The Experiment on Rapidly Intensifying Cyclones over the Atlantic was carried out over the western North Atlantic Ocean to provide temporally continuous comprehensive datasets from which to document the life cycle of extratropical marine cyclones. The most intense cyclogenetic event occurred on 4-5 January 1989 over the warm (>20°C) Gulf Stream current; the cyclone's central sea level pressure decreased by 60 mb in 24 h, from 996 to 936 mb. This study presents the synoptic-scale and mesoscale life cycle of this cyclone in two parts. Part I, presented here, describes the 24-h frontal-cyclone evolution through 6-h analyses of observations taken by specially deployed observing systems from air, land, and sea. The analyses of temperature, wind, and pressure about the incipient cyclone first illustrate the precursor signatures to cyclogenesis. The 850- and 500-mb temperature evolutions show a significant departure from the Norwegian frontal-cyclone model. In particular, the 850-mb analyses document 1) a storm-relative westward development of the warm front as a bent-back front into the polar airstream, and 2) the formation of a warm-core frontal seclusion in the post-cold-frontal cool air at the southwestern tip of the bent-back front. Analyses of sea level pressure provide a detailed account of cyclone intensification along the bent-back front. Infrared satellite imagery shows the evolution and immense size (∼5000 km) of the cyclone's cloud signature, and a 250-km-scale comma-cloud system in the vicinity of the warm-core seclusion situated at the southwestern tip of the large-scale comma head. Thermodynamic air-sea interaction diagnostics reveal large upward fluxes of heat and moisture from the sea surface into the marine boundary layer of the evolving cyclone. The maximum of combined upward flux approached 3000 W m−, several times larger than that typically observed in both extratropical and tropical cyclones. These fluxes exhibited extreme spatial variability, reflecting the mesoscale characteristics of the cyclone circulation.