서울 체감 -21도, 가장 혹독한 일주일…최장기 극한한파 온다

This headline-style sentence uses strong, dramatic expressions, which are common in Korean news. “체감 -21도” means the felt temperature, not the actual temperature. It refers to how cold it feels because of wind. The phrase “가장 혹독한 일주일” does not mean a literal record week, but emphasizes that the coming days will be extremely difficult due to the cold. “온다” (is coming) is often used in Korean headlines to create urgency and impact.