Complex sentence: While the typhoon was moving westward toward the Philippine island of Luzon, thunderstorms in the northern eyewall deteriorated as a result of unfavorable subsidence from a high pressure system to the north.
Split into simpler sentences: While the typhoon was moving westward toward the Philippine island of Luzon, the thunderstorms in the northern eyewall deteriorated. This was due to unfavorable subsidence from the high pressure system to the north.