Complex sentence: The JTWC then issued their last advisory on Halong as it had began its extra tropical transition the JMA then reported that it had completed its extra tropical transition and issued their final advisory on the remnant low of Halong.
Split into simpler sentences: The JTWC then issued their last advisory on Halong as it had began its extratropical transition. The JMA then reported that it had completed its extratropical transition and issued their final advisory on the remnant low of Halong.