Developing wave packets in the western North Pacific storm track are diagnosed observationally. An abrupt upstream edge to baroclinic-wave activity over the western North Pacific facilitates comparisons between the observational results and previous theoretical predictions on the spatio-temporal evolution of an impulse disturbance. Results for a sample of 291 cases show that the events are preceded by a sharply peaked wave packet, perhaps a vortex, that originates poleward of the Himalaya plateau. By filtering the sample to cases sharing a common ray path, the packets may be tracked for five days as they reach and cross North America.

The longitude--height structure of the mature packet his strikingly similar to impulse development in the linear Eady model, with the central portion of the packet dominated by deep growing waves with horizontal wavelengths of approximately 4000 km. Packet structure downstream from the peak is also consistent with linear theory: packet amplitude decays exponentially, wavelength decreases approximately linearly, and the leading edges moves near the speed of the background zonal wind. In contrast to linear theory, the packet peak also moves downstream near the background zonal-wind speed and, at later times, both the peak and leading edge move slower than the background zonal wind. The packets show an abrupt upstream edge, and no evidence of upstream development. Over the western (central and eastern) Pacific, packet length decreases (increases) due to a decrease (increase) in along-stream flow-relative group velocity, suggesting that packets are focussed (de-focussed) in these locations.

A sample of North Atlantic storm-track events produces similar results, and suggests that the Atlantic storm track is often seeded by wave packets that originate over the western Pacific ocean. In contrast, Atlantic packets refract equatorward and become trapped on the subtropical jet to the south of Himalaya plateau, suggesting perhaps less potential for seeding disturbances in the Pacific storm track.

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