Nepartak originated in a region of strong thunderstorm activity, associated with a broad trough of low surface pressure, which was situated around 640 km (400 mi) southeast of Guam by 0000 UTC on November 11.[1][3] With weak wind shear and modest divergence of air over the system, it began to mature, although initially the Joint Typhoon Warning Center (JTWC)[nb 2] downplayed the potential for tropical cyclogenesis.