sentences:  With the subtropical ridge to its north, Isa tracked to the north and gradually curved to the west.It slowly intensified, due to it being a large tropical cyclone, and late on April 13 JTWC upgraded the storm to typhoon status; at the same time, JMA continued to assess Isa as a minimal tropical storm, and did not upgrade it to a typhoon until April 16.Isa maintained a nearly due-westward movement, although tropical cyclone prediction models anticipated a quick turn to the north.