Fig. 1

a, b Zonal velocity (shading), relative vorticity (red contour; 1, 3, and 5 × 10⁻⁵ s⁻¹), and wind vectors at 850 hPa, c, d column-integrated water vapor (shading) and 200-hPa wind vectors, at 0000 UTC on 15 June 2008, e, f height–longitude section of relative vorticity at 0000 UTC on 16 (blue), 17 (black), 18 (green), and 19 (purple) June 2008, along the latitude across the center of the incipient disturbance, in the ECMWF YOTC (a, c, e) and the NCEP FNL (b, d, f). Contour lines of 850-hPa relative vorticity averaged between 6 hours before and after 0000 UTC on 15 (red), 16 (blue), 17 (black), and 18 (green) June 2008 are also depicted in c, d (1, 5, and 10 × 10⁻⁵ s⁻¹). Crosses in a, b, e, f indicate the location of the center of the incipient disturbance at 900 hPa, and triangles in e and f indicate the location of the center of the maximum vorticity in the free atmosphere