Core Viewpoint - The occurrence of severe convective weather, including hailstorms, in northern China during mid-September is unusual but not abnormal, attributed to the interaction of cold and warm air masses as the seasons transition [3][5][6]. Group 1: Weather Phenomena - Severe convective weather, including thunderstorms and hail, was reported in Beijing and surrounding areas due to the influence of a "supercell storm" [1][3]. - The Central Meteorological Observatory issued a blue alert for severe convective weather, predicting continued thunderstorms and hail in various regions [1][3]. - The phenomenon of hail is linked to the characteristics of the thunderstorm clouds, with the size and quantity of hail being influenced by the vertical motion within these clouds [6]. Group 2: Meteorological Analysis - The primary causes of the recent severe weather include the southward movement of the westerly wind belt and the influx of warm, moist air from the south, creating conditions favorable for severe convective weather [3][5]. - Although the frequency of such weather events in September is low, it is not considered rare, as similar occurrences have been documented in previous years [4][5]. - The distribution of hail is uneven, with localized areas experiencing significant accumulation while others remain unaffected, due to the small scale of the thunderstorm clouds [6]. Group 3: Future Weather Predictions - The likelihood of further severe convective weather events is expected to decrease as the transition from summer to autumn progresses, although isolated incidents may still occur [7]. - Predictions indicate potential thunderstorms in specific regions, but the overall probability of severe weather is lower compared to the recent events [7].

Group 1 - A strong convective weather event occurred in Beijing on the night of September 13, resulting in thunderstorms and hail, with some areas experiencing significant accumulation of hail, leading to a phenomenon referred to as "Ice Age" by netizens [1][2] - The average rainfall in Beijing from 16:00 on September 13 to 02:00 on September 14 was 6.8 millimeters, with the maximum rainfall recorded at 42.5 millimeters in Mentougou [1] - The weather conditions leading to this event included high temperatures in early September, creating favorable low-level warm and humid conditions, combined with the influence of cold air from above, resulting in unstable atmospheric layering conducive to severe convective weather [2] Group 2 - The convective cloud system intensified as it moved from mountainous areas to the plains, forming a "supercell storm" that is prone to producing hail, strong winds, and localized heavy rainfall [2] - Meteorological forecasts indicate that another rainfall event is expected in Beijing from the morning to midnight of September 15, with light thunderstorms and varying rainfall intensities across the city [2]

Core Viewpoint - The article discusses the blue alert for severe convective weather issued by the Central Meteorological Observatory, highlighting the expected severe weather conditions across various regions in China, including strong winds, hail, and heavy rainfall [2][3]. Summary by Sections - **Severe Weather Forecast** - A blue alert for severe convective weather is in effect from September 14, 08:00 to September 15, 08:00, predicting areas in Inner Mongolia, Northeast China, and northern Jiangnan to experience winds exceeding level 8, with some regions in Heilongjiang expected to see winds above level 10 [2]. - Short-term heavy rainfall is anticipated, with hourly rainfall exceeding 20 mm in several areas, and some regions in Chongqing and Guizhou may see hourly rainfall exceeding 50 mm, potentially reaching over 60 mm [2]. - **Impact Timing** - The main impact period for the severe convective weather is expected to be during the daytime of September 14 [3]. - **Defense Guidelines** - Government and relevant departments should prepare for short-term heavy rain, thunderstorms, and strong winds, while meteorological departments should be ready for artificial hail prevention [6]. - Outdoor activities should be minimized, and individuals should stay away from temporary structures [6]. - Livestock should be moved to sheltered areas, and windows and doors should be secured [6]. - Waterway operations should take precautions, and construction sites should protect materials from the weather [6]. - Drainage systems in urban areas, farmland, and fish ponds should be checked and prepared for potential flooding and landslides [6]. - **Recent Severe Weather Events** - On the evening of September 13, Beijing experienced severe convective weather, including thunderstorms, strong winds, hail, and short-term heavy rainfall, leading to public astonishment at the intensity of the hail [8]. - The hailstorm was attributed to the formation of a supercell thunderstorm, influenced by unstable atmospheric conditions due to the late southward movement of the subtropical high and abundant moisture in the lower atmosphere [8][9]. - The combination of cold air aloft and warm, moist air below created an unstable stratification conducive to the development of severe convective weather [9].