Millions of people in the UK are set to experience heavy rainfall tomorrow due to a shift in the jet stream. Following a record-breaking heatwave in May where temperatures soared above 35C, the weather has now normalized for this time of year. However, thunderstorms and rain showers have been forecasted by the Met Office. Meteorologist Aidan McGivern mentioned that a new weather system with a stronger and more southward jet stream will bring low pressure, leading to wet conditions in various regions on Wednesday. Despite some clearing in the afternoon, showers are expected to persist in Northern Ireland, central and northern England, and Wales. Highs of 21C in the south and 19C in the north are anticipated, with similar temperatures throughout the week. The BBC weather forecast for this week includes cloudy and windy conditions with outbreaks of rain, followed by a mix of sunshine and heavy showers on Thursday, a chance of thunderstorms on Friday, and cloudy skies with light rain possible on Saturday.