25 March 2021 Deep South Tornado Outbreak Preliminary Virtual Reconnaissance Report (PVRR)

A well-warned, high-risk tornado event was verified across portions of Alabama, Tennessee, and Georgia on March 25, 2021, producing 16 individual tornadoes, nine of which were EF2 or higher intensity (estimated wind speeds greater than 110 mph). Tragically, five fatalities were directly caused by the tornadoes, four in mobile or manufactured homes and a fifth in a non-permanent structure. A sixth fatality occurred in GA during search and rescue attempts. That more fatalities did not occur despite widespread damage was a testament to the National Weather Service and other members of the Integrated Warning Teams (IWT), with the high likelihood for tornadoes appearing in the convective outlooks several days prior to the event itself, and with prompt warning issuance and communication by local partners throughout the outbreak.

The majority of the damage occurred to single-family residences, although at least three schools and several engineered buildings were also impacted to the extent that classes were forced to go back to remote-only until repairs could be made. The affected buildings were constructed with no consideration of tornadoes. Roof structure failures were common, with many roof failures associated with subsequent collapse of the exterior walls. Most concerning was a number of site-built homes that suffered complete collapse, some with damage patterns suggesting the shear walls or wall anchorage assembly were the weakest link in the load chain, resulting in unsafe collapses.

This project encompasses the products of StEER's response to this event: Preliminary Virtual Reconnaissance Report (PVRR).

2021年3月25日，美国阿拉巴马州、田纳西州及佐治亚州（GA）部分区域发生一场预警充分的高风险龙卷风灾害，该事件经官方核实，共生成16个独立龙卷风，其中9个达到EF2级及以上强度（估算风速超过110英里/小时）。不幸的是，此次灾害共造成5人直接遇难：4人死于活动房屋或预制住房，另有1人死于非永久性建筑；佐治亚州还在搜救行动中出现1例额外遇难者。尽管灾害造成了大范围破坏，但并未出现更多伤亡，这充分印证了美国国家气象局（National Weather Service）及综合预警团队（Integrated Warning Teams, IWT）其他成员的工作成效——早在灾害发生数日前，对流天气展望产品中就已明确标注龙卷风高发的可能性，且事发期间当地合作机构及时发布并传播了预警信息。
此次灾害造成的破坏主要集中在独栋住宅，不过至少3所学校及多座工程建筑也受到严重影响，导致当地课程被迫全面转为线上教学，直至完成修复。受灾建筑在设计建造阶段均未考虑龙卷风抵御需求。屋顶结构损毁情况十分普遍，多数屋顶坍塌后伴随外墙倒塌。最令人担忧的是，多座现场搭建的住宅完全坍塌，部分建筑的损毁模式显示，剪力墙或墙体锚固组件是荷载传递链中的薄弱环节，最终引发了非安全式坍塌。
本项目涵盖了StEER针对此次灾害的响应成果：初步虚拟勘察报告（Preliminary Virtual Reconnaissance Report, PVRR）。