Analyses of four separate autumnal events of very heavy precipitation on the southern slopes of the European Alps indicate that each was accompanied by a strikingly similar anomaly at upper-tropospheric levels. It took the form of a narrow (~500 km), deep (~4 km) and elongated (~2000 km) streamer of intruded stratospheric air [sic. high potential vorticity air> extending north-south from the British Isles to the western Mediterranean. In each case the streamer translated comparatively slowly eastward, and the storm event ensued as its forward flank approached the Alpine ridge. Empirical indicators and theoretical considerations support the thesis that the streamer is an ubiquitous, dynamically significant, and distinctive precursor of storms on the Alpine southside. ¿ 1998 American Geophysical Union