to remain close to or over the UK for much of the period from October 19 to October 28, bringing plenty of fine weather, the Met Office says. Some rain remains possible as more unsettled conditions attempt to move in from the west or southwest.
Overall, drier-than-average conditions are more likely across most regions during this period, with temperatures expected to remain close to seasonal averages. Clearer, calmer nights could also bring overnight frost and patches of morning fog.
