(100 km) (c) (d)  (hPa) (a) (b) Birth of Typhoon Prapiroon Detectable 60 Hours Before Operational Warning Issued Xuyang Ge, Tim Li (IPRC), and Melinda Peng (NRL),. Mon. Wea. Rev., in press The origin of Typhoon Prapiroon in 2000 is seen in the output of a mesoscale Numerical Weather Prediction Model 60 hours before the JTWC identified it as a tropical cyclone (TC). As shown in (a) and (b), the genesis is associated with the Rossby wave energy dispersion from the existing TC Bilis. Parallel to the control simulation, a sensitivity simulation was conducted in which TC Bilis was removed. Analysis of the numerical simulations show that TC Bilis impacts the subsequent genesis of Prapiroon both directly and indirectly. The direct impact is through the conventional barotropic Rossby wave energy dispersion, which enhances the low-level wave train [(a) and (c)]. The indirect impact is through the upper-level outflow jet of TC Bilis. The asymmetric outflow jet induces a secondary circulation with a strong divergence tendency to the left-exit region of the jet (d). This upper-level divergence strengthens large-scale ascending motion, promoting favorable environmental conditions for new TC development. (d) Vertical cross section of the divergence tendency term in the nonlinear balance equation. The shaded area represents the jet core of TC Bilis on 23 August. (b) Horizontal map of energy propagation E-vectors obtained over 11-days (Aug.17 – 27). Dot represents center of TC Bilis on Aug. 22. (c) Vertical-radius cross-section of the relative vorticity field along the northwest-southeast oriented axis shown in (a). (a) TRMM rainfall rate (shaded) and 850-hPa wind (vectors) on 23 August, 2000 (2 days before Prapiroon genesis). “A” denotes existing TC Bilis and “B” the forming Praprioon. B A 100E 160E 130E E-Vector at 1000hpa