The major East Coast cyclone development and associated heavy snow event of 24-26 January 2000 was characterized by very low predictive skill in North American and European operational forecast models. Significant errors in storm position and intensity, as well as errors in the details of mesoscale frontal structure and precipitation distribution occurred in the 1 to 5-day forecast range. The storm produced 24-h snow amounts of 1-2 ft over large areas of the mid-Atlantic coastal zone and eastern slope of the Appalachian Mountains. Downstream development and associated Rossby wave dispersion initiated by cyclogenesis on 21 January east of Japan is identified as a principal component leading to the eastern U.S storm development on 24 January. Error in the forecasts of synoptic and mesoscale storm structure in this case was modulated by the interaction of these large-scale flows and downscale energy dispersion. Numerical simulations with the NCAR/Penn State mesoscale prediction model show that the 24 h of storm development was associated with the contraction of a tropopause-based PV anomaly from a synoptic-scale wave down to a mesoscale extruded filament. Vertical penetration of the upper PV feature from 300 down to 700 mb is shown to be a first-order component in the rapid intensification of the cyclone. Also, there is strong evidence suggesting from the satelite and radar images that this event might be categorize as a "instant occlusion" event in which the upper PV associated convection instantly merged with the preexisting surface baroclinic front and gain rapid cyclogensis. This rarely happens in previous studies and is very different from the frontal wave cyclogenesis model.