Complex sentence: On December 20, the Joint Typhoon Warning Center (JTWC) started to monitor a tropical disturbance that had developed to the southeast of Yap, when that broad and poorly organized system was located under moderate to high vertical wind shear.
Split into simpler sentences: On December 20, the Joint Typhoon Warning Center (JTWC) started to monitor a tropical disturbance that had developed to the southeast of Yap. The broad and poorly organized system was being affected by moderate to high vertical wind shear.