Southern England could be hit by thunderstorms sweeping in from the south west over the Spring Bank Holiday weekend, while the northern half of the UK is expected to stay dry, forecasters have said.
Experts have also predicted reasonable temperatures with the thermometer possibly getting as high as 23C (72F).
Travellers face a weekend of packed roads, disrupted rail journeys.
Motoring organisations reckon as many as 18 million cars will take to the roads over the weekend, with traffic heading for various big sporting events adding to the congestion.
More than 30 pieces of engineering work are scheduled on the railways, with services once again disrupted on the West Coast Main Line.
The prediction of 18 million cars taking to the roads has come from the RAC which suggested drivers allow 50% more time than normal for their journeys.
The RAC said that among the routes expected to be particularly busy were those to the West Country, to the Lake District and to Blackpool, with roads from northern England to north Wales and between the Midlands and Norfolk also likely to be congested.
It also warned of disruption on routes leading to sporting venues. Among the events this weekend are the Football League play-offs at Wembley Stadium, the Scottish FA Cup Final in Glasgow on Saturday, the England v New Zealand Test Match in Manchester and the Rugby Union Heineken Cup Final in Cardiff on Saturday.
RAC patrolman of the year, Iain Vale, said: "We're all desperately disappointed that none of the home nations are going to football's Euro 2008 tournament, but this weekend will be the start of many sports fans' summers.
"It's important for motorists to be aware of all the events which are on and to plan their routes around them, allowing 50% more time for their journey."