sentences:  It moved erratically at first, though after attaining tropical storm status it curved westward due to the subtropical ridge to its north.Isa very gradually intensified, and on April 20 the typhoon reached peak 1-min winds of 270 km/h (165 mph), as reported by the Joint Typhoon Warning Center; Japan Meteorological Agency reported maximum 10-min winds of 155 km/h (100 mph).After turning northward, it accelerated to the northeast, and merged with a larger extratropical cyclone on April 24.