The Impact of Forecast Error Growth on the Medium-Range Predictability of a European Cyclone

Forecasts from several deterministic and ensemble numerical weather prediction systems, initialized between 9 and 10 December 2011 consistently forecasted a strong mid-latitude cyclone impacting Northwestern Europe on 16 Dec. 2011. For example, 29 of the 51 members of the ECMWF Ensemble Prediction System (EPS) initialized at 0000 UTC on 10 December 2011 predicted that a cyclone with a minimum sea-level pressure below 950-hPa would make landfall in Northwestern Europe. This consistency among different models and run-to-run consistency gave many forecasters enhanced confidence that a significant cyclone could impact Northwestern Europe 6 days later. However, subsequent forecasts initialized after 10 December 2011 predicted a much weaker cyclone impacting Northwestern Europe, which ultimately verified.

The goal of this study is to use the available ensemble data to investigate the source of the noted large, medium-range forecast errors associated with this event. To accomplish this goal, two subsets of ensemble members were objectively created. One subset contains the 8 ensemble members that forecasted the strongest cyclone, as measured by minimum sea level pressure. This subset is referred to as the “strong forecast” subset. The other subset contains the 8 ensemble members with the lowest cyclone track error. This subset is referred to as the “good forecast” subset. Novel analysis techniques that employ these two subsets suggest that the differences in the end forecast stem from how the ensemble members handle the split flow that develops early in the forecast period in association with a deep trough off the West Coast of the United States, and the subsequent phasing of upper-level troughs in the northern and southern streams that occurs downstream over Eastern North America later in the forecast period. In the strong forecast subset, the configuration of the upper-level flow and phasing of the northern and southern streams appears to create an erroneously favorable environment for rapid cyclogenesis over the Eastern Atlantic Ocean on 15 December 2011. Further analysis will determine the role and relative importance that individual synoptic and sub-synoptic scale features play in creating both the forecasts for a strong cyclone and the forecasts that were closer to reality.