In this work, the extreme precipitation events in NW Greece are studied. Thedata used are daily precipitation totals recorded at the meteorologicalstation of Ioannina University for the period 1970–2002. 156 days withprecipitation totals above 35 mm (5% upper limit) are only considered. Itis seen that, a minimum frequency of extreme precipitation events appears inthe period 1986–1991, which is characterized by a high positive NAO index. Foreach of the 156 extreme precipitation days, at first, the mean sea levelpressure pattern over Europe is constructed by using 273 grid point values.Using Factor Analysis, the dimensionality of the 156×273 data matrix isreduced to 156×5 (84% of the total variance) and then, Cluster Analysisis applied on the results of Factor Analysis. Thus, the 156 cases aregrouped objectively to 11 clusters, revealing the main pressure patterns,which favour extreme precipitation in NW Greece. Seven of the patterns areencountered in winter and autumn, while three of them cover a period fromautumn to spring and one appears mainly in summer. In all of them the causeof the extreme precipitation event is a low pressure system centred west ofGreece or a low pressure trough extended eastwards or southwards up toGreece. In some cases the depression is so strong and extended that itcovers the whole Europe and the Mediterranean. In the single summer pattern,rainfall is caused by an extension of the SW Asia thermal low up to thecentral Mediterranean.
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