• Rossby-wave packet;
  • downstream development;
  • blocking high;
  • initial error


A singular value decomposition and a simple sensitivity analysis applied to an operational ensemble of monthly forecasts reveal that the prediction skill of a stratospheric sudden warming event observed in late January 2006 is particularly sensitive to uncertainty in the initial state in the vicinity of a developing synoptic-scale cyclone that was observed over the North Pacific more than two weeks prior to the peak of the event. For the first few days in the forecast, a local maximum of the forecast spread around the cyclone is translated eastward across the Pacific in association with observed downstream development of synoptic-scale disturbances. The spread then reaches the subpolar North Atlantic, where a blocking ridge was developing as the source of an upward-propagating Rossby-wave packet that gave rise to the deceleration of the stratospheric polar-night jet (PNJ). Following the observed wave packet, the maximum forecast spread is translated upward from the ridge and finally reaches the stratosphere, causing a large forecast spread in the PNJ deceleration. Copyright © 2010 Royal Meteorological Society