The Met Office has issued a warning for heavy rain affecting millions of people across eight regions in the UK on Wednesday, due to a shift in the jet stream. This weather change follows a record-breaking May heatwave that saw temperatures exceed 35C in some areas. Initially, some dry and bright weather is expected, but this will not last. Meteorologist Aidan McGivern noted that a stronger and south-shifted jet stream will bring low pressure, resulting in wet conditions starting in Northern Ireland, Wales, and the South West first thing Wednesday morning. Rain is projected to move northeast, impacting central, eastern, and northern England by the afternoon, along with central and southern Scotland. Despite some clearing in the afternoon, showers will persist in various regions. The forecast indicates that southern areas could see highs of 21C, while northern regions may peak at 19C. The rest of the week will feature a mix of sunshine and heavy showers, with a risk of thunderstorms in the north on Thursday and generally cloudy conditions on Saturday.