Weather forecasters are warning that "intense thunderstorms" including hail and torrential rain could strike almost anywhere towards the end of the week.
Shropshire and mid Wales enjoyed the hottest weekend of the year with outdoor attractions reporting big crowds and many heading for the Welsh coast on Saturday and Sunday.
But temperatures are gradually cooling throughout the week and the Met Office says thunderstorms could hit anywhere, any time from about 6pm on Wednesday to 6am on Friday.
It says the hot plume of air is causing instability in the atmosphere with eastern Wales and most of England potential storm spots.
As the sun beat down at the weekend many headed for lakes, meres and rivers to cool down over the weekend. Golf courses were among popular venues for those keeping fit.
However the Met Offices says that if there are thunderstorms, playing golf; fishing and being on boats in the water should be avoided.
"Whilst not all locations will be affected, some intense thunderstorms may occur during this period with torrential rain, hail, frequent lightning and strong gusty winds possible," forcasters said.
"Avoid activities such as golf, rod fishing or boating on a lake."
Cyclists are also being warned that gusty winds associated with storms and flash flooding could cause them particular problems.
While Shropshire could avoid the storms altogether, areas that are affected could see about 1.2 inches of rain fall in an hour.
Meteorologist Dan Stroud said: "We're likely to become increasingly changeable towards the weekend.
"There is still the potential for some more settled spells and some warm weather is possible later in the month, but it is generally turning more changeable."
The worst of any thunderstorms should have eased in time for the England Wales football match on Friday evening.
Pubs with outdoor screens are among those hoping that the weather stays dry for the game which promises to bring bumper customers to watch the match with friends.
However across the Welsh border rain could arrive in time for the Wales v Turkey tie on Wednesday evening.