sentences:  At this point, Isa became an annular typhoon, with a large eye and a lack of spiralform rainbands, while moving nearly due northward, through a weakness in the subtropical ridge.Simultaneously, JMA assessed the typhoon as attaining peak 10-min winds of 155 km/h (100 mph).Shortly after peaking in intensity, Isa began to weaken, and by April 21 it dropped below "super typhoon" status.