Thunderstorms and heavy showers could hit southern parts of England and Wales, as a split in bank holiday weather conditions will see Scotland and Northern Ireland basking in sunshine.
Yellow warnings for rain are in place for most of Saturday, with the Met Office saying homes and businesses could quickly become flooded.
Strong gusts, hail and thunder are expected to develop over the weekend and into Monday. Parts of Wales and the Midlands are expected to experience the most severe conditions.
The warmest weather will be felt across southern parts of England and Wales, as temperatures will escalate from about 25C on Saturday, and could reach 30C on Monday.
However, the heat is likely to trigger humid nights. Overnight temperatures will reach to about 8C in the north, and the low to mid-teens in the south.
Marco Petagna, a Met Office meteorologist, said there would “generally be a bit of a north/south split across the UK, with Scotland and Northern Ireland seeing the best of the dry conditions, with some sunshine especially towards the west of Scotland”.
He added: “There will be some very warm – if not hot – sunshine as well, but there will also be a risk of very heavy, thundery downpours at times.”
Northern Ireland is expected to reach highs of about 20C in Belfast.
The south east and western Scotland will bask in temperatures of up to 26C on Saturday, while the temperature could hit 29C in the south.
Wet weather could exacerbate transport disruptions taking place throughout the weekend. Drivers are advised to take care and be prepared for flooding on the roads.
Northern Rail services will also experience disruption after members of the RMT union walked out for 24 hours on Saturday. Passengers were encouraged to plan ahead and consider whether travel was necessary, the operator said.
Major engineering works were also taking place across several rail networks, with no trains at Swindon until Monday, while services to and from Birmingham New Street have also been amended.
Some services in Glasgow and Edinburgh were running a bus replacement service.