Observations and impacts of upstream blocking during a widespread orographic precipitation event

A unique combination of airborne and ground-based Doppler radar observations, stretching continuously from the Alps south to the Mediterranean Sea, is assembled to document profoundly blocked airflow and associated modification of precipitation processes upstream of the Alps during Intensive Observing Period 8 of the Mesoscale Alpine Programme. This diversion of incident southerly low-level winds in advance of a deep baroclinic trough served to reduce markedly the transport of warm, moist Mediterranean air toward the steep Alpine slopes. Heavy rains forecast in this flood-prone region thus failed to materialize, with lighter (though protracted) stratiform precipitation extending over the comparatively flat reaches of the adjacent Po Valley. Release of convective instability did occur, but was displaced well south of the Alps over the coastal Apennine mountain chain and adjacent waters of the Mediterranean Sea. Influences on a variety of scales that evidently led to development of this blocked flow are examined, and multiple expressions of upstream blocking are identified. These range from pervasive down-valley flows observed within multiple deep Alpine river drainages to a larger-scale cool easterly low-level flow channelled alongside the curving south face of the Alps, which was ultimately extruded southward over the Mediterranean. Observed thermodynamic profiles in these contrasting regimes are presented and related to the observed precipitation structure.