Analyses of four separate autumnal events of very heavy precipitation
on the southern slopes of the European Alps indicate that each was acc
ompanied by a strikingly similar anomaly at upper-tropospheric levels.
It took the form of a narrow (similar to 500 km), deep (similar to 4
km) and elongated (similar to 2000 km) streamer of intruded stratosphe
ric air [sic, high potential vorticity air] extending north-south from
the British Isles to the western Mediterranean. In each case the stre
amer translated comparatively slowly eastward, and the storm event ens
ued as its forward flank approached the Alpine ridge. Empirical indica
tors and theoretical considerations support the thesis that the stream
er is an ubiquitous, dynamically significant, and distinctive precurso
r of storms on the Alpine southside.