Complex sentence: The remnants of Ilsa drifted northwestward through the Stratocumulus cloud field of the eastern north Pacific Ocean, and at the same time, a mid-tropospheric trough slowly intensified while gradually undergoing cyclogenesis to develop into a cold-core upper-level low.
Split into simpler sentences: The remnants of Ilsa drifted northwestward through the Stratocumulus cloud field of the eastern north Pacific Ocean. At the same time, a mid-tropospheric trough slowly intensified while gradually undergoing cyclogenesis to develop into a cold-core upper-level low.