Complex sentence: The depression slowly strengthened as it tracked west-northwestward and then north-northwestward across the western Gulf of Mexico, and was subsequently upgraded to Tropical Storm Arlene on June 19.
Split into simpler sentences: The depression slowly strengthened as it tracked west-northwestward and then north-northwestward across the Gulf of Mexico. After briefly accelerating, the depression intensified enough to be upgraded to Tropical Storm Arlene on June 19.