When tropical cyclones move into the midlatitudes they often undergo “extratropical transition” (ET), i.e. they gradually lose their tropical characteristics and may transform into an extratropical low pressure system. The impact of the tropical cyclone on the synoptic-scale circulation in the midlatitudes can be significant. ET may excite Rossby wave packets that then disperse along the midlatitude jet stream. By this mechanism the impact of ET can reach near-hemispheric scales. Severe weather events have been linked to upstream ET systems. Frequently, predictability is severely compromised downstream of an ET system. The potential for severe weather to occur in regions of reduced predictability highlights the importance of achieving a better understanding of the physical processes that govern the downstream impact of ET.

Potential temperature (θ) on the dynamic tropopause (2 PVU surface, color), surface pressure (thin contours, every 5 hPa, dashed for 990 hPa and less), and wind speed at 200 hPa (thick contours, every 5 m/s, starting at 45 m/s). The initially zonal jet stream is located along the gradient of θ on the tropopause. The distinct minimum in surface pressure denotes the ET system. Very high θ values (white) indicate the outflow anomaly. The jet streak downstream of the ET system is evident and the wave-like pattern shows the developing ridge-trough couplet. A first indication of the “PV streamer” can be found on the eastern flank of the outflow air. (Fig. 2(b) from Riemer et al., 2008)