Why did Changsha encounter the coldest weather in 50 years?

At first, I thought it was caused by the cold front weather caused by the cold air mass formed by the Asian high originating from Mongolia and Siberia. But simply from the perspective of the Asian high and cold front heading south, isn't it a bit one-sided? In that case, there will be low temperature snowfall in eastern China every winter. But how to explain the reason for such a rare large-scale continuous snowfall this year?

After thinking, I think the most important reason for this abnormal weather phenomenon is La Nina phenomenon. Affected by La Ni? a phenomenon, the water temperature in the equatorial western Pacific Ocean is high, and the water evaporation increases, which leads to the abnormal meridional circulation in East Asia. In addition, since the spring, the northerly airflow has prevailed in the north of China, while the warm and humid airflow in the southeast is relatively weak. Therefore, cold waves and strong winds frequently appear in the north. We know that at this time, East Asia is being affected by the Asian high. Therefore, it has strengthened the power of Siberian cold air moving rapidly southward. After the cold air passes through the border, the temperature drops sharply, and after being blocked by Qinling and Daba Mountains, it goes south and stays in these areas for a long time. In addition, the warm and humid air flow from the southwest Indian Ocean to the north continuously increases the humidity and interacts with the cold air, thus forming continuous topographic precipitation (snow). Perhaps it caused the recent continuous low-temperature snowfall.