明晨起北京有中雨，气温“大跳水”，将是立秋以来最冷一天

Group 1 - The core viewpoint indicates that Beijing is experiencing a significant weather change with rain and temperature drops expected [1] - Rain is forecasted for the evening of September 4, with thunderstorms in the western and northern regions of Beijing, and a maximum temperature of 31°C during the day [1] - The weather will transition to moderate rain on September 5, affecting the morning rush hour, with temperatures dropping to a maximum of 24°C and a minimum of 19°C [1] Group 2 - The period from September 5 to 12 will see temperatures ranging from 24°C to 29°C for maximums and 19°C to 21°C for minimums, indicating a shift towards cooler weather as summer transitions to autumn [3] - The meteorological department advises residents to adjust their clothing accordingly due to the fluctuating temperatures, especially during the early morning and late evening [3]