A major storm impacted the subtropical Andes during 28–31 January 2021 producing 4-days accumulated precipitation up to 100 mm over central-south Chile. These are high accumulations even for winter events but the storm occurred in the middle of the summer when precipitation is virtually absent, conferring it an extraordinary character. Similar storms have occurred only 2–3 times in the past century. The January 2021 event included periods of high rainfall intensity, hail and lighting, causing dozens of landslides and flash floods with the concomitant social impacts and economical losses. Here we examine the meteorological drivers of this storm at multiples scales, its climatological context, the associated surface impacts, and some aspects of its predictability.

About a week before the storm development over central Chile, a large-scale perturbation in the central South Pacific set the stage for the formation of a zonal jet aloft and zonal atmospheric river (ZAR) that extended eastward until reaching the west coast of South America. The ZAR landfalled at 39°S and its subsequent northward displacement resulted in copious orographic precipitation over the Andes and adjacent lowlands, concomitant with a relatively warm environment during the first phase of the storm (28–29 January). During the second phase (30–31 January) the ZAR decayed rapidly but left behind significant amount of water vapor and the formation of a cut-off low (COL) in its poleward flank. The COL facilitated both advection of cyclonic vorticity and cold air at mid-levels, setting the environment for deep convection, intense rain showers, significant lightning activity, and hail.

An assessment of the quantitative precipitation forecast (QPF) from the operational Global Forecast System (GFS) indicates that the model captured well the 96-h precipitation accumulation (28–31 January) in terms of timing and spatial extent. However, specific zones with the largest accumulations varied as a function of lead time. The more stable precipitation during the ZAR phase was better predicted than the convective precipitation during the COL phase. Proper dissemination of these forecast and recently established infrastructure contributed to ease the impact of this extraordinary event on the general population.

2021 年 1 月 28 日至 31 日，一场大风暴影响了副热带安第斯山脉，在智利中南部产生了 4 天累积降水量高达 100 毫米。即使对于冬季事件，这些也是高积累，但风暴发生在夏季中期，几乎没有降水，赋予它非凡的特征。在过去的一个世纪里，类似的风暴只发生了 2-3 次。2021 年 1 月的事件包括高降雨强度、冰雹和照明，造成数十起山体滑坡和山洪暴发，并带来社会影响和经济损失。在这里，我们在多个尺度上研究了这场风暴的气象驱动因素、气候背景、相关的地表影响以及其可预测性的某些方面。

在智利中部风暴发展前大约一周，南太平洋中部的一次大规模扰动为形成高空的纬向急流和向东延伸直至到达南美洲西海岸的纬向大气河流（ZAR）奠定了基础. ZAR 在 39°S 登陆，随后向北位移导致安第斯山脉和邻近低地出现大量地形降水，同时在风暴第一阶段（1 月 28 日至 29 日）期间环境相对温暖。在第二阶段（1 月 30 日至 31 日），ZAR 迅速衰减，但留下了大量的水蒸气，并在其极地侧翼形成了截止低点（COL）。COL 促进了半层气旋涡度和冷空气的平流，为深层对流、强阵雨、

对来自全球业务预报系统 (GFS) 的定量降水预报 (QPF) 的评估表明，该模型在时间和空间范围方面很好地捕捉到了 96 小时降水累积（1 月 28 日至 31 日）。但是，累积量最大的特定区域会随着交货时间的变化而变化。ZAR 阶段更稳定的降水比 COL 阶段的对流降水更能预测。这些预测的适当传播和最近建立的基础设施有助于缓解这一特殊事件对普通民众的影响。