Analogues of a Flow-regime Conducive to European Forecast Busts: Are Mesoscale Convective Systems Catalysts of Synoptic Forecast Error Downstream?

Abstract:

State of the art global numerical weather prediction models occasionally exhibit large medium-range forecast errors - termed “forecast busts” - over Europe. Composite analyses of forecast busts identify flow regime teleconnections that give rise to atmospheric blocking downstream. One such flow regime involves a Rockies trough, mesoscale convective systems (MCSs) over eastern North America, and a coherent Rossby wave guide. Errors in representing MCSs and their convective environment have been hypothesized to be a stimulus for European forecast busts. However, the combination of MCSs and a Rockies trough are rather common features with respect to the number of forecast busts associated with the flow regime. Hence case to case variability may hinder a clear explanation of the role of MCSs in propagating forecast error downstream.

The forecasted evolution of two MCS cases are compared using ECMWF-TIGGE fields and ERA-5 reanalysis. The first case resulted in a forecast bust whilst the second, which was selected based on an analogous flow-regime, is associated with comparatively enhanced forecast skill downstream. The spatial and temporal characteristics of both the synoptic and convective environment are synthesized for each case to deduce whether discernible differences can be observed between the analogues. The MCS environment during the evolution of the forecast is also investigated using the FLEXTRKR: MCS database over the United States. The present study will aid future research aiming to climatologically investigate the role of MCSs in propagating forecast error downstream.