In September 2020, Western North America was impacted by a highly anomalous meteorological event. Over the Pacific Northwest, strong and dry easterly winds exceeded historically observed values for the time of year and contributed to the rapid spread of several large wildfires. Nine lives were lost and over 5000 homes and businesses were destroyed in Oregon. The smoke from the fires enveloped the region for nearly two weeks after the event. Concurrently, the same weather system brought record-breaking cold, dramatic 24-hour temperature falls, and early-season snowfall to parts of the Rocky Mountains. Here we use synoptic analysis and air parcel backward trajectories to build a process-based understanding of this extreme event and to put it in a climatological context. The primary atmospheric driver was the rapid development of a highly amplified 500 hPa tropospheric wave pattern that persisted for several days. A record-breaking ridge of high pressure characterized the western side of the wave pattern with a record-breaking trough of low pressure to the east. A notable anticyclonic Rossby wave breaking event occurred as the wave train amplified. Air parcel backward trajectories show that dry air over the Pacific Northwest, which exacerbated the fire danger, originated in the mid-troposphere and descended through subsidence to the surface. At the same time, dramatic temperature falls were recorded along the east side of the Rocky Mountains, driven by strong transport of high-latitude air near the surface.

2020 年 9 月，北美西部受到高度异常气象事件的影响。在太平洋西北地区，强劲而干燥的东风超过了一年中该时期的历史观测值，导致多起大型野火迅速蔓延。俄勒冈州有 9 人丧生，5000 多所房屋和企业被毁。事件发生后近两周，火灾产生的烟雾笼罩了该地区。与此同时，同样的天气系统给落基山脉部分地区带来了破纪录的寒冷、24 小时气温急剧下降和早季降雪。在这里，我们使用天气分析和气团后向轨迹来建立对这一极端事件的基于过程的理解，并将其置于气候学背景下。主要的大气驱动因素是高度放大的 500 hPa 对流层波型的快速发展，该波型持续了数天。波型西侧出现破纪录的高压脊，东侧出现破纪录的低压槽。当波列放大时，发生了著名的反气旋罗斯比波破碎事件。气团向后轨迹显示，太平洋西北地区上空的干燥空气加剧了火灾危险，其起源于对流层中部，并通过下沉下降到地表。与此同时，由于地表附近高纬度空气的强烈输送，落基山脉东侧的气温急剧下降。