Far‐upstream precursors of heavy precipitation events on the Alpine south‐side

This study examines the upper‐level flow pattern that foreshadows the comparatively frequent co‐occurrence of heavy precipitation events on the Alpine south‐side together with potential vorticity (PV) streamers aloft over western Europe. It is hypothesized that each streamer is itself preceded by a dynamically distinctive and a thermodynamically active flow associated with the space–time development, and the ultimate breaking over Europe, of synoptic‐scale Rossby waves. The ECMWF (European Centre for Medium‐range Weather Forecasts) ERA‐40 dataset is used to examine the nature of a streamer's coherent precursor pattern by deriving seasonal composites of refined Hovmöller diagrams, the upper‐level pattern of the PV and its gradient, and mid‐tropospheric diabatic heating. For the autumn and winter seasons, the composites reveal a significant and coherent wave‐packet precursor signal in the PV field that can be traced back ∼1 week to the central and eastern Pacific. Contemporaneously, the coherent signatures in the composite PV gradient field and diabatic heating fields are conducive to triggering and/or supporting the wave signal, and also contribute to subsequent downstream wave‐breaking. In spring the composite precursor PV signal appears over, and propagates eastward from, the Atlantic basin, and its limited zonal extent is attributed to the lack of a coherent PV waveguide. In summer, the signal appears essentially in situ in the Hovmöller analysis but extends across the Atlantic basin in the PV composite results. An indication is also provided of the extent to which the composite characteristics are evident in two individual events, and note is made of the study's implications for medium‐range forecasting. Copyright © 2008 Royal Meteorological Society