early on september 13, a low pressure area formed northeast of the northern mariana islands. the system gradually drifted north and steadily intensified until the next day when the jma upgraded the system to a tropical depression. later on september 14, the jtwc issued a tropical cyclone formation alert (tcfa) on the system reporting that the system could intensify into a tropical storm within 24 hours from then. convection rapidly consolidated the center with persistent, deep convection around the northeastern periphery, prompting the jtwc to initiate advisories on the system, designating it with "19w". soon, the jma also initiated advisories on the system, upgrading it to tropical storm "sonca". in the begging, sonca seemed to have intensified rapidly since formation, however, soon the storm weakened back to a minimal tropical storm because of dry air entering the llcc that caused it to elongate and weaken. however, that was not for too long as vigorous convection persisted over the well defined llcc with tightly curved banding wrapped in, sonca continued to strengthen gradually and the jtwc reported winds of at-least near the center. as sonca continued to strengthen, and the jma upgraded it to a severe tropical storm on september 17. later that day, sonca developed a large ragged eye with deep convective banding tightly wrapped into it. as a result, sonca strengthened more rapidly and by early the next day, it became a typhoon. on september 18, sonca reached a peak intensity of (1-min mean) and (10-min mean) and soon the convection around the northern periphery started weakening. being embedded in a baroclinic zone with low sea surface temperatures, sonca started its extratropical transition late on september 19. the transition took place relatively fast because of a frontal boundary and the jtwc reported that sonca became extratropical early on september 20, while the jma did the same later in the evening.