Complex sentence: However, it is often considered as a typhoon because of the damage it caused, which is yet the heaviest damage done by intense raining since the heavy rainfalls in September 2009, when Typhoon Ketsana hit Metro Manila.
Split into simpler sentences: However, it is often considered as a typhoon because of the damage it caused. It is yet the heaviest damage done by intense raining since the heavy rainfalls in September 2009, when Typhoon Ketsana hit Metro Manila.